I hadn't noticed before probably because use was pretty limited to some short, neighborhood type errands... but, when I drove to cars & coffee last Sat I noticed a definite missing at cruise (1800-2200 RPM range; wasn't on the highway).
Given that, here's what I'm thinking:
1. Retorque intake & headers - don't think they've loosened because I retorked them recently but, worth double checking.
2. Tighten exhaust clamps - possible they've loosened a bit - probably not related, but hearing some exhaust leakage.
3. Pull plugs & inspect to see what they indicate.
4. Re-check timing curve -- make sure full timing in by 2500-3000 & not bouncing around at higher RPM like it was earlier (I have a replacement timing light; hoping that's what it was or maybe that the problems w/intake being loose were causing that).
5. Once I've done that, what's next -- further adjusting timing or carb?
- Timing is currently at 10* initial but I've read SBF's tend to like 12-14 (Ford Racing tech also mentioned that the when i called about oil level too). Could use vacuum method of setting timing.
- Have also read missing @ cruise is likely carb related.
Anyway, i know this stuff is way easier if a methodical process is followed, just know know what that process should be. What steps should I follow?
Ok, did some checking - intake & headers hadn't loosened; they were good. Checked the advance...
idle (700rpm) 12* initial advance
1000 - 12*
1500 - 12*
2000 - 20*
2500 - 22*
3000 - 24*
3500 - 29*
4000 - 32*
4500 - 32*
Timing affect on vacuum - this doesn't seem like it could be right but it's what the gauge showed... What do you make of it?
measurements taken @ idle (although rpm increased some with advance) with air cleaner on...
12* - 6-7" vac
14* - 7-8"
18* - 8-9"
20* - 9-10
22* - 10-10.25
24* - 10-10.25
26* - 10.5-11
28* - 11
30* - 11.25-11.5
32* - 11.5
36* - 11.75
38* - 12
Also, and this also seemed too... vacuum drops when my electric fan turns on. I assume the rpm's also dropped but I didn't think to check the tack at the time. Why would that be? Fan is wired thru a relay with 10ga wire to a junction post that is fed by 10ga to the source (don't recall if I fed it from the alternator or the battery lead at the starter)
Vacuum seems very low. What cam do you have? What is your idle RPM? My 302 pulls around 18 inches at idle (900 rpm), yet the cam gives a power peak at about 5900 RPM. Often missing art cruise is due to too lean of a mixture, maybe tyou have a vacuum leak somewhere.
Cam is a B303. I think it should be way higher vacuum. Idle speed is 750. Can bump it up.
Vac readings above are with no vacuum advance (current distributor is mechanical only)
I did spray carb clean around base of carb & around intake. Engine didn't speed up any so don't think I have a vac. leak.
FWIW, I did a vac test on my 302/B303 cammed engine last week (to set idle mixture screws) and got about 16" Hg. Vac guage was "T"eed with distributor vac advance.
what are the chances my gauge isn't accurate? It's about 30 years old. Do they have a diaphragm or something in them that could dry out over time or from sitting for a long time (I probably haven't used it for 15 or so years till recently.
The vac guage I used was borrowed from a friend's retired mechanic dad - good quality but was decades old.
Maybe you are connecting the vacuum gauge to ported vacuum and not manifold vacuum.
Nope, it's on manifold vacuum (the lower of the 2 smaller ports on the carb).
That vacuum is pretty low Rob.
I would confirm it with another guage before going any further though. If it's really that low then your carb is going to be way out of wack.
Vacuum will drop some with your electric fan engagment. The alternator loads the engine and drops the rpm some causing the vac. loss.
If everything checks out ok set your timing to 38' total and we'll start setting up your carb.
Cheers
Fred
Alright. Had to go out of town twice in the past few weeks, but back now.
Intake torque was/is good.
header bolts were/are tight
swapped my MSD distributor w/o vacuum advance for a Ford Duraspark (Rich-Porter brand actually) for an 85 Mustang
swapped my plug wires (MSD cap used pin type connectors & Rich-Porter uses stock Ford cap w/regular female connectors)
Timing is now as follows (with vacuum advance disconnected & plugged at carb):
idle (~900) - 10*
1000 - 10.5*
1500 - 19.5*
2000 - 29*
2500 - 29*
3000 - 32*
3500 - 36*
And, vacuum is about 13.5 to 14" with gauge T'd into vacuum advance (connected to manifold vac).
Still have the miss when cruising at about 1800-2000 RPM.
I've read that the miss is likely a lean condition at cruise mode. Does that sound right?
One other thought since vacuum leak could also cause... I've sprayed carb cleaner all around the carb & intake gaskets - no sign of a leak. But, could having the "wrong" PCV valve be the culprit? I don't recall what application the valve I'm using is from & if it's wrong, what do I look for to get it "right"?
I doubt that a PCV is contributing to your problem - It's just a dumb 'ole ball check that allows pressurized crankcase vapours into your intake manifold. A bad PCV setup would manifest itself with oil seepage/blowby at gasketted joints.
I just grabbed any old PCV off the auto parts shelf that looked the part - a 90* black elbow that would fit into my valve cover grommet..
As far as verifying a lean cruise condition, getting exhaust A/F data from a wideband O2 sensor would tell you at lot of what is going on....maybe you can get access to one!
I've thought about that Graham. Have my MSD distributor on ebay. If it sells quick I'll be able to get one. In the meantime I'm thinking of just adjusting 1 step richer for cruise mode and seeing if that helps.
on the PCV - are you saying it should be effectively closed unless there's crankcase pressure sufficient to move the ball? Mine is definitely sucking vacuum thru it all the time - hard enough I put a double layer of tape over it when I was adjusting idle mixture for max vac because I was worried it might tear a single layer. I have it connected to the large center port on the Edelbrock carb.
Quote....are you saying it should be effectively closed unless there's crankcase pressure sufficient to move the ball?
Yes... It's a one-way check valve, so if air is flowing back thru the valve into the crankcase, it's malfunctioning.
If the PCV is removed from the valve cover (with the engine running), you should feel suction.
Man, the numbers stamped on the metering rods are tiny. Couldn't read them even with my readers!! Should have seen me huddled over my wife's magnifying lighted makeup mirror trying to read them -- even then I could hardly make them out. Installed rods one step richer for cruise mode this morning before work. Didn't have time to try them out; will do that this evening after work.
going one stage richer on cruise mode didn't help. Still missing @ cruise. What to check next?
The good news is I don't have any fouling on the plugs any more. They are all consistent - maybe a little darker tan than I expected, but at least they're consistent color.
Hmmmm. Got a PM from Kelly commenting on my cam. Called Ford to test his suggestion. Filled them in on the missing/bucking problem I'm having. Here's how the conversation went:
Ford: What RPM range?
Me: 1800-2200
Ford: That's the cam. That cam doesn't make power till 3000; what gear ratio?
Me: 3.54
Ford: Oh, yeah, you probably need 3.73; maybe 4.11
Ugh. In probably 30 seconds & in a very "matter of fact" way, they clarified what I've been working off & on to fix for a year. Granted, I've bounced around on different topics & fixed other stuff in the process, but wish I'd have known this earlier.
Net, I don't want those gears because I don't want to be revving in the 3k range cruising around. So, based on that response it sounds like I should be look to replace my cam.
Anyone disagree?
If not, suggestions for a cam to consider? Mostly street driven; no plans to drag race it; occasional autocross as in the V8 meets.
Did I say Ugh already? Oh, and thanks Kelly.
Rob,If your timing set has the 3 keyways and assuming you used straight up (advanced 4,straight up,retarded 4) you can move the power curve down 1000RPM by advancing 4 degrees, up 1000 RPM by retarding 4 degrees.
Thanks Jim. According to Ford, the B303 cam already has 4° of advance built into it. They're saying if I advanced another 4 at the timing gears it'd only move the power band about 100 rpm.
Rob, I have run a 4.56 with a 25.5 inch tire and was no where near 3000 RPM. Do you know what your OD ratio is? I have been running a 4:11 for several years now and if I recall correctly at 60 MPH I am at about 2200 RPM. A 4:11 with that cam might be an experience you don't want to pass up.
Don't panic yet, Rob. - my B303 cam is quite happy cruising the highways at 2200rpm (70mph).
Current distributor specs are 10* initial, and 26* centrifugal....Of course, you'd have to figure out the right spring rate combination to optimize the advance curve.
Car doesn't 'buck' until I get down close to 1500rpm
Well, our motors are pretty close to identical spec wise. I should be able to achieve the same then if I can get close to your timing curve.
Also, what carb are you running these days? If it's an Edelbrock 1405, what jets & rods are you running too?
Rob, Just checked my notes and I went 2 stages (8%) leaner on the primaries and retained the stock secondary. That makes the primary jet 0.100 (#1428) and the rod .075 x .047 (#1459).
Seems to work fine there but the engine didn't like another stage leaner.
That's very interesting Graham. You went two stages lean in both power & cruise modes. I thought I'd read that one primary cause of missing was being too lean. Going richer didn't help me & I'm now 3 stages richer in cruise than you. Our motors are similar spec although there are a few differences - but if your car isn't bucking or missing @ cruise, I think my issue is more likely to be timing related than fuel. I'm going to double check which centrifugal advance plate is in the distributor - was told it would be the 15L but didn't check it. Also need to see how much advance the vac adv. is adding in.
Update: Rich-Porter distributor advance plate isn't marked. There are two different size slots & it was on the smaller of the two. But not sure how much centrifugal that slot allows. I'm going to call them tomorrow to get the specs. But, I'm also going one to test it on the car to see how much it adds. There were different weights - light spring and a heavier one. The heavier one is a little loose; I think that's so the light one can do its work up to a point where the heavier one takes over. I'm also going to do the same with the vacuum connected to see what it adds & where.
Vacuum advance is pretty well a non event other than at idle - won't affect WOT total advance condition,
I knew it doesn't affect WOT but assumed it could/does affect light throttle cruise - thought was that is a higher vacuum condition so if the centrifugal advance isn't all in by the 1800 to 2200 range I have the miss, then the vac would be adding some timing still. Is that not right?
True, Rob - there can be some vacuum advance effect under part throttle and decel but that contribution would be hard to determine - you'd almost have to have a vac gauge hooked up to get a feel for how much and when.
I also understand that there is an internal adjustment available to reduce vacuum total advance (requires an Allen key adjustment through the vac nipple) but I've never tried it.
Perhaps some wisdom from "Fred" at this point ? As I recall 'the leaner the mix, the hotter the spark' ? How many volts ? Cheers, roverman.
Yeah, I've noticed Fred's been quiet. I'm imagining he's sitting back with a good Scotch amused by the follies of this neophite!
Hi Rob,
Don't get frustrated. When I'm tracking down a bothersome problem I always try to remind myself that the internal combustion engine is a hunk of metal at war with itself, angrily trying to rip itself apart and destroy its very existence. When you think about 5,000 explosions a minute acting against little metal objects hurling themselves to the heart of the explosion it gives you pause. Then a misfire that happens every 20,000-30,000 explosions seems to come into perspective.
If you could somehow identify the cylinder it's occurring in then you've got it narrowed down. If it's a random misfire then you have to look broadly at your entire ignition. Things like swapping out the msd, different spark plugs, a different brand of diz cap, etc., and hope it's not a valve. As I'm sure you're aware, the valves rotate in their seats as they operate so a bad valve may seat 80% of the time and allow a misfire the other 20%. I had that happen on my GTO and it drove me nuts until I just yanked the engine and replaced the wear items I hadn't when I built it to begin with, even though I did the valve job myself I missed it.
Good Luck,
NCtim
Oddly enough Rob that's exactly what I'm doing!
Actually just really busy at the shop.
Race season is starting and everyone has gone mental.
There is no reason that you can't run that cam at 2200 rpm. It's an aggressive grind but it's not that big.
What happens when you drop it down a gear and get the revs up a little?
Rob, WRT your Rich-Porter distributor centrifugal plates not being identified for degree advance, I found this on the 'net that may help by measuring the slot width.
To figure approximate slot width for a given advance figure Multiply the number of desired centrifugal degrees by .013" then add .150" to account for the width of the stop pin.[/ib]
8L slot = 16 degrees centrifugal advance = .358"
9L slot = 18 degrees centrifugal advance = .384"
10L slot = 20 degrees centrifugal advance = .410"
11L slot = 22 degrees centrifugal advance = .436"
12L slot = 24 degrees centrifugal advance = .462"
13L slot = 26 degrees centrifugal advance = .488"
14L slot = 28 degrees centrifugal advance = .514"
Great info Graham. But get this... Put the dizzy back in. Fired up the car & reset initial timing. After a few minutes I started hearing a squealing. Listen around & its coming from the dizzy. Loosen the retaining bolt just a little & squeal stops. Tighten again & squeals again. This just can't get much more "fun." When I bought it O'Rieley's said they haven't had any come back... Now I; wondering how many they've actually sold. Thinking I'm going to have them get me a real duraspark.
Fred & others - if I drop to a lower gear the missing goes away. I'm not sure, but I think it has more to do with load than RPM. I'll test steady cruise at higher rpm but I have noticed that, while less pronounced, it still misses in cruise at least up in the 2400 to 2600 range. When it's missing, if I simply accelerate a little - even very little - the missing stops till speed stabilizes @ new cruise speed.
As another test, I disconnected the vacuum advance & plugged it @ the intake to see if the missing improved w/o it (I don't recall the missing being as pronounced with the MSD). Net, no change - missing @ cruise was the same.
Rob, Maybe it's time to get it to a shop to do some diagnostic/scope work.
These new scopes can run gas emission checks/AF ratio condition, check electrical firing condition, indicate a rich/lean condition, indicate engine vacuum leaks, etc.
My reason for posting this now is the fact that my car is at one of these garages right now - first indications are that the electrics are OK but there is a lean condition at idle (possibly due to a vacuum leak.
Sometimes it's less frustrating to hand over a persistent engine issue to the professionals.
I hear you Graham. What sort of scope should I verify they have? Asking because I've chatted w the owner/proprietor of a local one man shop. He's a great guy who is more than happy to help. His business is really state inspections but he's known as someone who'll sort things like I've got going between other jobs. Not sure what sort of diagnostic scope(s) he has. He does have a rolling road & AFR that's used for the inspections -- don't know how flexible the software on that is to use it for other diagnostics.
Here's a couple of good articles from the straight six forum on distributor tuning:
http://classicinlines.com/IgnitionTiming.asp
http://classicinlines.com/spark.asp
Replaced the Rich Porter distributor with a rebuilt Ford unit. According to Cardone's web site, their 30-2831 is for an 85 mustang 5.0L 4Bbl, manual transmission, steel gear. ScotLand's computer says the same thing... But got two different ones and both had cast gears on them. Local NAPA got me a distributor for the same spec car. Box is clearly a Cardone box but has a NAPA sticker over the Cardone part # and instead it's listed as 48-2831... But it did have the the steel gear :). Aside from that, the difference was $79+30 core @ Napa vs 45+10 core @ AutoZone. So, twice the price, but the right thing.
Oh, and the NAPA one's vacuum advance is a good bit bigger diameter & it feels softer - like it will pull in with lower vac (& I compared with the adjustment on both all the way to the lightest setting). Initially I swapped it out with the rich porter one (also smaller) to get it timing set because NAPA one was hitting my A/C compressor but then I realized I could turn the oil pump rod & get the distributor to seat so the vac can is right in the middle of the compressor & thermostat housing. So, I may swap it back.
Now the good part... Car is definitely running better with the rebuilt Ford distributor. There's still some missing @ cruise, but not as bad as it was. I've decided to take it to the shop I mentioned anyway since BV8 is coming up so soon but it's definitely closer than it was.
almost forgot, pretty sure the squealing on the rich porter was coming from the bottom of the gear rubbing on the machined surface that you access the oil pump drive through. The bottom of the gear is now shiny - it was black before I installed it. That distributor is definitely going back.
Made a little progress - took my car to a shop. They were able to find a vacuum leak that I didn't. I was spraying carb cleaner at idle (about 900rpm). It didn't show up there. They found it by bringing the revs up a bit - small vacuum leak at carb to intake gasket - but enough that it kept running with idle air bleed all the way closed. Also AFR showed lean @ idle & when revved - don't know yet if that's due to the vac or if carb will need adjusting.
Your carb will need to be set up for sure Rob.
You need to nail down your timing as well.
But you must focus your efforts on one change at a time.
I generally set up the mixture first on a street motor because timing has very little effect on the mixture.
Set your total timing at 38' with the vacuum advance disconnected.
Disconnect the secondary linkage on your carb. You just want the primarys only.
Next go get or build something like this.
o2s001.jpg
o2s002.jpg
o2s003.jpg
This is just an autometer wide band o2 meter that I built into a hard case. It has some extended leads and a lighter plug for power.
I use some stainless welding rod bent up to hold the sensor a foot or so into the tailpipe or on some cars an unused o2 bung.
Plug into the lighter and you're good to go.
It gets used every day at the shop and has been drop dead reliable.
You will need an assortment of needles and jets such as the old Carter strip kit or the Edelbrock cal kit.
Find yourself a good straight level road that you can make some hot passes on without getting into trouble.
Warm your car up and make a full throttle run in third gear with a passenger watching the o2 meter. Obviously, back off if you hear any detonation or run extremely lean.
Start changing metering rods to dial in your fuel mix. If you run out of room on the rods, then change jets. (you are concentrating on the small tip of the needle at this point). You are looking for a consistent reading around 13-1 Once you are there, write down the rod and the jet size.
Next we'll set up the cruise mix.
Pull out your metering rod springs for now. You don't want the rods lifting for this part.
Gently take your car up to it's cruise speed and swap out rods until you can consistently read 15 to 15.5 to one, which should be good with your cam.( this is the thicker part of the metering rod).
Again copy down the rod and jet sizes.
Now reassemble the carb completely using the best rods and jets from the full throttle run.
Reconnect the secondary linkage.
Do the full throttle runs again and this time change the secondary jets only, to get the 13-1 mix.
Now the hard part. You need to calculate which rod and jet combo will net you your results from the driving tests.
Edelbrock has a chart that came with your carb that will help, or you can just do the straight area calculations(PiR2) of the jets and rods and go from there. (The secondary jets won't change from your test results).
A huge variety of rods and jets exist so just about any combo is doable.
Now you can do timing.( vacuum advance is still disconnected)
Mark off a distance on your road that will accommodate a full throttle run from 2000 to red line in third gear.
With a stopwatch, start adjusting your timing to get your fastest average run between your markers. Don't be surprised if it's less than you think. (IE 34' total) Take note of the total timing.
Next is cruise.
At speed (yours is 2200 rpm I think) in 5th gear, you want to dial in as much timing as the engine will tolerate. Probably somewhere around 46-50' @ 2200 rpm. You will know when the engine is happy. Take note of this number. Take a manifold vacuum reading at this point and a ported vacuum reading as well.
Now lock down the distributor so that at max advance it reaches the reading of your best power run.
Rev the engine up to 2200 rpm and note the timing. Is it stable? Remember the smaller and larger advance springs? At 2200 rpm you want the smaller spring to be maxed out and the larger spring to be just starting to take up the slack so that the timing stays relatively stable at your target rpm. Some experimenting here with different springs will make a big difference.
Now you connect the vacuum advance.
Ported or manifold is your choice, but I have never used manifold vacuum on an engine that was tuned well.
You want to adjust the can so that it's all in at the vacuum reading that you took at cruise.
You also need to file, bend, weld, paperclip or chewing gum the vacuum stops to get the timing reading that worked best @cruise rpm.
The cruise "manifold" vacuum vacuum reading should be slightly higher than the metering rod springs rating. Your springs should be pretty close if I remember correctly.
And there you have it!
Should run nice and get decent mileage.
All in the time it takes to enjoy a good scotchy scotch.
Seriously, there is a lot more to it and you do get a feel for it after a while, but this should get you closer than you are now.
Cheers
Fred
QuoteAll in the time it takes to enjoy a good scotchy scotch.
: )
Thanks, Fred Great info!
I'll save that with my notes from Dan Jones.
Me too, Carl. I wish Fred would move to the sunny south so I could just take him my car when it's almost done.
NCtim
Agree, that is excellent info. Fred, as always, thank you for taking the time to write it up. Excellent info.
Lookin a Freds' latest pose, he's heard how much "fun" West Hollywood is ? Party-down, Fred. Cheers, roverman.
Thanks guy's, always happy to help out where I can.
The picture is new years in Jamaica.
Apparently I had a good time.
Cheers
Fred
Time for an update... Looks like I had two problems... 1) a vacuum leak between carb & intake. Didn't show up spraying carb cleaner but I was doing that at idle. The shop I took it to did it while revving it some & it was evident then. When the carb was pulled you could see where the leak was on the gasket. 2) I was running lean. I'd been checking the plugs after the car had been idling a while. They checked them after higher rpm's an plugs showed white. It still ran lean after the gasket was replaced but odds are the vacuum leak was making it worse.
It got a little interesting at that point. The shop I took it to generally only works on Holley carbs. Called around a bit and got same message from the couple other places that cater to non-stock, non-FI modern cars. Shop said they'd do the Edelbrock just wanted me to know it might take an extra hour or two. My gut said it wouldn't be hard to tune but I didn't have the time this week & was out of runway. Given that and the fact that I'd be in a similar situation if I needed help again, I decided to go ahead and swap my carb. My rationale was the extra $100 (Holley minus what I expect my Edelbrock will sell for) would be more than offset by shop rates - if not in this shot, most likely by the next.
With the carb swap & vacuum leak fixed, the car is running much better. Has noticeably more power & not feeling any miss.
FWIW, the Edelbrock/Carter carb is about as simple to work on, as they get. Glad it worked out. roverman.
That's what I've always heard too Art. But I figured I can learn the theory on either one and make some adjustments, but when I need help it should be nice to have the one the shops around here seem to prefer.
"With the carb swap & vacuum leak fixed, the car is running much better. Has noticeably more power & not feeling any miss."
Rob,
Glad you're happy now. I know that feeling all too well. Sometimes you kick yourself spending that $100 but when it works you run in the house and yell "Honey! Ya wanna go fer a ride?!!!"
That's a feeling you'll remember for a loooong time and I sure hope you shared that with someone. This thread is going into my archives.
Cheers,
NCtim
Rob, luckily I remembered that you were having this problem. I started having almost exactly the same symptoms on my 351 Ford in my Jaguar. Except for my engine being a 351, I have the same ignition, manifold and carburetor that you have.
My engine was running fine and THAN the stumbling started. I was simply convinced that my problem was electrical, so I started throwing money at the problem and started making all of the changes that you did and also had the same results that you had, which were none.
Finally in desperation I read your final fix for your problem and even though I still knew that my problem had to be electrical, I threw on and old carburetor and presto problem resolved.
When I finally rebuilt my Edelbrock carburetor, I found a small sliver of fuel line hose that was blocking the fuel flow to the left side of my carburetor and after the rebuild all was fine.
If it wasn't for your description of your problem, I am sure that I would still have my head stuck in the engine compartment looking for my NON-Existent electrical problem.
My only saving grace is that my wife was driving the Jaguar when my problem started, so at least I can BLAME HER.
Of course if I would have been smart enough to read your final fix, than I would have saved a lot of time and money.
Spent a few hours with a guy who has a few drag cars and a performance shop/carb tuning business today. Smaller air bleeds, smaller jets (primary & secondary), larger squirters on the secondaries, and a couple of small holes in the primary throttle plates and it's like night and day. Noticeably more power. No more bog on transition. Smoother in cruise. Way better throttle response through the range. And, idles smooth @800 rpm (before i was about 1000 to get a consistent idle). Starts easier too. To be fair, I was there a few hours but he did my carb in probably 45 mins between other customers.
Most impressive to me was he made all of the changes based on the car specs (engine, cam specs, car weight, gear, some info about rpm at a few speeds) and was friendly and chatted while working.
My be my imagination, but the car even sounds better.
That's great, Rob. Nothing like having one dialed in right. The holes in the primary butterflies is an old hotrodder trick to help a car with a big cam.
I see that Rick has a successfully modified double pumper (mechanical secondary) carburetor on his car. That is quite an accomplishment as these are the most difficult to tune of any carburetor. Reasons follow:
Street carburetors come jetted from the factory with the assumption that there will be a certain range of air flows through the carburetor. The air flow (velocity) creates a pressure drop that sucks fuel into the air stream and the calibration settings assure that, under steady state conditions, the air/fuel ratio will be optimal over the expected range of air velocities. To achieve these expected air velocities, the carburetor must be sized correctly for the engine. A street tuned Ford 302 needs about 475 CFM of air at maximum output, while a Chevy 350 needs about 540 - 550 CFM. Since the Chevy 350 is by far the most popular engine, there are many carburetors around 600 CFM that are perfectly calibrated for the Chevy 350. By contrast, there are very few 500 CFM carburetors for the Ford 302. A reasonable compromise is to use a 600 CFM carburetor on a 302 that has a vacuum operated secondary system. This means that the carburetor will open no farther than necessary and so acts as a 500 CFM carburetor on a small displacement engine, thus delivering optimized steady state air/fuel ratios. But there is a problem under acceleration. Opening the throttle quickly causes air flow to increase more rapidly than fuel flow since air is lighter than fuel. This results in a lean condition, and is compensated for by the accelerator pump which squirts raw fuel into the manifold. With a big engine, the higher air flow restores the steady state fuel flow faster (remember, the primary throttle bore is fixed, only the secondaries respond to the vacuum signal) than in a small engine which leads to the strange result that the small engine often needs more fuel delivery from the accelerator pump than the bigger engine. So accelerator pump tuning is generally required when using a large, vacuum secondary carburetor on a small engine. This means a faster delivery pump cam or pump linkage setting, generally combined with larger pump discharge jets. Note that the accelerator pump fuel delivery is by spring pressure, and the operating linkage only frees up the spring to do its job. With larger jets, the fuel flow is higher and the spring delivers more fuel in a shorter period of time. About the only way to mess up a vacuum secondary carburetor, other than radical jetting changes, is to decrease the spring tension on the secondary side so that the secondaries open too far and too fast. By contrast, a higher spring tension may restrict top end power slightly, but the effects are minor.
But with a double pumper the situation is not nearly so forgiving. In this design the secondary throttles are forced open mechanically, and if the carburetor is too big for the engine (a virtual certainty for a 302 as there are few if any double pumpers in the 500 cfm range) the air velocities are below the design criteria over much of the operating range. This means that many modifications must be carried out to get acceptable operation, such as increasing jet sizes to deliver enough fuel at low air flow velocities, larger/faster accelerator pump deliveries for both the primary and secondary pumps, and often smaller air bleeds (smaller air bleeds allow fuel to start flowing through the jets at lower air velocities). So modifying an oversize double pumper for street use is well beyond the capabilities of an amateur tuner. Rick was fortunate to find a professional who knew what he was doing.
The question becomes as to what double pumpers are good for. The answer is racing, especially drag racing. Because the secondaries are opened mechanically, they will open a fraction of a second sooner than vacuum operated secondaries. This can reduce quarter mile times by a tenth of a second or two, which often makes the difference between winning and losing a race. But, for street use, the effect will not be noticed.
Good writeup Larry, very informative as usual, and helps clear the fog for many of us amatuer carb tuners. Most instructions are no where near so clear on what the air bleeds do and the logic of the acc pump shot. (And then there is the power piston...)
As for not noticing the difference on the street, probably but it sort of varies. The vacuum secondary is calibrated for a certain pressure drop across the throttle and will maintain that drop. At about 1-1/4" of mercury which is about what 4bbl carbs are flow rated at that amounts to around 5% of the vacuum the engine can produce. It isn't much of a stretch to say that a 5% increase in output would be noticed, and a double pumper can at least theoretically decrease the pressure drop to near zero (fuel metering gets to be a problem, so efi has a potential advantage here). How much of that 5% you can actually get I don't know, but that is probably most of what accounts for the better drag strip performance. Street driving tends to suffer of course so we generally just pay for driveability with a performance loss. Yet another reason for efi, but all fuel injection systems are not created equal. Many will not flow as well as a 4bbl carb and intake.
Jim
More carburetor information (if anyone is interested).
Conventional carburetors have (at least on the primary side) three different metering circuits: idle, transition, and main. At idle, there is vacuum and fuel delivery only on the idle circuit; as the throttle is opened, the idle circuit drops out and the transition circuit comes in, further opening of the throttle shuts down the transition circuit and the main jet system takes over. Interestingly enough, these circuits do not deliver raw fuel to the carburetor air stream, but rather an emulsified mixture of fuel and air. This is due to the emulsion tubes which are between the fuel source and the discharge. All carburetors have at least two emulsion tubes, one for the main system, and one serving both the idle and transition circuits. As an alternative, some carburetors have three emulsion tubes where the transition system has its own tube. The emulsion tubes are built into the metering block. At the bottom is a replaceable jet (main circuit) or a fixed orifice (usually) for the idle and transition circuits. At the top is an air bleed, and in the middle are a series of very small holes (usually 3 to 5) open to the fuel in the float chamber. About ¾ of the way up, there is a passage from the emulsion tube to the discharge system (main, transition, or idle). When vacuum is drawn on the emulsion tube it draws in both gasoline and air, forming an emulsion that is then delivered to the carburetor throat. A larger air bleed means there is more air in the emulsion and also that more vacuum has to be applied to the emulsion tube to get gasoline out of it. A smaller air bleed obviously causes opposite effects.
The most critical circuit for drivability and fuel economy on a mild engine is the transition circuit. With a light car and a big engine, most cruising actually uses the transition circuit more than the main circuit, so main jet changes have a surprisingly little effect on highway fuel economy. But transition circuits are generally calibrated on the rich side so as to avoid drivability problems in "universal" applications. The only practical way to tune the transition circuit is by changing the air bleeds. On most carburetors this means drilling the bleeds larger, but once they are enlarged there is no going back. The solution is to buy a carburetor with replaceable air bleeds (more on this later). It is relatively easy to determine if the transition air bleed is correct on a two circuit carburetor. The clue is the idle mixture screw setting. The screws should normally be set between about one and 2 turns open. If the setting is less than a turn, the emulsion is too rich and a larger air bleed is needed. If the setting is more than about two turns open, then a smaller air bleed is needed. If air bleeds can be easily changed, it is best to try for one where the idle screws are open as far as possible. With a three circuit emulsion system, transition circuit air bleeds can be enlarged until there is a stumble on light acceleration, or a surge during cruising at low throttle openings. In any case, before tuning is attempted, it is important to be sure that the idle speed screw is not set to the point where the throttles are opened into the transition circuit during idle. If more air is needed for idle, a trick is to drill small holes in the throttle plates to deliver the extra air (but some carburetors also have auxiliary idle air bleeds that can be set to avoid the need for drilling). One problem with drilling throttle plates is that this also leans out the transition mixture and smaller air bleeds are needed to correct this (very difficult to accomplish on a carburetor without replaceable air bleeds).
Keep in mind that for street use, a properly sized carburetor will have properly calibrated emulsion tubes and reasonably close air bleeds. But air bleed tuning is worthwhile, as well as accelerator pump tuning. Generally, only large, expensive carburetors have replaceable air bleeds and these have no place on small, street tuned V8s. But there are three reasonably cheap (under $300) small (around 600 CFM or less) carburetors with replaceable air bleeds. One is the Summit brand, which is patterned after the Autolite 4100 carburetor which is, even now, considered one of the best, if not the best carburetor for small Ford V8s. The other two carburetors are the HR and Slayer series made by Quick Fuel. the Quick Fuel carburetors are especially interesting because they have other premium features not otherwise found in this price range such as three emulsion tubes, replaceable idle feed restrictors, etc. They also have a novel control of the vacuum operated secondary throttles. Normally, this is done by changing springs in the vacuum opening unit. A stiffer spring delays the opening point, preventing too much air at low engine speeds. But the stiffer spring also limits the ultimate amount that the secondaries open. What Quick Fuel does is control secondary opening by a needle screw in the vacuum signal line. This unit can thus use a very light spring, and control opening by delaying the buildup of vacuum through the restrictor screw. These vacuum units can also be purchased separately (about $40) and be bolted on to any vacuum secondary Holley carburetor.
So, if one wants to experiment with carburetor settings, I recommend buying a Quick Fuel carburetor because any changes can be easily reversed. The difference between the Slayer and HR series is only in the secondary metering functions. The HR has a full secondary metering block with three emulsion tubes and idle circuits. The Slayer series secondary has only a main jet circuit. As long as the cam is not too radical, all the necessary idle air can be supplied by the primary side of the carburetor and the Slayer series will be fine (at a significant cost savings).
A final note on power valves. Power valves are almost universally confined to the primary side of the carburetor. With a power valve, the primary jets are sized to give a lean mixture, which is fine for cruising, but not good for power. The power valve function is to richen the mixture when power demands are high by delivering extra fuel to the main system (so the power valves open when the manifold vacuum decreases). Power valves can be obtained that open between about 3 inches of Hg to about 12 inches of Hg. The usual setting for a street engine is 6.5 inches of Hg. This is right for most engines, but if the cam is radical and idle manifold vacuum is low, a power valve that opens at a higher vacuum is necessary so that it does not open too soon. Note that in power valve equipped carburetors the primary jets are always smaller than the secondary jets. With the power valve open, the primary side then delivers fuel equivalent to the secondary side. It is thus important to maintain the difference between the primary and secondary sides when changing jets, otherwise the carburetor mixtures will not be balanced. So, if the primary jets are changed by 2 jet sizes or more, it is best to also change the secondary jets by the same amount.