exhaust backfire

[rficalora]

Still getting some run on and exhaust backfire when I shut the car off.  

For those not familiar, it's a Ford 302 with aluminum GT40 heads, B303 cam, and Edelbrock carb, 10* initial timing, mechanical advance.

What should I check first?

[flyinlow]

sounds like your carb is adjusted a little too rich and probably have some carbon buildup in your cyl heads
Its easy to clean the carbon, warm up your engine and remove the air cleaner lid, use a soda can full of water and open throttle enough to keep running while dribbling water down the primaries then take it out on the street and drive it hard letting the engine rev.
getting the carb adjusted correctly is important also just dont go too lean

[mgb260]

Rob,Throttle plates might be open too far, check the secondary stop also.

[rficalora]

How about excess fuel?  I noticed fuel dribbling from the nozzles at the front of the primaries.  Read that means the floats are stuck.  Make sense?  What about if fuel pressure is too high?  Gauge says 5lbs but no idea if it's accurate.  Thinking it's easier to back down the pressure a little than getting at the floats.

[Bill Young]

Rob, you're moving air over the venturies, enough to siphon out the fuel, but not enough to fully atomize it. Throttle plates have to be open a bit but I wouldn't think the floats were stuck or the needles leaking, I'd lower the float level a bit to increase the amount of vacuum needed to siphon fuel out of the bowls to start with. Drop it about an eighth of an inch to start and see what happens.

[rficalora]

Ok, I pulled the top of the carb - have a few questions...

there's a little varnish around the float bowl vents and the upper part of the metering rod hole where the cover is.  Seems very thin but want to clean it while I have it apart.  Is it ok to use carb cleaner with the accelerator pump attached or will it damage the rubber (or whatever that material is)?

There's carbon or something (a thin black build up) around the middle inch or so on the metering rods.  Will clean them but assume that's a good sign there's more & I should follow Kevin's water suggestion.

The floats seem to be set right on the "up" travel - right about 7/16" from the surface the gasket sits on when the carb lid is held upside down.

Not sure about the amount the floats are dropping.  Thought I'd read that should be close to 1" but not sure.  Mine drop almost 1 5/16" on one side and 1 1/4" on the other.  Do I need to adjust that?

It doesn't look like the throttle plates are fully closed but the only thing that appears to be affecting that is the idle speed screw.  Should they be fully closed and still be able to get a good idle?

[Bill Young]

Rob, it's not how far they drop, that would be limited by the depth of the float bowls when installed anyway. It's the distance they are from the face of the carb top when it's inverted, that's what the little fiber gauges that come with a rebuild kit check. Check the height with a small rule or even cut some light cardboard and make a gauge to fit between the float and the top surface. To lower the float you'd increase the distance the float rests above the carb surface by bending the little lever that pushes in the needle. I wish I had pictures, I'll try to find something on line about how to rebuild a Carter style carb and send them to you. Try this link to see how the floats are adjusted. http://www.laroke.com/larryk4674/2009/AFB_Rebuild_Guide.pdf
Figure 8 shows the float height adjustment and how it's measured. That's the critical adjustment as it determines the fuel level in the bowls, too little and you can get fuel starvation at high rpm or wide open throttle for sustained periods, too high and you may get gas dribble into the venturies because even with low vacuum there's just enough to siphon gas out of the bowl. In your case I'd make sure the floats are set to normal spec to begin with then lower them (actually raise them as seen in figure about 1/8" and see if that stops the fuel dribble you're seeing in the venturies.

[rficalora]

Well, I just got back from a test run & it didn't backfire when I shut it off.  It was just about a mile so we'll see if I fixed it or not with the next couple of runs.  I called Edelbrock to see if there's any sort of extended warranty on their "Endurashine" finish and while on the line their support guy started diagnosing the fuel dribble...  He was pretty clear that only 4 things can cause it...
1. Float level on the rise.  Should be 7/16".  Mine were pretty close but I went ahead and adjusted them a bit - maybe 1/16" higher at most.  I think this was 1/2 of my problem.
2. Fuel getting into the floats. Mine were dry.
3. Dirt in/on the valve or valve seat.  They looked clean but I sprayed some carb cleaner, let it set a few mins and then sprayed a little more.
4. Throttle plates not fully closed.  I think this was the other part of the problem because when I put it back together, it was still dribbling some - but less.  I decided to reset idle air mixture to where they were out of the box thinking maybe my idle screw was forcing the throttle open (looked like the only mechanical thing keeping them from closing 100%) to compensate for too much gas getting in on the idle circuit.  It was just a guess, but once I reset the idle air mixture, I was able to back the idle speed screw out a little and maintain idle - and the rest of the dribbling into the venturis stopped.

Assuming this did fix the over rich/backfire, I'll follow the book to adjust the idle air properly.

The next item is I now noticed a little bit of hesitation or missing when I was accelerating in 1st and 2nd and a little popping from the exhaust on quick deceleration.  Likely causes for that?

[Bill Young]

Probably need a bit more ramp on the accelerator pump cam as I remember about the stumbling under acceleration, leaning out just a bit most likely. I'm not sure what options Edlebrock carbs have, but Holly has a lot of different pump cam profiles, Edlebrock should as well. You might also need to work on the timing and adjust the vacuum advance  for a bit less vacuum to retard that input a bit, sounds like it may be over advancing when the vacuum rises as the throttle plates close.

[rficalora]

Thanks Bill.  The Edelbrock has 3 positions that the accelerator pump linkage can connect to.  It's easy enough to try adjusting that, but the more I think about it, the more I think it felt more like ignition - like it was missing.  Would the accellerator  pump make it feel like that or would it feel like it's bogging down?  As far as vacuum advance, my distributor is all mechanical advance; no vacuum advance.

[DiDueColpi]

Hey Rob,
After running the engine that rich you have probably fouled up the plugs pretty good.
I would start with a good inspection of the ignition system before I went any further.
Pull the plugs and make sure they are clean and not blackened.
Double check the rest of the tune and check your oil for fuel contamination.
Warm up the engine fully and set the idle speed with the screw at the throttle stop.
Pick one of the idle mixture screws and adjust it for the highest idle speed that it will provide.
Now do the same for the other idle mixture screw.
Reset the idle speed with the throttle screw and go through the idle mix screws one more time.
Now turn both mixture screws in 1/4 turn. this should cause your idle speed to drop. If it doesn't then you are still too rich.
Now that you are idling right, get a vacuum gauge and check the idle vacuum. If it's anything less than 8" you will need to change your step up springs.
For your cam a pink or possibly orange spring would be appropriate.
Once you get to this point see how the car drives. If it's still off we'll have to start rejetting before we play with the accel pump.
Keep us informed and we'll go from there.
Cheers
Fred

[kstevusa]

Rob, sounds like you are making great progress.  Bet the car looks great also.   Maybe EFI was not too big of a project,heh ?

[rficalora]

Nice Kelly!  But honestly this kind of stuff is fun to me.  I like learning by doing.  I plan to switch to FI sometime down the road, that's why I ran the high pressure fuel pump & return line (with a bypass valve to cut the pressure down for carb use.  I'd only messed with SU and ZS carbs in the past.  The Edelbrock & all the stories about carb problems intimidated me so I put off messing with it.  When I pulled the top off yesterday, I was pleasantly surprised with how simple it was.

Fred, I had the plugs out when I used the piston stop to find TDC.  Now that you've reminded me, they all looked ok - a light grey - except one.  #5 plug was black and I meant to clean it but I had my son put the plugs back while put the radiator back.  

Hadn't thought about fuel getting into oil.

Guess my weekend is shaping up!

[flitner]

An air leak in the exhaust will cause that to happen as well, when I had a header gasket blow out on me and run up the rpms in 1st then engine brake a little ways it would backfire, 10* initial timing sounds low for a mechanical advance, how about trying the vacuum gauge trick as was discussed over on MGE to get optimum settings. Just a thought!

[ex-tyke]

.....to change your step up springs. For your cam a pink.......

 I'll second Fred's recommendation on the step up spring change to compensate for reduced available vacuum with your B303 cam - that was the first thing that was changed on my Edelbrock 600cfm and B303 combo. (pink if memory serves)

[DiDueColpi]

Tan is the color that you are looking for Rob.
Unless you are running a 10% or more blended fuel in which case they will just turn white.
Grey generally indicates contaminants that are burnt onto the plugs.
Clean them all or better replace them. If you clean them it needs to be with a media blaster not a wire brush.
Short gap them @ around .028" until you get the carb sorted out.
Once the carb is close then you can open up the gap and fine tune everything else.
Cheers  
Fred

[rficalora]

Our gas does say "Contains up to 10% Ethenol" year round.  Not sure if we have any sort of winter mix down here but I doubt it; there's never any mention of it at the pumps or anyone talking about it.

[Bill Young]

The mixture in the pumps isn't ever mentioned, but almost all parts of the country now get specific mixtures for summer and winter with more volitle elements for winter driving to help it atomize, and less volitle elements in summer to help reduce evaporation and air pollution. Fuel containing 10% ethanol will burn cleaner on the plugs. Since we're starting to get more blended fuel around here I'm starting to use products like Sta-Bil to help reduce the corrosive effects of the alcohol.

[roverman]

Methanol-yes, 10% ehtanol- not likely. I suggest a peep at the" Performance E85" webbsite. Very informative  without being corrosive/ lol.  roverman.

[mgb260]

Bill is right about Stabil. Not so much corrosive but alcohol draws moisture and separates from the gas. If not driven regularly or stored ,highly recommended.

[rficalora]

Didn't have a lot of time this weekend to work on the car, but did do a little...

Pulled the plugs (#8 was a pain -- had to shift the steering u-joint to get a socket on it), cleaned, regapped (.028 for now per Fred).  They looked good to me except #5 but I'm interested in your opinion of the color & especially likely reasons for #5 being so fouled compared to the rest.  I guess I need to watch the valve/spring on that cylinder and check compression?  What else to check?

A couple of additional updates...
  *Carb is an Edelbrock 1405 (600 CFM)
  *Plugs were gapped way too much.  They were around .044; I didn't check them all, but #5 was a bit more - about .050
   I'm hoping that's why it was fouled so much more.  What do you think?
  *I'm going to change the oil -- didn't smell a strong fuel smel in it but did smell some.

[DiDueColpi]

Hey Rob,
Your engine is probably just fine.
What happens is, when your fuel mix goes way rich the plugs soot up like # 5 and the engine starts to misfire and stutter.
The misfire happens because the soot is a conductor and shorts the plug out.
Back in the old days this didn't happen as much.
Mostly because the ignition systems couldn't tolerate today's wide plug gaps. And partly due to today's very high additive load.
With the shorter gap the plug will still fire even when fouled.
Once the engine starts to warm up and the plug starts to fire the soot burns off leaving the grey contamination that you see on the other plugs.
That's why you short gap the plugs while you are dialing in your fuel mix.
It makes the ignition less of a variable while tuning.
Once you get the mix closer you can open the gaps back up so that the engine will tolerate a leaner idle and cruise setting.
What does your ign. consist of? It just might be happier with the smaller gap anyway.
Cheers
Fred

[rficalora]

Ignition is MSD6AL; MSD 8579 distributor; plug wires are Taylor 8MM (one of which has the silicone melted from coming in contact w/header, but it's not #5.  I have new plug wires to replace them) and, plugs are AGSP32C.

Note, there is something wrong with the MSD box I think -- the tach output on the side is suspect (either that or I have two bad tachs).  I will be testing the tach output tomorrow or Wed evening after work but am pretty sure it's broken or has come loose.. the 1/4" spade connector feels loose in the box and neither my dash mounted tach (autometer) nor a SunTech one I've used under the hood when timing in the past show any RPM's.  According to the MSD tech, if that output is bad, it shouldn't be affecting the rest of the ignition - including the info just in case you guys have had different experience.

[rficalora]

Did some adjustment today...

1.  Unrelated, but because I needed a tach, I checked the tach output on the MSD box & it was working... So next checked the voltage at the other end of the wire... Or at least went to do that...  That's when I realized I'd connected my Speedo sender to the tach & vice versa.  Tach now works & assuming Speedo will too but didn't go for a drive.

2.  Checked the advance.  Remember, mechanical advance only, no vacuum advance on the distributor.  Result:
    - 10° @ idle (600 rpm)
    - 10° @ 1000 rpm
    - 10° @ 1500
    - 17° @ 2000
    - 20° @ 2500
    - erratic, bouncing between 30 & 10 very rapidly @ 3000

3. Idle reasonably smooth @ 500; have it set to 600 rpm; 1/4 turn in of an idle air screw drops rpm by 50-75 rpm

4.  Manifold vacuum @ idle was generally moving between 8 & 9.5.  Every few seconds it'd drop down to 7 for about a second and then return to about 8.

5.  There's a noticeable popping in the exhaust on deceleration if I quickly rev it to about 2000 rpm and immediately drop it back to idle, but it did rev better and transition from idle was smoother (no real load, just opening the throttle by hand in the driveway)

6. My son was playing with a non-contact thermometer @ noticed the header primaries were all running 250-310° except #5.  It was about 190.  Not sure if the ceramic coating might be thicker there but since #5 was the plug that was way more fouled, thought I'd mention it.  Engine has only been run for about 20 minutes since cleaning the plugs (blast cabinet) & gapped them at .028; I didn't pull the plug to look at it again.

I haven't changed the step-up spring, rods or jets from the way the carb came (orange spring, .100 mains, .070x.047 rods.)

So, what do you recommend as the next step?

[DiDueColpi]

I would turn in both idle mix screws 1/4 turn re adjust the idle speed and take it out for a drive.
That should help out with the backfiring and make the idle more stable.
The erratic timing at 3000 is a concern but it might just be your timing light.
If it's really moving around, you will feel it on the drive.
You might need some more initial timing but lets get the carb set up first.
I'm surprised that your carb has orange springs. These are probably a little too weak. Sadly pink is your colour Rob.
The orange springs will probably cause a flat spot on moderate acceleration. They won't affect full throttle power though.
cheers
Fred