strange vacuum reading

[Roverboy]

Hi,

Being old skol, I rely heavely on a vacuum gauge for my diagnostic and post build analysis.  However I came upon a strange reading after one of my mild 4.0 rebuilds.

Everything looks good except that when I blip the engine, I expect to see the gauge run towards zero, then back up to bout 25 when I release and back to 20 or so (steady).  However, with this one, when I blip , it will just go down to about 15, then back to zero (while holding rev) and then to 25 after the release and a steady 20 at idle.  

The build is a rather standard;

rebored and sleeved to spec a 4.0, crank polished, main cap to 2thou as per specs, rings hand lapped, stock rods/piston, block decked at 3 thou and heads planed with composite head gasket, heads cleaned and polished new stock valves, springs etc etc..  Custom grind on cam ( low rpm torque), HEI, edelbrock, Holley 470 CFM, hedders coated and wrapped.  

Onboard diagnostics;

wideband, water temp, oil pressure, vacuum, EGT (single side) , tack  (all autometer sport comp)


The only thing that I think could explain this is that I have too much back pressure..  

Please chime in

[BlownMGB-V8]

Sounds like you have a bog. I'd check the carb for correct accel shot and tip-in.

Jim