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Old 09-05-2015, 12:20 PM
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340 with bouncing vac at idle

Heya, glad to be part of the forum.
I have been reading up on the forum and I am totally confused with whats going on with my 340.


I had the engine built 2 years ago,
1967 340 Block
.060 Over
Keith Black Pistons
Isky Cam .500 lift 160 dur
Edelbrock 600 cfm 4 bbl afb
comp valve springs
comp pushrods
340 six-pak heads "J-Heads"
Edelbrock performer rpm air-gap intake.


The car is a 1975 Dodge Coronet custom 2-dr hartop
a-833 4-speed
lol 2.71 gears, I converted it to manual but still have same rear diff


It ran awesome for a while, now im having issues with carb stumbling for fuel/air? just as I roll outa gear.


I have tried accel pump tuning, u name it, I am aight at edelbrocks. but **** its got a bouncing vacuum at idle, that's it. I bring the rpms up a lil bit and steadies out. Runs like a bat outa hell after I roll outa gear. I just don't want it to stumble when I roll outa gear.

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Old 09-05-2015, 07:43 PM
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You have either a lack of timing, too much fuel pressure or a big vacuum leak.
What's your initial timing?
Don you have a fuel reg set at 5 psi?
Have you checked for vacuum leaks?
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Old 09-06-2015, 06:26 AM
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Bouncing gauge at idle like that is a valve problem or a weak cylinder (rings).


Do a compression test, then post back.
Be sure to do a wet/dry test and write the results down
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Old 09-08-2015, 11:33 AM
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I know how to do a comp test, but what u mean by wet/dry?
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Old 09-08-2015, 11:38 AM
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Dry means testing it just after you pull all the plugs.
Wet means testing it again after you squirt some oil into the cylinder.
If the compression goes up a bunch with the wet test, then you'll suspect bad rings.
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Old 09-09-2015, 12:35 PM
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"I bring the rpms up a lil bit and steadies out"

At what RPM does it steady out? What do you mean roll out of gear? Do you mean roll out from an idle in gear?
Normal manifold vacuum on an engine at idle speed is around 18 to 20 inches. the bigger the cam the lower the vacuum.
are your rockers adjustable? What is your lifter lash?

As your Idle picks up your advancing the timeing, So if vacuum levels out at a faster Idle reset initial timeing to a higher initial setting. Do you have a vacuum advance and are you running it off ported vacuum or manifold vacuum?
Are you running a ZINC addative or high ZDDP oil?
A wet dry compression check is needed but isn't necessarily bad valves or rings if you get low readings, it may be Mopar type HY. lifter lash ! The lash is set when lifters are fully collapsed when setting up valve geometry if you do not have adjustable rockers! It may be a couple of your valves are just off their seats causing a bouncing needle because of normal age and small changes in engine expansion and circumstances in lifter pump up as lifter internal tolerances change over time and when you pick up your idle the lifters at higher speed close the valves better! When new everything was tight and breaking in so a too tight lash ran good and in time during driving that lash could actually get tighter and the valve to seat seal is decreased ! If properly lashed this can't happen because the HY. lifter lash tolerance adjusts automatically to changes in its surrounding applied forces, expansion, pressures, heat, cold, valve train speed, valve spring weakening ETC. A couple of week valve springs can cause a bouncing needle too! Poor quality seats that have been pounded in by valve action can also cause valve leak! and change lash settings! A open cylinder during storage when an engine just sets not being turned over for a long period ( over a winter) can cause valve and seat and cylinder wall corrosion and not seal properly! A sticking valve at low Idle can cause needle jump and at a faster idle the valve doesn't stick and the needle levels out!

Jester (Chris)
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