I'm Stumped ! what else could it be ?

[87BlazeLS]

Ok, We installed the new HEI GM performance distributor new wires ( 500 ohms per foot ) Still same persistent stumble .
did vacuum checks and all is steady and stable . Mixture screws set at 1.5 turns out , 12 " vacuum idle about 850 in park.
Timing is right at 13 BTDC. Seems fine when warm at idle. As soon as you start part throttle rpm you can feel the stumble as well as hear it. Driving under load it is more pronounced. As long as your in the throttle in runs good. Back off to part
throttle and stumble occurs. Vacuum advance hooked up or not it still does it. Brand new carb from summit racing replaced 600 EB and the same results. so it's not carb related. It would have sworn it was electrical. Not in the distributor or wires.
Checked hot lead to distributor ign on and get 12.86 volts.

Fuel pump pressure is around 6 psi.

I cannot find any reason for this issue. Brand new GM crate 350 290 h.p

I still think it's electrical in nature but how do I find out what ? Am I missing something maybe at the fuse box or wiring harness level ?

Any suggestions would be of great help here. As I have been with this far to long and running out of options.

jake

 

[hurst1979olds]

What kind of fuel pressure do you have? Also check when its stumbling.

 

[87BlazeLS]

stated above ( around 6 psi ) Stock GM mech. fuel pump.

Carb is brand new Summit Street /strip 600 like Holley

checked for leaks with carb spray and got nothing . Carb base, manifold, vacuum lines , ect.

 

[87BlazeLS]

Timing and or idle mixture ,idle speed, vacuum advance hooked up or unhooked and plugged.

Seems nothing affects the resulting miss. The fuel pump pressure does swing a good bit between 3 and 8 psi but steadys out around 6 psi when the speed is held up around 2000 rpm.

I was told this is normal for this type pulse pump.

I have looked at the plugs and they look fine. Not sooty and not white. More like grey /tan.

This is crazy. I have been trying to find this demon for over 2 months and something is being over looked.

I hate to think it is something internal in a brand new crate motor. Oil is clean and clear. No over heating or bubbles.

The only Thing I can think of outside the electrical box so to speak is the valve lash adjustment.

Oh yes the motor was broken in on a zinc additive added to the oil for the cam.

 

[pontiacgp]

I'd try a new set of plugs....

 

[tc1959]

Maybe you have something arcing under a load. Get a spray bottle with some water in it, Have a helper power brake the car in gear while you spray around all the plug wires, plugs and the cap. If you have a problem it will show up right now.
We have done this for years on cars with coil packs I hasn't failed me yet. Just my 2 cents... I hope you find the problem.

 

[87BlazeLS]

Plugs are new AC Delco and they look fine. burn wise.

What would cause arcing ? Every component in the electrical to the motor is new. New battery wires, All wires from fuse box
to motor, ect. Only does this at just above idle not all the time.

If you hold the rpm at around 1500 to 1800 you can see the motor shake as the miss occurs.
Above that RPM it acts fine. Good power no miss or hesitations.

 

[tc1959]

Plugs are new AC Delco and they look fine. burn wise.

What would cause arcing ? Every component in the electrical to the motor is new. New battery wires, All wires from fuse box
to motor, ect. Only does this at just above idle not all the time.

If you hold the rpm at around 1500 to 1800 you can see the motor shake as the miss occurs.
Above that RPM it acts fine. Good power no miss or hesitations.

Have you thought about having it put on a scope to see what the primary and secondary ignition are actually doing.
That would also help narrowing down what, If any cylinder might be the problem with a Cylinder balance test.
It could be something as simple as a defective plug or as serious as a broken valve spring.

 

[tetomas]

What kind of intake are you using? Does it have an EGR valve on it? If so the valve may be jumping open under light load and causing the stumble. Also check the PCV valve, and the brake booster, easiest just to plug the vacuum lines and test for the stumble. The trouble sounds like a vacuum leak to me. What trans are you running? Does it have a modulator valve? There is another possible place for a vacuum leak. Hope this helps.

 

[87BlazeLS]

TH 350 trans modulator is also new. Plugged all vacuum lines individually for zero net results.
Intake is E B Performer dual plane RPM No EGR .

Motor Has No polution controls what so ever. I ran carb spray all over the base of the carb and manifold with zero net results.

Still stumped !