slight stumble with factory 402

[2500HD]

I am looking for help with a slight stumble from 5 to 9 mph and then again from 25 to 31 mph. It is very slight with light throttle and more prevelent with medium throttle. I first thought it was a bad carrier bearing but have ruled that out after inspection and new exhaust was installed.

To cure I installed a pertonix and removed the points. At the same time I installed new cap, rotor, coil, plugs and plug wires. I did not think my vacuum advance was working so I installed a new one after the first test drive and it did not solve the issue. Plugs are gapped at .035 and there is 12 volts to the coild and the pertonix. Timing is set to 9 or 10 degrees. It starts quick and idles smooth.

Any ideas would be appreciated.

[John Fabris]

With all that you replaced, they only other things I would check would be the Carb, or ignition wiring. Carb may need to be cleaned or rebuilt or the ign wire could have a bare spot that grounds out as you are moving. I assume that your transmission is good, not low on fluid.

[Lee H]

Q-Jet? If yes it's the power valve. Make sure the replacement is E85 (corn alcohol) resistant. Also, verify you have 12 volts going to the pertonix.

[2500HD]

Originally Posted by John Fabris

With all that you replaced, they only other things I would check would be the Carb, or ignition wiring. Carb may need to be cleaned or rebuilt or the ign wire could have a bare spot that grounds out as you are moving. I assume that your transmission is good, not low on fluid.

I think I am out of options except for the carb. Ignition wiring is all good. The trans is not low on fluid and goes into reverse and drive good so I assume it is good.

Thanks for the input.

[2500HD]

Originally Posted by Lee H

Q-Jet? If yes it's the power valve. Make sure the replacement is E85 (corn alcohol) resistant. Also, verify you have 12 volts going to the pertonix.

Yes it is the factory Q-jet. My step dad mentioned the power valve. I guess it is time to learn about Q-jet rebuilds. Other than the power valve the carb works great. I do have 12 volts to the ignitor.

Can the power valve be replaced without total rebuild?

[Zoomin]

Have you looked under the chassis? You might have a Prius stuck under there....

[DANTIP]

I agree with the potential of the Q-Jet being the culprit. It sounds like the carb is having trouble with the "transitions" from idle-circuit to full power. Either do some reading on Quadrajets, or find someone that knows them in order to correct the "stumble".

[rsavage]

A Q jet has a "power piston" that the primary metering rods hang off of and a separate accelerator pump assembly. Holleys have power valves. My past experience (and I am certainly not professing to be an expert) is that if a motor stumbles when you try to give it gas off idle circuit, the accelerator pump may not be giving a good enough shot. In any event, you should change the filter in the carb filter inlet first, and if that doesn't work, rebuild and clean the carb

[geezer#99]

Yup. A power piston which is adjustable too!! Could be his carb is a little lean. Metering rods too deep in the jets will cause a lean stumble too when in transition to the power mode.