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#1 |
Registered User
Join Date: Aug 2005
Location: DFW
Posts: 268
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slight stumble with factory 402
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.
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1970 Custom Original 400/400 Now ZZ454/4L80E/Edelbrock Pro-Flo 4/3:73 Eaton Posi PS PB and cold factory air Early Classic 4.5/6 Front 20X8.5 Riddler 4.75 inch bs Pirelli 255/40/20 Rear 20X10 Riddler 5.5 inch bs Toyo 295/40/40 Rear end is original wide 6 lug converted to 5 lug |
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#2 |
TOTY 2006
![]() Join Date: Dec 1999
Location: Magalia CA USA
Posts: 2,270
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Re: slight stumble with factory 402
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.
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#3 |
Registered User
Join Date: Jun 2010
Location: Upland Ca
Posts: 4,143
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Re: slight stumble with factory 402
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.
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1972 C10 SWB, Air, PS, PB, 350/350THM. Second owner. 1965 Corvette roadster, 44K miles, 327/365 SHP, 4 speed, side exhaust, knockoffs, teak, second owner (bought in 1970), Have ALL numbers matching components. My frame off restoration thread: http://67-72chevytrucks.com/vboard/s...d.php?t=556703 |
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#4 | |
Registered User
Join Date: Aug 2005
Location: DFW
Posts: 268
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Re: slight stumble with factory 402
Quote:
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.
__________________
1970 Custom Original 400/400 Now ZZ454/4L80E/Edelbrock Pro-Flo 4/3:73 Eaton Posi PS PB and cold factory air Early Classic 4.5/6 Front 20X8.5 Riddler 4.75 inch bs Pirelli 255/40/20 Rear 20X10 Riddler 5.5 inch bs Toyo 295/40/40 Rear end is original wide 6 lug converted to 5 lug |
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#5 | |
Registered User
Join Date: Aug 2005
Location: DFW
Posts: 268
|
Re: slight stumble with factory 402
Quote:
Can the power valve be replaced without total rebuild?
__________________
1970 Custom Original 400/400 Now ZZ454/4L80E/Edelbrock Pro-Flo 4/3:73 Eaton Posi PS PB and cold factory air Early Classic 4.5/6 Front 20X8.5 Riddler 4.75 inch bs Pirelli 255/40/20 Rear 20X10 Riddler 5.5 inch bs Toyo 295/40/40 Rear end is original wide 6 lug converted to 5 lug |
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#6 |
Man of Mini-Talents
Join Date: Sep 2009
Location: Oklahoma City, OK
Posts: 4,295
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Re: slight stumble with factory 402
Have you looked under the chassis? You might have a Prius stuck under there....
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#7 |
Registered User
![]() Join Date: Sep 2001
Location: Fort Wayne, Indiana, USA
Posts: 1,887
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Re: slight stumble with factory 402
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".
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#8 |
Registered User
Join Date: Aug 2009
Location: Alden NY
Posts: 2,705
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Re: slight stumble with factory 402
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
__________________
1961 C1 Corvette 1959 El Camino 350 TPI, 9" 4 w disc 69 Blazer K5 - sold July '20 2021 Durango RT 5.7 |
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#9 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,628
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Re: slight stumble with factory 402
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.
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