Rochester 4g off idle issue

[Guest Pletch]

I'm stumped and looking for advice. I've got a stumble/dead spot just off idle. Other than this dead spot it idles great, accelerates great and maintains speed great.

What circuit is between idle and cruise that could cause this?

[Guest riv guy]

I'm stumped and looking for advice. I've got a stumble/dead spot just off idle. Other than this dead spot it idles great, accelerates great and maintains speed great.

What circuit is between idle and cruise that could cause this?

Try your accelerator pump

[Guest Pletch]

I have and I've also confirmed it's not the accelerator pump because it happens at a steady throttle position. It's only during the transition between idle and cruise.

[old-tank]

The Rochester 4GC is junk! Our currently available ethanol laced fuel will dissolve the cement on the bowl plugs which can leak and catch fire. Also the ethanol fuel when it dries in a carb will leave a white powdery residue. That is what seems to clog the 4GC: rebuild it, install and it performs just fine; let it sit for a week and you will have poor idle, stumble, surging until you disassemble and clean again. The Carter WCFB seems to tolerate drying much better. Anyhow study the service manual and concentrate on the low speed circuit.

Willie <!-- google_ad_section_end -->

Edited October 11, 2012 by old-tank (see edit history)

[carbking]

Try this link:

THE CARBURETOR SHOP / Troubleshooting

The Rochester 4GC is one of the finest 4 barrels made. You should be able to make it run well.

Jon.

[Guest Pletch]

thank you for your advise so far. I've played with the primary jets so far with no improvement and will play with the power jet next. Once I find an answer I will post it.

[Guest Pletch]

okay, I found that it is not from the carburetor. The issue goes away when I disconnect the vacuum advance. I think the spring in the advance is weak and applying to quickly.

Once I have this confirmed I'll let the group know.

[old-tank]

Since you disabled movement of the breaker plate, look inside the distributor for loose or shorted wires. The original fabric coated lead from the points to the junction on the side of the distributor is usually the problem.

Willie

[Guest Pletch]

one more update to this. I found the stumble happens between 8 and 12 inches of ported vacuum. below 8 inches the diaphragm is not moving. I didnt mention that I installed petronix ignition so the old wires have been removed, I did take the cap off to confirm the wires are okay and they are. I also checked the breaker plate to make sure it is secure and not loose.

once again I would like to hear any advice and I will post my final resolution. Thank you for the help so far.

[old-tank]

With what you described, I'm still thinking it is an ignition problem. Things to try:

1-Replace the pertronix with stock points/condensor

2-Swap the coil with a known good one. With the pertronix you should be using a stock coil with the ballast resistor. You can jump across the resistor to give a hotter spark for testing purposes. Even though the pertronix tolerates some wear in the distributor, excessive wear in the shaft or breaker plate may affect function.

Willie

[carbking]

Since you have mentioned that you have installed the p-conversion; have you upgraded to an alternator?

Jon.

[Guest Pletch]

I dont have an Alternator, I'm still running a generator. Is this a problem?

[carbking]

Any electronics like stable voltages. An alternator will give a more stable voltage than a generator at lower RPM. Electronics can be irratic with less than the desired voltage.

If it were mine, I would put the points back in the distributor, and see if the issue disappears.

Jon.

[Guest Pletch]

Problem solved thanks to the help of the members here. The petronix was the issue, after installing new points and condenser the stumble is gone. I do hear a light rattle noise from the distributor and it appears to be coming from the breaker plate which may have been the issue with the petronix not working once the vacuum advance engaged.

That will be a project for a future weekend.

thanks again for your help.

[old-tank]

Thanks for the update. That makes a half dozen 55&56 Buicks that I know of that had more problems created by Pertronix than solved. I can't blame you for trying it out since I have one my shelf too. After removing it and replacing with the points/condensor I have had no problems (just changed points/condensor at 30,000 miles).

Now get out there and drive it regularly before that 4gc give you problems.

[Airy Cat]

I have used Pertronix ignition in all of my Buicks now for the last 20 years and I have had absolutely no problems with them!

[NTX5467]

Might it be that a higher output generator might help the voltage supply issue? Or that the voltage regulator might need its contact points cleaned for possibly better operation? Or a little tweaking of the voltage setting on the old regulator, too?

I tend to concur that in the prior times, ANY Rochester carb was a pretty bulletproof carburetor. But I also know that current fuels can significantly degrade the "cement" around the sealing check balls (staked and cemented to seal drilled passages in the float bowl area of the carb) enough such that the check ball exits its prior position and then allows the carb float bowl's contents to flow onto a warm engine . . . with related underhood fire hazzards. One of our chapter's late members saw that happen to a '56 Cadillac at a local car cruise event (many members with their fire extinguishers saved the day for the Cadillac's owner!) and THEN discovered that his 4GC on his '58 Super was getting ready to do the same thing, just that he smelled gas first! So, I'm going to concur with BOTH sides of this discussion on 4GC "Quadra-Jet" Rochester carbs.

Respectfully,

NTX5467