Acceleration problems

[dn010]

Carburetor: Rochester 4GC, slotted gasket, brass float, no level sights in body, rubber accelerator pump

Vehicle: 1957 Cadillac Seville 365CI

The Problem: Stumble/hesitation/dead spot on acceleration when warm only/choke full open.

Details: I have rebuilt this carburetor a few times now with the same results. When cold the car accelerates fine, no issues. It idles well cold and warm. Once warm and in gear, I slowly accelerate the car and it hits a point right away where it wants to stall, I pump the gas and it gets past those RPM’s and accelerates fine as long as it’s not the slow RPMS. If I slam down on the gas it passes that dead spot, accelerates quickly and smooth but I don’t want to chirp the wheels every time the light turns green. So far: Ignition timing checked/adjusted, advances checked, valves/springs checked-one badly bent and disconnected pushrod replaced with broken lifter-1 disconnected pushrod reconnected, valve timing checked, tried with vacuum plugged and unplugged. With the vehicle in park, engine warm and slowly accelerating the engine, it will slightly stumble at a higher RPM but not bad. I tried the original 4GC (rebuild) that came with my car and another 4CG I had in my parts room, this one has the sight holes in the body and the body gasket with no slots. I also had to use the bowl cover of the original 4GC on the second one-so both the carburetors were tested with the same bowl cover. This bowl cover is clean and in working order, no gunk in the fuel passages, the power piston seem to work fine, the only issue is part of the gasket over the main well is broken and missing. I checked and adjusted everything and still have the issue. BUT when I hold the choke with about a 1/8” gap, the engine accelerates fine past it’s issue RPM and until it requires more air.

So, is it a problem of the gasket being broken? Idle adjustments or float levels? Should I replace this with a Carter unit? Any and all help is greatly appreciated and I thank you kindly!

[Cliff Ruggles]

Although not a quadrajet, the symptoms indicate a lean condition.  Basically, from the description, there is a point in the fuel curve that isn't supplying enough fuel.  Since the problem occurs right off idle, and with slow throttle movements, I doubt if the accellerator pump is at fault.  There are many other things can cause a lean condition, including not enough fuel in the bowl(s), power piston not moving up, clogged up internal fuel passages, air leak(s), etc.

I suspect part of the problems may be simply because of the lower BTU content, and lower octane levels of modern fuel.  4GC parts are not easy to obtain these days.  If you have access to them, I would start by installing one size larger main jetting on the primary side, and see if it helps.  I would also replace any torn gaskets, and blow out all of the internal fuel passages while you have the top off the carburetor.

We get very, and I mean very few calls for 4GC carbs these days.  We took one in for overhaul this year.  That's the first one we've had in here in at least 10 years!.....Cliff

[dn010]

I have two 4GC's, and I basically made one out of the two. The first was soaked, cleaned with carb spray and then blown out with high pressure compressed air. Every passage was tested to make sure the air blown in was blowing out the correct place and at a nice steady pressure. The carburetor was then rebuild, installed and ran. Today I compared the intake manifolds of the 365 (I have 2 of these engines as well) and noticed that the crossover was clogged with carbon right under the throttle body. Cleaned the crossover passages and it slightly improved the dead spot/hesitation in that it takes a few more minutes of running/warming up to start the hesitation/dead spot and was also a bit less severe as before but it is still there.

New question- would you recommend dumping the 4GC and go with a Quadrajet or even a Carter? I don't exactly need to go original carb on this engine/car. Also, if it is recommended-Does the 4GC have the same intake bolt pattern as the Quadrajet? Lastly, the car's [manifold] heat control valve has been removed, will this have any effect on the issues at hand? Thank you kindly