Carburetor Rebuild...Won't idle

[Robert Larson]

Due to frustration, I have pulled the Carter WCFB and purchased a replacement carburetor, rebuilt by Holley, a Rochester 4GC. I know it is incorrect, but I thought it would give me a good runner while the original is being restored.

However, once hooked up, it allows for a quick start up but won't idle at anything less than 1500 RPM. Obviously I have a problem. I have replaced all vacuum hoses and haven't touched the adjustments from what was set by Holley.

The idle adjustment screw doesn't even come in contact with the choke piece. Could it be in the choke and throttle rods? Is this just an example of a bad rebuild by Holley or is there an adjustment I need to make.

I appreciate any ideas and maybe the best is to take it back to the shop I bought it from....

[Duke W.]

Re: Carburetor Rebuild...Won't idle

Are you saying it won't idle at less than 1500 when it's fully warmed up, or are you talking about the cold fast idle?

It could be an issue with the carburetor, depending on what engine it was designed for.

Carburetors meter fuel at idle and off idle based on manifold vacuum. Once there is some load on the engine it meters fuel on the basis of air flow volume, which creates a metering signal at the venturi nozzles.

Because idle and off idle fuel is metered off manifold vacuum rather than air flow volume, the idle and off idle metering circuits have to be tuned to the displacement and manifold vacuum characteristics of the specific engine. Idle and off idle calibration is "tuned" with tiny fuel metering orifices and air bleeds and the off idle transfer slots in the throttle bores. It's somewhat of a black art. Within a limited range, the idle mixture adjusting screws can compensate, but if you install a 4GC from a 390 CID Cadillac engine on a 283, it's probably going to be way too rich and the idle mixture screws won't have enough adjustment range to compensate.

Also, air flow can be an issue in addition to fuel flow. An engine that idles at about 14" or more manifold vacuum has achieved what is called "critical pressure ratio". This is the pressure RATIO (not pressure difference) of the abolute downstream pressure to absolute upstream pressure and the critical ratio is 0.528. If atmospheric pressure is 29.92" Hg. then the the downstream pressure at the critical pressure ratio is .528(29.92) = 15.79" and manifold vacuum is 29.92 - 15.79 = 14.1".

The significance of the critical pressure ratio is that once it is achieved, velocity past the throttle plate is sonic and no matter how much the manifold vacuum is increased, air flow will NOT increase unless the available flow area is increased.

This is why medium perforance engines have a very steady idle speed. Conversely, because manifold vacuum on most SHP/FI engines is not sufficient to attain the critical pressure ratio they will not have as steady an idle speed, and the lack of achieving critical pressure ratio is the primary cause for the "lope" we are so fond of.

Well, I may not have solved your problem, but you got a fluid mechanics lesson that explains why some engines have a smooth idle and some don't and why a Cadillac carburetor won't work on a 283.

Duke

[Kris Schmalenberg]

Re: Carburetor Rebuild...Won't idle

Sounds like you may have a vacuum leak. Any ports on the carb that may be open? How does the 4GC mate to your WCFB manifold? Could it be leaking there?

[Robert Larson]

Re: Carburetor Rebuild...Won't idle

All ports are met with a vacuum hose... one from the distributor and one from the exhaust manifold.
As far as the match to the manifold, the gaskets and carb were exact and fit right on. Thanks though.

[Robert Larson]

Re: Carburetor Rebuild...Won't idle

Even when it is heated up well, it won't idle below 1500.

As there were so many 4GC's produced, I just wonder if it might be the wrong match. The carb was to replace a 327 250hp carb... by the books this was the correct carb.

I will call Holley and see if they can shed any light.

Thanks.

[William C.]

Re: Carburetor Rebuild...Won't idle

Many Chevrolet 4GC carbs have the part number of the carb (last 4 digits stanped in to the flat surface of the float bowl cover (air horn) on the drivers side near the rear screw hole. See if you can find that ahd if so post to see if we can verify the original application. A 327 carb should work on a 283 if you are running a hydraulic lifter engine. I suspect the rebuild may not have used the correct gasket in the throttle body to float bowl junction and the idle fuel is blocked, but at this point it's purely a guess.

[Clem Z.]

Re: Carburetor Rebuild...Won't idle

disconnect all the rods that attach to the throttle arm,accerator pump,throttle linkage from the firewall,auto trans rod if you have one etc and see if the idle goes down. you need air to make it idle fast so the butterflys could be open too far

[Robert Larson]

Re: Carburetor Rebuild...Won't idle

Thanks Bill, but the cover has no numbers on it. Back along that edge is the copy "4 JET". It also is on the passenger back top. Up front, driver side it does say GM, but that is all other than RP for Rochester.

I called Holley and their response was to back off the fast idle screw...located under the choke.... However, there isn't a screw there to back off as instructed... just another open port (in the base of the carb) that if left open sucks in lots of air! So I have blocked this port with a correct thread bolt.

[Robert Larson]

Re: Carburetor Rebuild...Won't idle

Thanks, I will try it... Actually I did release the throttle linkage, as well as the linkage from the throttle to the accel pump... and it basically didn't seem to make a difference...