I have a 1979 corvete 350 engine.
The carb I have is for a 1970-74 Oldsmobile with a 455 engine.
The float is curentaly set at 3/8" which is good for the Oldsmobil but for the corvete the rebuild kit says 1/4".

My question is should I set it to the 1/4" for the 1974 corvette or the 3/8" for the carb?

I am asuming the 1/4" for the corvette but wanted to check.

The main reason I am replacing the gaskets is the carb is the gas is running out of the carb when it is off. Making you have to crank the engine for a long time if the car sits a few days.

Thanks
Chris

 

Dont know if its any good to you but my 79 calls for a 15/32 adjustment from toe of float to face of air horn, works a treat at that distance.

Cheers, Dennis.

 

Set the float level for whatever the proper spec for the oldsmobile carb is. The float level is not corvette specific it is carburetor specific.

Get a copy of Lars Q-jet tuning paper, he discusses proper float level setup.

 

Chris,
Changing gaskets won't likely fix this problem. I have the same issue as have thousands of others over the years.
The commonly stated reason for this is leaking fuel bowl plugs, and many if not most old Q-jets already have some JB weld stuck over these plugs.
The problem is that many experienced and knowledgeable Qjet rebuilders reject this explanation, saying that this is actually a very uncommon cause of the draining fuel bowl problem.
According to Lars Grimsrud and others I have spoken with, the more common causes are:
1. Bad fuel pump check valve, allowing fuel to drain back and siphon the carburetor fuel bowls until empty.
2. The fuel bowls on Qjets are vented, and modern fuels reportedly have a much higher vapor pressure than older gasoline, so apparently it evaporates much more quickly.
3. Bad float.
4. Porous casting, so that the carb body itself is apparently "leaky" in some cases.

maybe a few others.
some can be fixed, some require replacing carb
it is a very frustrating problem

I'm not an expert, but this is what I've learned. Read the tech papers by Lars Grimsrud to learn more, and he may chime in to lend his experience.

good luck and let us know if/how you get it fixed

Fred

 

Set the float to .375" (3/8").

Changing gaskets won't solve your fuel dsissapperance problem - you have other issues.

Lars

 

Thanks for the tips,
How can a float go bad? I have a foam one is there. Is there something to look for to check it?

I put epoxy on the plugs as the book I was following said this was a good idea.

The valve the float pushes on was scratched, looks like the valve was forced into the seat some how. I was hoping this was the issue.

You are right it still is hard to start, took about 5 seconds of cranking for it to start.

 

My 74 vette doesn't run right. It would die at low rpm an back fire when I step on it. I took my car to a mechanic. He said that I need to replace my carb. I said that Rochester Q-jet is not a good carb. he recommended that I should replace it with a holly or Edelbrock carb.
I need help. Any suggestions?

I called Jegs auto part, they (technical guy) said that I only need to replace the float in my carb. What is the float? I don't know any thng about the rochester quadraject carb.

where can I get LARS Q-Jet Tuning paper?
Thanks

 

Send an e-mail to v8fastcars@msn.com and request the papers and ask questions. Lars is usually very helpfull.

 

The phenolic floats 'go bad' by becoming fuel soaked. They are supposed to be a "closed cell" foam material that cannot absorb fuel. But, after years of use, they can break down and allow fuel to get absorbed. That makes the float heavier and requires more fuel flow to get the valve shut down. If it gets soaked 'thru', the float will no longer raise far enough for the valve to shut at all.

ALWAYS install a new Nitrofyl float when doing a rebuild...unless you have a brass float. And, if you have a brass float, always shake it to see if there is any fluid trapped inside it. If there is, you can drill a small hole to let out the fuel and then resolder that hole to seal it; then, dunk it in a mug of water and look for blubbles (indicating another leak site).

Although Lars recommends never to use a brass float, Cliff Ruggles suggests that you DO use the brass type. I don't know the rationale for either's respective position on that matter. I only know that I always use brass floats and always check new ones before I install them. I've never had a brass float go bad.

 

Like was said, use the settings for the carb, not for the car.

To address the problem with gas running out of the carb, this occurs mainly because of fuel back-flow. The back flow occurs with fuel pumps having a small return line nipple on the bottom which goes back to the tank. This is on all C3 vettes after approximately 1969. There is a small orifice of approximately .058" diameter internal to the fuel pump which allows fuel in the pump's output chamber to flow back to the tank to help prevent vapor lock. Also, for a year or two prior to this, there was a special fuel filter which had a fuel return line going back to the tank. If fuel can run backwards out of the carb, it will follow this path and then drop to the same level as that in the fuel tank.

So, how does it get out of the carb?... Awhile after the engine is shut off, the fuel level decreases gradually due to evaporation; as the float settles, it pulls the needle up with the small needle clip, like pulling the plug in a bath tub, and then, the fuel back flows through the feed line into the fuel pump and thence into the return line to the tank. HOWEVER, if an internal carburetor paper element fuel filter is used, there is a back flow check valve in this filter which prevents this back flow. On the other hand, I do not like the internal paper fuel fuel filters because of their small orifices into the filter check valve which might limit fuel flow at WOT; therefore, I prefer to use an external filter.

Adding to the problem is the design of the brass needle-seat tube that the needle rides in - if this tube has side windows or holes just above the seat, when the needle rises as the float goes down, the fuel can back flow to a lower level in the carb than if the tube was non-windowed (i.e., solid) to the top. (Supposedly, the presence or lack of windows in the needle tube has no effect on fuel flow rates). Therefore it is better to use a non-windowed needle-seat tube to stop the back flow at a higher level when back flow does occur.

Another prevention item is to eliminate the clip connecting the float arm to the needle. Therefore, when the float settles, it won't pull the needle up off the seat and let the back flow start. It has virtually no effect on the slowing the fuel flow into the bowl when the engine is running.

With the dozen or so quadrajets I've rebuilt, I always test for bowl leakage by filling the bowl with gasoline before I reassemble the throttle plate and air horn and let it sit for several hours to see if any fuel drips below. Even without epoxy, I still haven't found a leaker.

Lastly, if it's been more than a week since the car has last run, I simply set the choke and count potatoes - invariably the engine bursts to life before I can reach 5.