EP1978

I finally got around to switching out the original engine in my 78. I plan on rebuilding it eventually and putting it back in the car. In the meantime I've put in a Chevrolet Performance 350/290 HP with an Edelbrock Performer intake manifold and most of the original equipment (Including the original Quadrajet Carburetor) from the old engine. However, I'm having a problem with the idle where it seems to idle fine in park and neutral, but as soon as it's put in gear, the idle slows and stalls the engine. I'm not the original owner of this car, so I don't know what the Carb has been through and appears to have never been rebuilt. I've talked to a few different people who seem to believe that either the carburetor needs to be replaced/rebuilt or that I'm having a vacuum leak somewhere. One thing that I believe may be the culprit is the carb gasket. It was intended for the original intake manifold and carburetor, but I now have a new manifold. What do you guys think could be the problem here?

Brady


Here's a picture of the new engine I took halfway through the swap.

GUSTO14

Do you have the Carburetor Idle Stop Solenoid connected to a power source. In the picture, it doesn't appear that it is.



It also doesn't appear to be the original one for you car. It looks more like the one I have on my '73. It may work as well, but here's the correct one for your car if that one is not functioning. https://www.paragoncorvette.com/p-35...-solenoid.aspx

EP1978

So it looks like my solenoid is broken. I'll look into getting another one, but could this be the direct cause of my idle problem? From my understanding, it only serves to increase the idle when the A/C is turned on.

GUSTO14

Throughout the years there were varying reasons given for their use. My '73 (4-speed no A/C) uses it as an anti-dieseling solenoid so it comes on as soon as the engine is started and closes when the engine is shut off to close the carburetor butterfly completely and prevent dieseling. In some of the later cars, it was used to bump the rpm up when the A/C comes on such as here.

I would just try to see if it works first of all by adding 12 volts to it. If the solenoid activates when you put 12 volts to it, you know it is good and then it's a matter of determining when the car normally activates it.

Is the carburetor you have the correct/original one for your '78? If you don't know, there is a number stamped into the housing that identifies what it originally came on.







Good luck... GUSTO

Big2Bird

Correct

lars

"90% of carb problems are ignition problems."
The most common cause of the engine dying when put in "drive" is that the vacuum advance control unit, if hooked up to manifold vacuum, is mis-matched for the vacuum that the engine pulls in "drive." This will cause the timing to retard about 15 degrees when the car is put in "drive." To check for this condition, simply yank the vacuum hose off the distributor and plug it (disabling the vacuum advance). Re-set idle speed to the desired idle speed. Now drop it back into "drive" and see if it stalls the engine. If the engine does not stall in this configuration, your vacuum advance unit is incorrect - e-mail me for my paper on how to select a vacuum advance. If it still stalls, you may have a carb problem (most likely a lean condition of the idle circuit).

Lars

EP1978

Thanks Lars, this was the problem exactly. I just checked using your method and it didn't stall. I'll email you for the paper now.

lars

Every once in a while I get lucky with a diagnosis...
Good job on the troubleshooting, and good luck with correcting the vacuum advance system - it will run really nice once you have timing and vacuum advance correctly set up and matched to your engine. I have mailed out papers to everyone who has requested papers today, so let me know if you have not received anything.

Lars

EP1978

It seems I've overlooked a bit of information for my engine regarding timing advance.




I guess this means the vacuum advance isn't necessary after all. If this is correct, how should I approach setting the timing? Will this make any difference?

Big2Bird

That setting is for smog/millage/lower octane fuel. Follow Lars paper.

lars

They say that with the assumption that most people don't use a correct vacuum advance. A vacuum advance that pulls in too much vacuum (as most do) can cause engine damage resulting from detonation, so they tell you not to use it in order to limit their own warranty claims. Failure to use vacuum advance will result in a hot-running engine with poor idle characteristics, poor off-idle throttle response, and poor fuel economy. But it will have lower emissions, if that's your primary goal...

Use the vacuum advance. Just use a correct unit with correct specs as outlined in the info I have sent you (vacuum advance should be limited to 12 degrees).

Lars