starfire

Rebuilt the carb on my '78 Starfire. Lots of documentation on how to do it, except: idle mixture screw adjustment. All I can find is they say don't make an adjustment to these unless rebuilding the carb. I tried backing them out 2 turns each (swag), then setting all of my adjustments. Is that the right thing to do? Is there a technique for setting the idle mixture screws?

Thanks,

Octania

Adjust for best idle and vacuum reading. Back in 1/8 turn for economy.

johnnyjaws

screw the mixtures screws in all the way not hard to break the tips off back out 2 1/2 tuens go from there 1/4 turns at a time till you get best idle out of it I alaways go another 1/4 turn out

twintracks

I was taught to turn them in until you start to loose RPMs, and then back them out 1/2 a turn. May not be right, but it seems to work well on most.

MDchanic

If the carb in your '78 is original, it's not a 2GC, it' a 2MC or a 2ME.

- Eric

starfire

Eric, the carb on my car is original. It is a 2GC. That's what came on the Chevy 305's in the H-Special bodies in 1978.

MDchanic

Okay then. I had not been aware that GM used that carb past about 1975, but I've been wrong before.

- Eric

starfire

You'll have to get in line way behind me if we're lining up in order of how many times we've been wrong, but yes, the 2GC was used, I think all the way up through 1980 (but I'm not positive on that), in the V-8 H-bodies.

johnnyjaws

Some carbs have block off caps u have to remove

starfire

No caps on mine. I can use a flat head screwdriver or a 6 point socket to adjust mine.

I find it very odd that the adjustment information is not to be found in any of the factory manuals. In the three replies I got on this thread, they are all slightly different on how to do it. Seems almost like witchcraft instead of science

techg8

I use a vacuum gage hooked to manifold vacuum

With a stock carburetor, I start 1.5 to 2 turns out from fully seated.

Adjust the screws for maximum vacuum reading achievable at idle rpm. Adjust one mix screw, see the result then match the other one. Reset idle rpm if needed.

counterclockwise turns richen. Clockwise turns lean the mix.

If when you richen the mix the rpms go up, the engine likes the additional fuel. If the rpms do not rise, youv ereached saturation point where more fuel will only hurt. If you go too far rich, you'll get a wavy uneven idle.

I look for the point where if I go rich a half turn on a single screw, there seems to be no effect on rpm, but also where if I go half a turn lean, the rpms will drop a little. I view this as the "tipping point", where youre getting max vacuum

As mentioned above, some people like to go a fraction of a turn leaner from there, I believe primarily to avoid fouling plugs at idle.

MDchanic

Ken above gives a good description of the standard way of adjusting mixture, and this is what I would probably do.

HOWEVER...
The reason why your '78 manual doesn't describe this procedure is because the carburetor is supposed to be properly set and sealed at the factory, and so never need further adjustment.

In order to adjust it properly, you technically need an emissions sniffer to check the levels of the appropriate gasses. There will be a procedure for this somewhere, either in one of the GM submanuals that they published in these years, or in an aftermarket manual, such as a Motor manual.

I wouldn't lose sleep over it.

- Eric

starfire

Ken,

Thanks for the step by step. Can I assume this is done with the engine fully warm?