Anyone ever have a carb that changes it's idle speed on it's own?

I've been playing games with mine. It was installed and set to around 700 (4 speed manual trans). Drove it home that day (2 hrs) and it I noticed that when it warmed up, it now was at 1100. I took it back to the shop and they adjusted it back down to about 750. A few days later I noticed again it was back to 1100.

Seeing as that shop is 2 hours away, I decided to find a closer location. I found one and took her over there. They adjusted the idle, hooked up a gauge and adjusted the mixture, etc. When I drove him it was back down to just under 800. Two days later, it's back to 1100 again!

Anyone have a clue as to why this keeps happening? It's getting really frustrating. I keep suggesting that the carb is bad, but both places have said it's not.

At $2.45/gallon, it'd be nice to get this problem resolved!

|UPDATED|6/9/2004 11:57:03 AM (AZT)|/UPDATED|

had a simular problem some time back. it turned out to be a leak on my intake that would open up when it warmed up then seal when it was cool. It drove me nuts adjusting it going for a drive,stopping at a light-- pulling over readjusting the carb idle down -- only to find a little later the beast would not idle at the next light.

It could also be the the linkage or the throttle shafts are binding up after it has warmed up. The way to check is just pop the hood when it is doing this and reach in and see if you can push or wiggle the linkage to see it it bumps forward a little. It may be so slight you wont even see it move, but will here the change in engine speed. If it changes then you might be able to find the problem. The cable may be sticking inside the sleeve as well. So when tuning it under the hood your your rapping the throttle back and forth by hand so the cable wouldn't come into play.

I've also seen the throttle shafts unbind and release there position when loosening up the carb hold downs. this means they were not torqued evenly so when the engine warmed up onside tightened up more then the other causing everything to bind up. They should only be torqued to about 15 to 20 inch lbs apiece in a criss-cross pattern.

then again all that may be required is a stronger throttle return spring. That is if you have one. A lot of people take them off or forget to put them back on because the carb throttle linkage already has a throttle spring on it. One should be added if its not there anyway as a safety precaution.

Maybe this will help you locate your problem.

All very good advice.
It could be as simple as the spring on the idle screw has lost it's tension, and allows the screw to move. This does not happen often, but it does happen. Just remove the screw and pull the spring to increase it's tension, and reinstall. This may not be the problem, but it can happen. Common on mixture screws. Have only seen it a few times on idle screws.

Check your throttle return springs, on a 76 there should be two of them(one inside the other) perhaps one is broken.