Stock 76 Carb with new high idle problem...

[deluxe]

I don't have problems with my carb but a new problem has decided to show up. Recently after the truck is hot, it won't drop to normal idle. If I stop the engine and restart it, sometimes the idle will be correct but not often. Even better, if I start the truck with the low idle, it'll get reset to high at some point during the drive and I will be at a light with a hot truck and a high idle.
From my reading this has something to do with the electric choke. I consider myself mechanically inclined but have to admit I know little about carbs. Can someone explain to me how the idle works from cold to hot and maybe help me figure out what has changed on my carb over the last 10 starts. Thanks in advance... Man I love this place.

 

[Skuzzlebutt]

When it's doing the high idle thing when the engine is warmed up to normal operating temp, Get under the hood and look at both idle adjustment screws. The end of the cold idle screw (on the choke mechanism) should not be touching anything. The hot idle idjustment should be pushing on the throttle arm. This is how it'd supposed to be. If the cold idle screw is pressing on the cold idle cam, the hot idle screw should be be lifted off of the throttle lever. This is how it should be when the engine is could, but if it's hot there is something amiss with the choke.
Another cause could be in the distributor. If it's dirty or gummed up inside, the advance mechanism may be sticking in the advanced position instead of the idle position of 8 to 12* BTDC. During accelerationclimbs up to 25 or 30*. If it does drop down when the engine slows down you'll end up withan abnormally high idle.
Since the problem is intermittent, I'm leaning more towards the distributor as the source of the problem.
I'm thinking that if the choke was out of adjustment, it be that way all the time. A sticking dizzy would sometimes unstick then stick a gain.
I'm not sure of the best way to clean a dizzy. Maybe douse the innards liberelly with WD-40 while moving the advance by hand, with the cap removed of course, and the ignition turned off.
BTW, what carb do yo have.

 

[wildbill]

;D ;D ;D I would think that your fast idle cam is sticking or bent start there good luck. Bill %)

 

[pinenuts]

Another possibilty

On my 76 CA 302 there is a temperature sensitive ported vacuum switch on the coolent outlet elbo. It normally switches the vacuum source for the distributor advance from the carburator vacuum to the manifold vacum under high temperature conditions. Because the manifold vacuum is higher at idle, it increases the dist. advance, causing an increase in idle speed. this is supposed to cool down the engine. If this switch were malfunctioning or intermittant it might cause your problem.

 

[deluxe]

Thanks for all the feedback

Thanks a lot guys.
I disconnected most of the vacuum connections that would match the description you gave me a long time ago so I doubt that is the culprit since this has crept up just recently. I really like the sticky distributor advance because it would match my problem very well. Especially since the engine seems to have much more power when it is in this state. I have also noted that I get deiseling and backfires after I turn it off and a gross amount of advance might account for that change as well. The really good news is that I have been planning my duraspark swap for a while now and planned on doing that this weekend. Hopefully things work out and this problem will go the h3ll away. Thanks again everybody.