Idling high and can’t get it lower

[1977C20Silverado]

My sbc 350 is idling high. I have the idle screw backed out all the way and have pulled the throttle back and it is still high. I was thinking it might be the choke because it’s not closing fully, it’s the electronic one, and someone I know told me it might be the thermostat but I wanna check what yall have to say first. If anyone’s had any issues like this and has any input it’s appreciated

 

[Dmack]

How high? Intake or carb leak will cause it to run high. I used the wrong base gasket after a carb rebuild and it wouldn't idle below about 2k. Major leak.

 

[1977C20Silverado]

How high? Intake or carb leak will cause it to run high. I used the wrong base gasket after a carb rebuild and it wouldn't idle below about 2k. Major leak.

I don’t have an rpm gauge so I’m just estimating but probably ~200-400 rpm’s higher than normal. Kinda jumps the whole truck when put into gear. But after being put into gear, it drops down to its normal rpm. Then after going down the road a little it starts doing high rpm’s again and at idle I’m going 30 mph. And I haven’t done any work on the carb either, but the gaskets also might need replacing so it could be that. I also didn’t mention this because it wasn’t happening, but there is now a misfire on one of the passenger side cylinders. I got new spark plugs so I’m gonna see if that fixes it because the misfire stops when I give it throttle. Could the one cylinder not firing right be affecting the rpm’s?

 

[TotalyHucked]

Get a can of brake clean and spray around the base of the carb and the intake gaskets. If the truck idles up, you've got a vacuum leak there

 

[Ricko1966]

Get a can of brake clean and spray around the base of the carb and the intake gaskets. If the truck idles up, you've got a vacuum leak there

Use carb cleaner,throttle body cleaner,water,or propane. Brake cleaner burned makes some very toxic phosgene gas.Water is hard to use to find leaks you have to get the area satured to seal the leak temporarily, propane blows away too easy,carb cleaner is my favorite.Also one little extra bit of advice if you're spraying carb cleaner etc. Keep some rags handy,and don't do it if know you have plug wires arcing. The rags are to snuff out flames if the cleaner catches fire. It probably won't and it won't be big. But have some rags handy anyway.

 

[1977C20Silverado]

Update: I replaced some vacuum lines, but that didn’t really drop it much. Then my starter went out and I replaced it, and it was idling lower. Idk why but yeah

 

[fast 99]

Is the throttle plate tab against idle speed adjustment screw?
If not check that the fast idle cam isn't keeping it high.

One of 2 things is going on, either air is entering from sources not controlled by carb
or throttle plates aren't closed. If your sure throttle plates are closed start checking for vacuum leaks.

Also remember intake gaskets can leak on the bottom in lifter valley. There's a procedure to check that.

 

[PrairieDrifter]

I guarantee it's the choke. If the choke doesn't fully come off, the fast idle will stay engaged. Especially on an electric choke that isn't properly adjusted.