Fast idle after engine is warm..can someone give advice plz

[DuaneMike]

1986 gmc Sierra classic 305 4bbl carb.
Idles fast even after engine is warm.

 

[Bextreme04]

1986 gmc Sierra classic 305 4bbl carb.
Idles fast even after engine is warm.

More details? Have you tried adjusting the idle set screw on the drivers side of the carb?

 

[DuaneMike]

I haven't tried that yet as I'm inexperienced with carbs but I'll check it out. The problem just started after it got very cold here -40+ C. It was working fine a couple days ago. I believe it's a factory carb for the 305 121000kms

 

[Bextreme04]

I haven't tried that yet as I'm inexperienced with carbs but I'll check it out. The problem just started after it got very cold here -40+ C. It was working fine a couple days ago. I believe it's a factory carb for the 305 121000kms

Weeeeeeellllllll that might have something to do with it

Are you sure it's getting warm enough to kick the choke off? If the choke isn't pulling off, it will stay on the fast idle cam. Try taking the air cleaner off after you think it is fully warm and see if the choke is still on.

 

[DuaneMike]

Kk I'm sure it is warm enough as it was blowing good heat and we went for a cruise down the highway for 20 mins still a fast idle after that.

 

[75gmck25]

There is a small rotating cam and the fast idle screw. When the choke is on, the cam rotates so a small lever is under the throttle linkage and it keeps the throttle on high idle. You can adjust the screw to set the high idle. Then when the choke is off the cam rotates and allows the linkage to drop to curb idle.

The choke might be misadjusted and not completely moving the linkage. Or the cam may be poorly lubricated and it is sticking in cold weather and not rotating.

if you look at the lower front of the choke housing from the front/side of the truck you should be able to see the fast idle screw.

Bruce

 

[Bextreme04]

Kk I'm sure it is warm enough as it was blowing good heat and we went for a cruise down the highway for 20 mins still a fast idle after that.

On a newer truck like yours the choke should be electric and has nothing to do with the engine temp. That’s why I said to physically check it. It could be sticking, or the cold air might be keeping the choke from pulling off due to some kind of problem with the electric choke, or you could have an electrical issue that is keeping it from functioning at all. Pull the air cleaner, start it up, see if the choke pulls off after a few minutes. If you have had it running for 5 minutes and it still hasn’t pulled open, you need to look at the choke for why it isn’t working.

 

[DoubleDingo]

I haven't tried that yet as I'm inexperienced with carbs but I'll check it out. The problem just started after it got very cold here -40+ C. It was working fine a couple days ago. I believe it's a factory carb for the 305 121000kms

-40C/-40F, the only temperature readings that are exactly the same in C and F. Being -40 degrees, that will keep the engine cold for sure, so the fast idle probably won't kick down. Try a block heater to keep the engine warm when it gets that cold.

 

[Ricko1966]

Warm it up. Turn it off take off the air cleaner lid. Hold the throttle in the floored position with 1 hand while reaching into the air cleaner housing to fully open the front metal flap on top of the carbureator. Use a wooden clothes pin clipped to the choke plate or air cleaner stud to hold it open. Start the truck. Is the problem gone?

 

[TubeTruck]

Take the air cleaner off, douse the whole carb in 1-2 cans of carb cleaner while working all the linkages. I bet some stuff is stuck.

 

[80BrownK10]

Weeeeeeellllllll that might have something to do with it

Are you sure it's getting warm enough to kick the choke off? If the choke isn't pulling off, it will stay on the fast idle cam. Try taking the air cleaner off after you think it is fully warm and see if the choke is still on.

I was going to ask, is the choke plate open or closed when it's "warm".

 

[80BrownK10]

Take the air cleaner off, douse the whole carb in 1-2 cans of carb cleaner while working all the linkages. I bet some stuff is stuck.

More like frozen!! I'm sure any oil or residue that may be on things is quite gummy at that temp.

 

[Robert Bare]

There is nothing wrong with your truck/carb. They were never designed to operate in these conditions.No matter if you had the older "air heat" choke, or the newer electric choke. Being a mechanic, have seen this a bit. Even my old 78, have backed down the choke adjustment, quite a bit, to the point I have to restart it 3 times, so it stays running in cold.Now,i call cold anything below 0. I am probably to the point I shall install a manual choke cable.Just had 2 weeks of 0 to -30,same issues, even to the point, even though I have backed down the "choke" adjustment", it would come back on, when plowing snow.To the point, choke would start to fall back into the carb, and lug the engine down, overrich smoke coming from the exhaust, and lugging down.I have tried to avoid installing a manual choke cable unit,but is the way to go. you have to remember, back in the day,people did not drive in this weather, we are spoiled from what fuel injection does.

 

[Ricko1966]

There is nothing wrong with your truck/carb. They were never designed to operate in these conditions.No matter if you had the older "air heat" choke, or the newer electric choke. Being a mechanic, have seen this a bit. Even my old 78, have backed down the choke adjustment, quite a bit, to the point I have to restart it 3 times, so it stays running in cold.Now,i call cold anything below 0. I am probably to the point I shall install a manual choke cable.Just had 2 weeks of 0 to -30,same issues, even to the point, even though I have backed down the "choke" adjustment", it would come back on, when plowing snow.To the point, choke would start to fall back into the carb, and lug the engine down, overrich smoke coming from the exhaust, and lugging down.I have tried to avoid installing a manual choke cable unit,but is the way to go. you have to remember, back in the day,people did not drive in this weather, we are spoiled from what fuel injection does.

I grew up back in the day and everybody drove even when it got super cold. They didn't call school off because it was too cold, they didn't close businesses or tell the police take the day off its too cold. 99percent of cars I 've owned in my life were carburated and I always went to work no matter how cold.If it won't drive in the cold something else is going on. No heat riser, plugged heat riser passage, wrong heads, manifold, air cleaner, inop. Air cleaner heat flap,etc.etc. I worked as a flat rate mechanic 1981?- 2006 my Dad never even owned a a F.I. car tell I was grown and gone, not once at any shop did we ever tell anyone oh it's just too cold.Not once did my Dad skip work because it was too cold.

 

[Mike turbo]

Ck the secondary pull off.thays what kept mine on