Dieseling / Run On

[Gordo_AZ]

I have a 86 GMC Caballero 305 that has just developed engine run on for the last couple of weeks. I have a new Rodchester 700 cfm OEM carburetor that's two months old, Idle speed is set to about 650-700 in drive, about 1000 rpm in park. Idle mixture is set to 3 full turns. I have replaced spark plugs, gaped em at .45 ..replaced with new wire set, new cap and rotor. I thought it was carbon build up, put one full can of Sea Foam in the tank with fresh full tank of fuel and bought another can for pouring down the carburetor at high idle to break up the carbon. I feel I have removed any carbon build up. My car is not overheating, just had the radiator cored and cleaned out, new thermostat 160 degree. The Caballero only runs on when engine is hot, about 190 degrees. My timing seems to be fine the car runs great and has plenty of power, just this dieseling has me baffled. Any ideas out there?

Update!

Starting to think it's winter gas mixture, I've checked everything else. Anybody out there experiencing problems with winter formula fuel?

 

[jlcustomz]

A band aid for dieseling when shutting off engine is to leave in gear till it dies.

If it's not carbon buildup , then it's between lean carburetion & ignition timing issues. Sounds like it started with new carb. Any possible vacuum leaks which could include ruptured diaphram in distributor vacuum advance or egr valve if you have either one of them on your model. Need a vacuum gauge if you don't have to see what reading you have at idle. If vacuum advance, is it hooked up to ported vacuum & not full time vacuum port?' Lastly confirm you have timing set correctly using correct method for your application. You may want to try a couple degrees less advance as a last resort.

My first guess still is that something is causing fuel mixture to be too lean, which causes higher combustion temperatures.. Could aslo try slightly lower idle speed.

Hope this helps you think.

 

[Peter]

your idle speed is way too high hot, its no wonder its dieseling. A engine without a big cam has no business idling at 1000 in park.

 

[Gordo_AZ]

Peter,

I think your right, there's a factory sticker on my fan shroud that has a bunch of info on it. I went ahead and actually read it :lol: it said to idle speed to 600 rpm in gear, so I did so. It reads about 800 rpm in park and I took it for a test drive and got the temp up to about 180 degrees and it did not diesel. I think this might have done it. I will try and get it up a little hotter around 190 degrees later this week and will know for sure if this fixed it for good. I did not loose pick up speed or power by turning the idle speed down either, thanks for your advice. I will post later this week letting you know if this did it for sure.

 

[Gordo_AZ]

Peter,

That was it, I tuned down the idle speed on the carb and no more run on. I got the engine nice and hot, has not dieseled once since the adjustment. Thanks!