Carbed 4 cylinder and dieseling


I have a 1980 Toyota corolla with a weber downdraft carb installed by previous owner.

My problem is that when you shut it off it runs for a few seconds and will either A. Diesel, rev up and die. B. Backfire loudly and shoot flames. C. Backfires but sounds more like a burp.

It also idles rather high.

Any ideas? I've had two carb tuners look at it and they did their best but to no success.


Pontiac Pilot
Agree sorta, it is not a carb issue. Dieseling is compression ignition caused by carbon build up and or hot spotting in the heads. So you have a cooling issue or carbon build up or both. Make sure your timing is set correctly and flush your cooling system with some good cooling system flush. Try a lower temp thermostat, maybe a thermocouple controled electric radiator fan. Take a bore scope to it and see if you have carbon build up.

Webers are easy as hell to tune, changing jets is a breeze. Google for how to info.


...irrelevant user name..
agree, when my timing belt was stretched, my truck dieseled sometimes on shutoff... check your cam timing, ignition timing, and cooling system... then seafoam the hell out of the engine(through a vacuum line on the intake) to eliminate carbon buildup, then look at the carb... see how unrelevant it is to your issue?

Middle School

middle school
Dieseling isn't a carb issue, generally. It's an ignition issue ....
agreed. its likely timing, though ive seen it happen with really bad gas. or appear too, when in reality it wasn't dieseling, really just running like goatse'd butthole.


Well-Known Member
I agree that the Timming could be off and/or the temp is too high. However there is an rare exception that I have confronted on and off for 15 years. You mentioned that the Idle is too high? Posible vacuum leak and lean fuel mixture can cause that. Can create a ventury effect. Look bown the barrle of the carb while idleing. If you see dropplets siphon at the ventury while at idle speeds then your Idle mixture screw is too lean...Lean mixture equals Hotter combustion chamber temps. First and fore most you need to isolate any vacuum leaks. Take a empty bottle of dishwashing soap and add water. While Idleing sqirt water around the base of the carb to find a leak...also any intake gaskets, vacuum hoses and any component that requires vacuum. When you hit the a leak you should hear a slurp sound and notice idle speed change..Fix the leak then move on the checking mixture. Go back to Idleing and softly lay your hand over the carb. but don't seal it completely. Your just trying to restrict air flow into the carb. not block it completely. If it is still too lean then slowing the air flow witll raise the idle speed. Adjust the Idle mixture screw by backing it out 1/4 turn intervals untill you get Fifty (50) RPM drop when you place your hand over the carb. IF it is too rich then it will stall. Remember your not choking it to death with your hand just restrickting flow.

Another Carb issue is Air Horn gasket leaking due to warpage. Most carb. come with three primary parts..Air Horn (top), Mid-section (middle) and Throttle Plate (bottom). Any leakage between any of these components will/can syphan extra fuel and distrupt mixture control. Grab the top of the carb and see if it moves if you try to twist it. if so fix it.

Timing could still be a problem but in order for it to Diesel it still has to get fuel to continue. No fuel then no dieseling.

Good Luck. Bob.


Well-Known Member
Oh just FYI. This is something I use to teach to my students....Always ALWWAYS..tune in this order...
Set and adjust,,Points, Timming, Fuel mixture then Idle. I've fired many technicians for finishing a tune-up with timming as there last step....Remember Points (1) effect Timming(2) effect Mixture(3) effects Idle speed (4). You have to have a solid base to build. Idle cant change mixture, Mixture can;t change timming and timming can't change dwell (points). Good Luck.