Supera

New User
Hi, I have a 51' Super A that is dieseling when hot.
The engine as been rebuilt with 3 1/8" stepped piston in the 80's. The compression is still good and quite equal around 130 psi. It's easy to start and runs perfectly.
I tried running it with supreme gasoline 92 octane, it helps but still dieseling.
Is there something to do or check to cure my "little boy"?
 
very simple fix... slow the idle to 400 rpm or less or as slow as it will run and no problem. if it cant suck fuel past the throttle plate it cant keep running.
 
A few things can cause that, if its not been used hard and has corbon on the valves and pistons, that stays hot and makes it diesel. Timing set too fast will also, as well as idling too fast.
 
well the idling too fast is what causes dieseling which means running without a spark. and this happens because the throttle plates are partly open at idle sucking fuel into the cy's. timing has little to do with it but it should be set to spec. so to cure this problem you need to have the throttle plate pretty much fully closed. this way no fuel can be sucked into the cyl's and the hot carbon igniting the fuel. vehicles in the 1980's use a soleniod on the carb to completly close the throttle plates once the key was turned off. dont really know why people keep on saying its the timing when there is no spark happening.
 
Big misconception on Dieseling, timing advanced has NO impact, carbon has minimal impact, it is likely idle speed.

Let it idle for a minute at about 600, then slow the idle WAY down to about 400, then stop.
 
If setting the idle down to 400 doesn't get it to stop, just put it in road gear, apply the brakes and let out the clutch as you shut it off. Much
better than blowing fuel through the air cleaner. Jim
 
Thanks guys for all the good advices. I love this forum.
The slow idle is around 500 rpm, i'll bring it down to 400 rpm.
I'll also tcheck if the throttle plate isn't completely shut.
If with all that the "little boy" is still dieseling, well, i'll do what I always did, killing it in gear.
 
as you are slowing the idle down it is closing the throttle plate. if its completely closed the tractor will not run. but you can also do that... back out the screw so the plate is closed or tractor stops . then when you retard the throttle the tractor will just stop after you idle for a few minutes. "having the throttle plate closed is the cure for dieseling" so fuel cannot be sucked into the cyl's on shutdown. the rpm must be so slow that the tractor is barely running, less than 400 rpm even. it appears your missing the point on rpm and throttle plate , they go hand in hand.
 
I set mine that way, then only use last click on the throttle as a "shut off", just remember to open it a click or two for real idle.
 
Well, with all those informations, can't believe I won't cure it.
Thanks Rustred for the precision on throttle plate and idle rpm.
I now understand how a carburator works. I'm more an "injected" guy then a "carburated" guy.
Have all a nice day.
 

Hey John, good advice. That's what I do every time I shut a tractor down for the day or for an extended period. That way, the float is in the "down" position, and the next time when you turn the fuel on, you fill up the bowl. If you leave a full fuel bowl for an extended period, and the float sticks, result is no fuel, and carb bowl removal. A float stuck open isn't good either, but it is definitely easier to "rap" the side of the carb and get it to close than it is to get it to drop........

In any case, I'll be the "crab butt" and say the following:

Fix the problem and get rid of the carbon..............quit circumventing the problem. A correctly timed engine will run at the appropriate temperature (ie. not too hot). A correctly set carb mixture and a clean air cleaner will not run rich and cause the cylinders and piston to carbon up. When you turn off the ignition, it will stop running. If it doesn't, something is igniting the fuel mixture, which usually is hot carbon build up. Even with your stepped pistons, your compression isn't high enough to diesel on its own, something else is igniting the fuel mixture

To everyone else, my up front apologies, but dieseling in a low compression, low RPM engine comes from 1 thing and 1 thing only: hot deposits in the combustion chamber. Fix the problem, don't adjust around it. If I cut the spark at full throttle on my tractors, at operating temperature, they stop..............they don't keep running.

Rant over.
 

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