841 gasser 'dieseling' after shutoff

Hi, All,

subject line says it all

- carb rebuilt
- valve train replaced and regapped

after ignition is shut off tractor will run more
or less indefinitely on 1-2 cylinders; eventually
one will fire before tdc and the engine will run
backwards and that puts a halt to it. I can kill
it under a load or put the throttle wot and the
influx of cool gas seems to kill the dieseling.

Temp gauge barely into green.


Anything else I should do?

thanks in advance

Bill
 
I think dieseling can be caused by a couple of things. One is carbon build up in the combustion chamber that gets hot enough to ignite the gas/air mixture even after the ignition is off.
You might take the tractor out and work the holy snot out of it to see if you can blow out some carbon.
Another thing that I think will make them diesel is too lean a fuel mixture. Try enriching it a little and see if that helps.
 
You, know, if I was smarter I would have also remembered to say that there is occasional backfiring out the carb when I shut it off.

When I get the tin back on I'll work it a bit and I'll get some insight into the mixture.

thank you!
 
Idling at too high an rpm at shutoff can result in dieseling as well as too advanced initial timing or a problem with the advance mechanism in the distributor
 
Back when we had leaded gas, higher octane, that would elimate most of the dieseling.

It could be caused by the tips of your plugs getting too hot. Look at the plugs, if they are white, try a cooler plug. If they are black, you definately have a carbon build up, increasing the compression ratio, increasing the temp, carbon is glowing like a glow plug.

I have a 20 hp kohler command that had the same problem with carbon on plug, no adjustments on carb, so I went with a hotter plug. However, in the summer, I may have a little dieseling. So I let engine idle for 5-10 before shutting off or put it under load to prevent it from dieseling.

Sometimes dieseling causes engine to run backwards, that may cause back firing. Bad intake valves may also cause back firing.

I have experimented a little with trying to change octane by adding a little, 5-10% E85. Not sure, but I think it may cool off the gas a little. I know a farmer who makes his own E30. He claims E30 gives him the best bang for the buck in his older gas trucks and tractors. Now that up for discussion. Not my opinion, Just his. He farms 7000 acres too.

On the other hand, I know a guy who's grandkids race small shifter cars. They look like smaller midget race cars, bigger than 1/4 midget. He burns methanol and cools it off with what he calls top lub. Like mixing 2 cycle oil with methanol. Then he add something to boost the oxygen. Oil to gas decreases oxygen in fuel.

I'm no rocket scientest, but in the winter, I have to turn the main jet on my Jubilee out one full turn to richen up the mix. There is something in the winter blend, that makes my tractor run lean or the cold air has something to do with it. Not really sure.

So, if you are running lean, tell by the plugs, richen it up, that should cool the fuel it off. Try a hotter or cooler plug, depending on your problem. Try cooling off you gas by adding a little a very small amount E85 or 2 cycle oil. And then there is always timing. I had a 1962 XKE jag. It needed leaded 98 octane +. When I couldn't get it, I retarded the timing to keep engine from pinging. Then there is always putting tractor under load before you shut it off.

2 cycle gas and oil has a very short shelf life. A guy gives me his old left over mixed fuel. I add a very small amount of it to the fuel in my old gas tractors. So, I can say a little 2 cycle oil doesn't hurt a thing.
Post back and let us know what works.
George
 
Not the same engine as yours but I had a 158 CI gasser in my 3000. I had put a new Zenith on it to replace the old Holly.
Then I switched the engine to a 201 CI for a few years. Used the same carb on both engines but did not adjust it for the larger cube engine.
It ran fine but was backfiring when I shut it off.
So I adjusted the carb a bit and enriched the mixture.
Backfiring stopped.
 
Run on (dieseling) upon ignition shut off is nearly caused caused by killing the ignition when the throttle is open too far (idle RPM too high), at least prior to EPA gasoline.

Other causes are, spark plugs of (much) higher heat range than recommended, overheated engine, improper ignition timing, rarely, carbon build up in combustion chambers, etc.

NEVER kill the ignition on a hot engine before returning the engine to proper idle RPM. Run on is not always in normal operating direction. When the engine runs on in reverse direction, the oil pump is also operating in reverse direction.

Dean
 
Hi, Dean,

- shutting it off at pretty low idle (400ish)

- I have the POs sparkies in there; will AL437s do?

- hadn't considered the oil pump running backwards... but I thought the valve timing would keep it from running backwards

thanks!

Bill
 

Someone gotta 'splain to me how ignition timeing can cause diseling AFTER the ignition is switched off.
 
My limited understanding: Advancing the timing increases the
maximum pressure in the cylinder, which increase temps. This
them leads to hot spots in the head which continues to ignite fuel-
air mixture even after ignition is off. Retarding the timing places the spark ever laters after
TDC, reducing max pressure and temp.

Experts, tell me if I got it right.
 
(quoted from post at 20:54:43 03/17/14)
Someone gotta 'splain to me how ignition timeing can cause diseling AFTER the ignition is switched off.

It would be what affect incorrect timing would have on combustion chamber temps BEFORE the switched were turned off...
 
That usually means it's turning backwards and further points to too advanced timing either from the initial setting or faulty dist advance mechanism
 
idle does little at mid rpm.

enrich the main mix.. normally you want to adjust the idle mix for best rpms at idle
 

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