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Run-on fix

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simpzimmer

simpzimmer

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Another problem I'm having is run-on. I noticed some models have a run-on adjustment, but my 79 Spit does not that I'm aware of. I thought maybe I had carbon build up and I've tried the cleaners but I've still got run on. How would you all deal with this problem?
 
Hi,

Run-on is caused when two things happen... fuel/air mixture continues to be drawn into the engine and there is something acting to ignite it once it's in the cylinder. The source of ignition can be glowing carbon embers, as you have noted, or glowing sharp edges, an overheated spark plug, or even high compression. Run-on is sometimes referred to as "dieseling" because it's really very similar to the normal funtion of a diesel engine, which has self-sustaining combustion once a glow plug in the combustion chamber has heated up.

The easiest things you can try are swtiching to higher octane fuel and/or retarding the timing a little. Also try a fuel with more cleaning agents in it, such as Chevron. Another possibility is cooler spark plugs (first confirm the ones in the car are correct specification). A "cooler" plug is one that helps remove more heat from the combustion chamber. Some octane booster fuel additives might help, too, but I wouldn't expect much.

More involved fixes include checking if the engine's compression ratio has been increased a lot by milling the head and, if you wish, reducing the compression with a thicker head gasket. Also, it would probably help to pull the head and properly decarbon it (the cleaners can only do so much) and, while it's apart, check that there aren't any sharp edges in the combustion chamber that might heat up and glow, i.e. act as an ignition source.

Yes, some cars have anti-runon valves. Essentially these release the remaining vacuum in the intake manifold when the car's ignition is turned off. Doing that stops the engine from drawing in more fuel/air mixture, which is partially what causes the runon. If your intake manifold has a means of doing so, you can add an anti-runon valve that's wired to the ignition (closed when the ignition is "on", open when it's "off"). Personally, I think that adding an anti-runon valve might be sort of a last resort, after all other possible causes have been addressed and doing so hasn't resolved the problem.

Something to avoid is carburetors with anti-runon fixtures. Some SU were fitted with butterflies that have a small valve (not sure what to call it) in the center. This really messes with the flow of the fuel/air mixture into the engine and isn't a very good solution to the problem.

Hope this helps. Let us know what solves the problem.

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Alan,
Great explanation...probably the most helpful summary of solving dieseling I've ever read!
 
I've never had run-on in 15 years since we switched to lead-free gas. When I removed the head with 43,000 miles on it to convert it for lead-free gas with inserts for the exhaust valves, there was no carbon build up and since then I've driven another 44,000 miles and no problem. I thought that lead-free gas prevented hot spots or carbonising.

Don
 
Alan - are there any disadvantages to a cooler plug?

Bruce
 
Hi Sam, Don and Bruce,

Sam, you are way to kind!

Don, I really don't know... seems to me that unleaded has lower effective octane and runs hotter, but I might be wrong. I know unleaded valves and valve seats need to be able to handle higher temps, as well as be harder since the lead in old fuels provided lubrication that is now lacking.

Bruce, too cool a plug will tend to foul. If you try a cooler plug, watch for that and any missing. If there is any fouling or missing, better switch back.

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Or, you can do like most people, simply turn off you engine with the car in gear and the brake firmly applied.

Bill
 
I don't even think about it anymore, I do exactly what TR6Bill does.
 
Fuels are so much cleaner burning than they were 35 yrs. ago, as Don has noted and just not from the removal of lead. My first job out of the army was working in a lab and my race car building mentor one day hands me a plug that looked more like a cave formation and could I please analyze the crud. Well I couldn't, but my boss did and while it turned out to have about 40%? lead oxides, the rest was more like a hot water tank deposit, calcium, magnesium, some silica. And from a car that only burned 'super' shell - remember platformate. What awful chemistry they had, shell that is, but great advertising.
But now you have to go 4 times as far to even accumulate a little bit of scale. And Bill do you have the clutch mostly disengaged?
Tom Lains
 
No different than engaging it to get the car going from a stop.
In fact, less stress on the whole setup.

Bill
 
Allowing it to diesel is much more harmful than stopping using the clutch/gearbox.
 
I'd posted the same problem with run on last summer and had the same sort of suggestions you've see here. My TR4A does not run on now, what I did was a tune up, carbs and ignition. My TR was running hot, to the point it actually overheated on one trip. I also revamped and improved the cooling system and as I said, no problems now.
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