Jason wrote:
>Since we're talking about dieseling, running-on, or whatever you want to
>call it, maybe some folks out there can tip me off to why my car does it.
>It only runs on after it's good and hot, and if I let it idle a bit before
>shutting it off, the situation seems to improve. It's an intermittent
>thing...
>
>I know this can be caused by a LOT of things, (mixture too lean, coke in
>cylanders, etc.) but I guess what I'm looking for is a difinitive way to
>track down the true cuase of the phenomenon, rather than beating my head
>against a wall. If I have learned anything about these cars, this search is
>in vain: Is pure trial and error the only way, or are there any more
>symptoms I should look for to try to narrow it down?
As I have commented on the situation of dieseling and plugs in a prior
thread...
>>Be all that as it may... I also have a dieseling problem. I don't know if
>>it is plug related or not, but I'm interested in trying a cooler plug to see
>>if that will help. I assume that's the direction I want to go in? What is
>>the secret to knowing what plug is one or two steps cooler than the N9Y?
>Plug temp refers to the RATE at which the plugs dissipates heat from the
>center electrode back thru the thread and into the block. Installing a
>plug that is a heat range or two higher than recommended is the same as
>installing a glow plug.
>
>>What other implications are there in switching to a cooler plug?
>NONE, it does not affect the spark in any way.
>
>Carbon build up was also mentioned as a possible problem for dieseling.
>Unless you are burning a huge amount of oil, carbon does not build up in a
>sufficient amount to cause dieseling. You need a hunk of carbon that is
>large enough to hold and give off enough heat to cause ignition of the
>air/fuel mixture.
>
>Personal case: When I first bought my 73B it was running N9Ys. I changed
>them out for another pair during an initial tune up. I was having
>dieseling problems. I put AC Delcos in and guess what... no dieseling.
>Upon rebuild I found that there was a bit of carbon build up, (1/4"), on
>all of the valve flats. Plugs DO make a big difference.
As you can see there are quite a few refrences to a diesel motors. The
reason is that you motor is acting in the same way. When you shut the
motor off the cylinger that has been thru the compression stroke, yet not
recieved a spark, has enought heat inside it to ignite the air/fuel
mixture. This heat source usually comes from HEAVY carbon build-up or
running too hot a plug, (take out the N9Ns if you are using them...see
above). If neither of those two problems are present then the fuel that
you are using has a flash point that is too low and you have to go to a
higher Octane rating, (octane figures indicate flash point only).
Hope this answers you question.
John Knight
'73 MGB (undergoing restoration)
"If uniqueness makes us who we are... why be like everyone else?"
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