Compression and running on

[AK260]

 

Dear Z gang,

 

We have a member here whose L28 runs-on with SU carbs.

 

He is running a compression of 210PSI and believes this is normal behaviour for high compression combined with using SUs.

 

Mine with 205 PSI, high quality UK RON 99 fuel and SUs does not run on anymore after being rolling road tuned.

 

Is anyone out there on SUs getting run-on after turning off and what dynamic compression do you run?

 

Or indeed is anyone pulling 210PSI (or more) with SUs and NOT running-on ?

 

Would be interested to hear your experiences.

 

Apparently it doesn't happen with DCOE carbs, is that correct in your experiences?

 

Does anyone out there have a good explanation of why it happens on SUs rather than just that "it is common"!

 

The DCOEs like SUs will continue to feed a pumping cylinder so is it just a case of DCOEs running richer and hence cooler?

 

 

 

[Zed Head]

Nothing specific to SU's but my understanding of "run on" or dieseling as we used to call it is that it's caused by hot spots in the combustion chamber.  Why it was called dieseling.  So, he either has dirty combustion chambers or sharp edges that are staying hot.  High cylinder pressure would be more susceptible to dieseling.

The common way to avoid it was to rev the crap out of the engine just before shutting down to blow all of the carbon out and get some fresh cool fuel and air in there.  

[Mark Maras]

 I agree with ZH. Probably carbon and residual unburned fuel that is causing the run on. It could be a rich mixture (carbon in the cyls.) that is the root of this problem. What do the plugs (color) look like? I can't see why DCOEs would be any different than SUs if both are properly tuned.  

[26th-Z]

High compression engine...fuel could cause the problem, carburation could cause the problem, hot spots could cause the problem, but I would look at the timing.  Run-on is more often a timing problem, especially if the fuel is low octane for the high compression.  Check your spark plugs also.  They may be too hot?