My 425/427 motor tends to want to continue running when I shut it off. I thought that this would stop during cold weather, but it still happens. I have factory equivalent cam(Wolverine) and flat-top pistons(TRW 2300F)...I think the comp ratio is near 10 to1 (or maybe lower). I have tried several different plugs, but seems not to matter. I have a set of AC R43XLS which I will try next. The carb is rebuilt recently. I am currently running 91 octane premium (Cal. gas..ugh) but it is stoked up with an appropriate amount of pure TEL additive from Podell.
Timing is exactly at factory specs. There is NO pinging while driving. The car runs absolutely perfectly except for this phenomena.

Is it possible that there is a build-up of carbon which is at the root of this? Is there a way to address that? (I am aware that some C5's have a pinging problem which is fixed by cleaning out the carbon using top oil...a factory recommended procedure). But, since there is no pinging at all, this doesn't add up.

Any expert opinions on root cause ??? :confused: :confused:

Thanks,
Mark

 

Your suspicion of carbon buildup could be one cause. Sometimes the carbon retains heat. Others that I can think of could be advanced ignition timing, high engine temps, octane level and too high a curb idle. I'm sure others will add their ideas and experiences. Until you get it corrected you can always keep the car in gear and let the clutch out a little when turning it off.

Rich Lagasse

 

Thanks, Rich. I forgot to mention in my post that I also recently installed a new aluminium Be-Cool radiator and since then the car runs very cool. The idle speed is at about 900 rpm and is smooth with a little roll (probably the cam)

The technique of keeping the car in gear and letting out the clutch a little, is actually what I have to do now to stop the motor. Otherwise it just keeps bouncing along.

Quite a mystery to me.
Mark

 

install a idle speed control solenoid on the carb the sets ithe idle speed with the ignition on but closes the carb completely when you shut of the ignition. you can get them from holley. this should cure your problem because now fuel is being pulled into the engine on rundown. :chevy

 

Your engine has ported vacuum advance. Reconfigure it to get full manifold vacuum to the vacuum can at idle by teeing off the choke vacuum break line or other source of full vacuum.

The increase in total idle advance will allow for better idle quality and reduce EGT and combustion chamber temps. which will very likely stop the run-0n.

Duke

 

In addition to the other comments, if your idle speed is set too high it usually results in the throttle plate being open slightly at idle (thus allowing the dieseling of the engine to continue to pull fuel and air from the carb). Just backing down on the idle a little (and thus closing the throttle plates) may be all you need to completely solve your problem. :smash:

 

The factory idle setting for my 427/435 was 750 rpm. The carbs have to be in good condition and adjusted well to hold that idle setting without stalling. If I "cheat" the idle up to 800 rpm to avoid rebuilding the carbs---it'll diesel every time.

 

Do you have the stock Holley?

And this is an aftermarket camshaft (wolverine).

The previous mention of the advance should be mapped out to ensure it and all it's functions are operating. The base timing could be set to high, or the vacuum can stuck open??

As for the Holley, if it is not stock, it may not have the correct bleed air holes.
If you find that when you lower the idle and the engine shuts down without the run on you may have the fuel continuation as previously mentioned. An easy cure for this is to drill a very small hole in the throttle plates on the primary side of the carb. Start small and work you way up. This allows the plates to close all the way to there designed posisition. This will ensure that the engine gets the idle air and the transfer slots are covered up. then the carb will be metered off the idle circuit only. Do you have a problem with the idle mixture screws having little input on the mixture settings? This is a good indication that the transfer slots are feeding the engine and not isolating the idle circuit. Another quick fix is to raise the back throttle plates slightly and bleed air this way. I have a holley 1850-2 as a back-up carb for my vette. This one-size-fits-all application like most generic holley carbs have to small of bleed air passages and I had to drill the throttle plates to get the mixture set with that carb. The stock 2818 seats just fine.

Mark

 

Again, thanks for the ideas. I will go ahead and experiment with idle speed settings to see if that helps. Also, will try backing off on the base timing advance. The carb is absolutely stock and has recently been completely rebuilt by professionals (not me). If the idle speed does not affect this, I will try the vacuum line changes (unfortunately, there is not an easily available engine manifold access). Last ditch is to try for the solenoid controlled idle from Holley.

Will let you know what happens,
Mark