ddtully

I have a big block 427 motor on a 66' corvette and I can't seem to get the dieseling to stop after turning off ignition. This forum has been very helpful with some suggestions but I am now desperate for a more definitive solution. I have spend so much time in the garage trying to correct this issue.

So much appreciated,

David

Greg

Idle too high or timing too far advanced?

Sky65

Throttle plate open too far. This usually happens when not enough base timing or vac advance not used or not connected to manifold vac.

Tom

marshal135

Greg is right on.
If I start my 68 427 briefly with out allowing it to warm up it will run on at shut off.
A properly warmed 427 and properly timed should not run on.
A 4speed car should have a warmed idle of 750 rpms and initial timing should be 16-18*
Marshal

pigfarmer

My '69 350 did that. Idle was just a bit high.

bashcraft

This is your problem.

domenic tallarita

No one will like this, but my 425 has the factory timing and idle speed.
It will diesel every time I skimp on octane. I can get it to stop when using 91 octane and retarding the timing, but loose mega HP.

Dom

bashcraft

The throttle plates are open too far. Do whatever you have to too close them up a bit.

domenic tallarita

I respect that good advice, but it runs soooo good with the right octane and stock settings. I guess being a old Chevy mechanic I wanted a engine
just the way it was and ran back then with all the same parts that GM built them with.
It did it with the stock Holley also. I adapted a Q-jet to it and upped the power and fuel economy, but it still wants good gas.
I did get some advice here on the forum about changing the vac can and distributor springs, and that might be what I need to close them a bit.

Dom

ddtully

Thank you Greg as I have not yet considered to check my timing or idle and will give that a try.

ddtully

I am going to check this today

ddtully

Thank you Greg. Others have been suggestion this too and will be giving this a try to see if I can get the right amount of idle/mixture to prevent the run on.

REELAV8R

I don't disagree that timing could be too advanced leading to detonation leading to hot spots in the combustion chamber, however I would suspect too low of initial timing first.

When the initial timing is set too low it leads to very hot exhaust and combustion chamber and forces you to open the throttle blades farther to get it to idle making dieseling more likely. Too much heat in the combustion chamber and a throttle opening seems to be the cause for run-on or dieseling from what I'm seeing with guys here.

If there is a lot of carbon built up in the combustion chamber this can be a contributing factor as well. Do you plugs look pretty sooty or black when you pull them? If so then the chamber is likely in a similar condition if it has run for any length of time like that.

Water poured slowly into the carb while holding the RPM up to maybe 3000 or so on a fully warmed up engine can clean carbon out.

I can't get mine to run-on even if I hold the throttle at 2500 RPM and turn the key off. But I'm running aluminum heads (less heat in chamber) and have the timing set to 19* BTDC for the cam I'm running which allows me to close the throttle blades for idle.

69427

Usually the issue is excessively opened throttle blades (regardless of idle speed) due to the timing being too retarded from optimum.

PAmotorman

you got it as you need heat to fire the gasoline that that is sucked into the camber. closing down the butterflies will stop the fuel and air but you still have something hot enought in there to light the fire

ddtully

Can you recommend the optimal BTC setting?

lars

E-mail me for my papers on that subject.

Lars
V8FastCars@msn.com