[AG]

I built a new motor for my brother, 464 ci with Eddy heads and OF HR cam with headers, MSD 6AL box, MSD RTR distributor and converted to 10 SI alternator (car is a 1970 LeMans). The engine runs on after it is shut off and I can't figure out the issue. The motor runs cool, about 160-170 F even on a 90+ F day. The initial timing has been changed from 10-14 degrees and it doesn't matter. The idle is at 800 rpm. The idle AFR is about 14-15 and the cruise AFR is fine. It has a Holley 850 and the throttle blades are adjusted to show about 0.040"-0.050" of the transition slot. The plugs look lean but the AFR is 13.5-14 cruising. The compression is 10.4 with AL heads so it's similar to 9.4 with iron heads. I have checked all the obvious and I'm wondering if it is ignition related or maybe the alternator is back feeding. How can I check the alternator without just pulling the hot wire off while running.

[leeklm]

I am assuming you need to install a diode on the alternator wire (brown one i think?) to prevent back feeding. While the engine is running after shutting key off, pull the 2 wire connector off alternator. If engine dies, you need the diode.

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[george kujanski]

So is it Dieseling (run-on) charaterized by stumbling and smelling, or is it just not shutting off?

Not shutting off can be due to the 10SI alternator....you need to add a diode (like a 1N4007) to keep the alt from backfeeding and keeping the MSD box alive. Add the diode in series with the alt terminal 1 with the diode arrow pointing to the alt.

george

[AG]

It is dieseling. I thought it was electrical related and I will try the diode. Is the 1N4007 a generic designation of the diode type or is it a PN?

Thanks both for the responses.

[george kujanski]

it's an industry type number...1N4001 thru 1N4007 will work, the 1N4001 is a 100V rated part, and they go up from there 1N4007 is 1KV as I recall.

Dieseling happens with a hot engine, low octane, higher compression ratio, advanced timing, and throttle open too far as I recall, can you slow it down a bit more from 800?

Back in the day, there was a anti-dieseling solenoid that held the throttle open for idle, and let the throttle plates close with ignition off....no air, no dieseling.

george

George

[AG]

I can try a lower idle, but if the diode works then it is fixed. My race car idles at 1300 rpm and shuts off no problem ever.

[leeklm]

I was thinking the alternator was causing a full on condition, not dieseling... i have same issue with my engine if I run anything lower than 93 octane from our local BP station. My initial timing is low, same issue with multiple carbs. If I do run something like 91 octane, car runs perfectly fine street or strip, but will diesel a little at shutdown.

I run about 1050 rpm in park and 850 in gear. I am about 11:1 compression with kre heads. If not running 93, I will turn the car off while in gear, which eliminates the diesel .

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[AG]

Not full on, but dieseling. It diesels bad, not a little and we tried straight 93 octane and it didn't matter which is why I think it is electrical. It's a 4 speed so he puts it in gear and dumps the clutch to shut it off, not good. My race car is 11.2 CR and I run 93 octane and it never diesels even when it's 95 F out. Your car runs well but I can't believe it is 3320 lb race weight.

[leeklm]

"Your car runs well but I can't believe it is 3320 lb race weight."

That weight is stripped down, no seats or center console. Also lightweight wheels all the way around, but includes full exhaust.

I ran across the scales at BIR last week fully dressed with front street tires and was 3,400.

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[Cardo]

I would get some higher octane fuel before chasing anything else. Dieseling by nature is combustion without spark. I doubt if it's electrical.

[NAPA68]

I chased the same problem in a 71 455HO (9.0:1 compression) I recently bought. Checked and adjusted the float level (on a Cliff Ruggles build).......no change. Colder plugs........a bit better, but still dieseled.

Bumped the initial timing to 18 degrees initial (38 overall) and was able to close the throttle plates to slow down the idle..........SOLVED!

Now having said that, you need to watch a couple of things. First, the starter has to be able to carry a good amount of initial timing. Secondly, you need to see where you are on the top end of the timing curve. My experience dictates 36-38 all in is optimal, provided you are not pinging under load, nor do your plugs indicate you have detonation on the top end. If you have anything such as I mentioned, you need to limit the mechanical advance in the distributor.

All of these different combinations make the tuning a bit of the braille method. Time, patience, and paying attention to the small details makes all of the difference.

Hope that makes some sense to you as well as helps you with your problem.

Tim

[Scott65]

Have you checked the balancer timing marks to verify they're accurate? Maybe you have more advance than the timing light is showing

[AG]

I checked balancer TDC when I built the motor and it was off 1 degree. I've tried colder plugs and it didn't change. Already tried 93 octane and it didn't change. Throttle blades have no more than 0.050" of the transfer slot showing and it idles at 800 rpm. I can try putting more static advance and closing the throttle blades more.

[Scott65]

How cold of plug did you try, and what type? Projected tip maybe holding onto heat?

[PontiacJim1959]

Originally Posted by AG

I checked balancer TDC when I built the motor and it was off 1 degree. I've tried colder plugs and it didn't change. Already tried 93 octane and it didn't change. Throttle blades have no more than 0.050" of the transfer slot showing and it idles at 800 rpm. I can try putting more static advance and closing the throttle blades more.

Rig up an electric carb throttle solenoid - that is their purpose and they came factory. Set your idle with the solenoid energized, then when the key/power is shut off, the solenoid retracts and will completely close (or close to it) the carb primary throttle blades. Problem solved.

["QUICK-SILVER"]

AG

Brother's engine could easily have higher octane requirements at idle than your hot rod engine.

Two things:
Compression figured from intake valve closing.
And intake charge dilution from exhaust during overlap.

Comparing cranking compression between the two might tell a little. But it doesn't tell you about intake charge dilution that gets worse when cams have more overlap.

Brother may not be good with 93 where you are.

Also wouldn't rule out too much by-pass air from a vacuum leak. Like can happen when you dont use a metal shim adapter and two gaskets with aftermarket spread bore intakes and square bore carburetors.

Clay

[Kenth]

Engine run-on was not common for 1967 or earlier engines using full manifold vacuum for the igniton vacuum advance. Not uncommon to have 26° BTDC at idle speed at the time for complete combustion events.
Retarded ignition timing started 1968 for emission reasons causing larger openings of throttle blades and why curb idle speed solenoids were used to prevent engine run-on at shut off.
The added heat in combustion chambers due to the retarded timing made the engines run-on, from to self-ignited residues from not fully burnt A/F mixtures at shut off.
Conclusion, the easiest way to cure engine run-on is added timing, from vacuum advance or added initial timing with shortening the mech and vacuum advance settings to prevent WOT and part-throttle ping.

Hope this helps some.

[AG]

I will check out some of the suggestions as a process of elimination.