Dieseling

[Joel T.]

Hi Guys;

From time to time, without a pattern that I can readily pick up, my 1963/340 will diesel after I shut off the ignition. Happened last night coming back from "classic car nite" at our local diner (gets you a 20% discount off dinner!). I stopped for gas and she kept running... a few seconds, like 2 to 3. When I went to start her again after my fill up, she was a bit flooded, and once she did kick over, the idle was low... like 500 RPM...

I drove the car home and put her in the garage.. By this time the idle was back to normal (around 1000 RPM)... She shut off without a peep.

Stock distributor with the correct vacuum can (within Joe's 2" rule), stock timing, valve lash etc. Stock carb recently rebuilt...

Any ideas?

Thanks in advance,

Joel

[Roy B.]

Re: Dieseling

Carbon build up on top of the pistons

[Michael B.]

Re: Dieseling

Unless you have a cam that wont allow a lower idle, 1000 RPM is too high and can cause dieseling. Try lowering that idle to spec and re-adjust the idle mixture.

[Duke W.]

Re: Dieseling

Assuming you have an original Duntov cam, or even any other OE or aftermarket mechanical lifter cam, use of the "two-inch rule" (which I coined, but whatever...) should yield the B28, which has different specs than the OE 340 HP VAC.

So what's actually installed?

It should idle at 800-900, or something is not right.

As previously stated, lowering the idle speed should reduce the "run-on" tendency, and you can always let out the clutch in gear (with your foot on the service brake and/or firm application of the parking brake) as you turn off the ignition to keep it from happening.

Duke

[Stuart F.]

Re: Dieseling

Definitely high on the idle, question is why? Is your base adjustment (off the high idle cam) set too high, or are you still on a cam step. The manual choke on an AFB is always suspect as they tend to be lazy after years of use (weakened spring or sticking choke pulloff piston) even on a rebuilt. I've delt with these same issues on mine for years, routinely cleaning out the pulloff cylinder of white corrsion and using emory cloth on the piston, freeing up the linkage, etc. I've finally gone to an electric for everyday use that both pulls and pushes the linkage (closed and open). The piston itself only sees about 1.5 to 2.5" hg pull which, even with the heat from the clean air tube, is marginal at best to do the pulloff job. I have made a correct manual choke work better by increasing the heat (sort of) by using an older steel tube (not the chrome) and pinching it with a vice grips near the manifold to restrict the air flow (slow it down) further (similar to the shape of the heat tube that's in the exhaust manifold - same principle). You can also add an insulating sleeve over the tube, but all these are not appearance correct measures. If appearance is of the utmost, then you'll have to adjust for full opening of the choke and live with the harder initial starts. You do not want the choke hanging up partially closed when the engine is full warm as this will give you dieseling, and you can't get WOT either.

The E-Choke kit I have uses a sensor mounted to the blind tapped hole in the intake manifold behind the carb, then is wired to the ignition controlled hot wire on the wiper motor right behind it. My wires to the choke housing, and behind the sensor, are all enclosed in plastic wire loom tubing and clipped to the carb so it all looks factory - but what year? I leave the fresh air tube in place on the exhaust manifold so I can re-connect the stock system in short order. Maybe some day I'll fully kick the originality habit and remove it, because the electric works so much better and has eliminated a lot of grief I've known over the years.

Stu Fox

[Douglas G.]

Re: Dieseling

I have a 1963 with the 340 HP.I was using low test fuel and had the dieseling problem.I switched to high test or premium fuel and my problem was solved.Worth a try if you are not already using premium fuel. Good luck.

[Duke W.]

Re: Dieseling

A '63 327/340 would detonate like a banshee on regular unleaded unless the CR was considerably lowered from OE, which was actually closer to 10.5:1 with the OE double head gaskets than the the 11.25:1 advertised number.

Duke

[Richard T.]

Re: Dieseling

Joe after you check all your adjustments try some upper cylinder top oil added to the gasoline at each fill up or like Duke says let the engine idle down and turn the motor off in gear. Having the motor run after the key is turned off is not good for the motor it puts stress on the rods. I have found this to be a common problem with high compression Corvette engines especially engines with miles on them and carbon on the pistons.

[Joel T.]

Re: Dieseling

Assuming you have an original Duntov cam, or even any other OE or aftermarket mechanical lifter cam, use of the "two-inch rule" (which I coined, but whatever...) should yield the B28, which has different specs than the OE 340 HP VAC.

So what's actually installed?

It should idle at 800-900, or something is not right.

As previously stated, lowering the idle speed should reduce the "run-on" tendency, and you can always let out the clutch in gear (with your foot on the service brake and/or firm application of the parking brake) as you turn off the ignition to keep it from happening.

Duke

Duke;

I'm a convert, I follow your "two inch rule" religiously... in fact I have a few other "converts" here in NJ as well... I quote you often and people actually think I am pretty smart!

Seriously, yes, there is a B28 in there... The cam is an after-market solid lifter from Competition, supposedly slightly more aggressive than stock. (I have the model and specs in my files) Idle vacuum is a tad lower than stock which you and I covered in prior posts... I know that it will idle OK at 800-900... but we went a tad higher based upon the lower vacuum...

My concern was/is really the carb... She has a tendency to flood from time to time for no apparent reason... (i.e. run it, shut it off, go to start it and it is flooded... foot to the floor to get her going) so I was wondering if I might be getting post shut off fuel leakage which could be contributing to the dieseling problem as well..

Regards,

Joel

[Stuart F.]

Re: Dieseling

Joel;

I have covered in previous threads my experiences with the AFB's on my 63. I have long had a heat soak problem which I was able to limit only after I put a L-75 300 hp spacer under the carb w/2 L-76 gaskets. That lowered the heat at the carb body by some 35 to 45 degrees (checked w/ IFR gun). To do this, of course, requires a hose in place of the chrome fuel line from the filter as well as a longer fresh air heat tube to the choke. Otherwise, I think I tried every way possible to limit the problem w/o changing from stock appearance. I continue to use TEL additive with my fill ups of crappy (10%) premium gas. It's a fact of life in Florida so I've learned to live with it. I live near an international airport - but forget trying to get avgas there. I run an aggressive advance curve w/ a B-28 VAC as well and have no spark detonation or dieseling ever. I also have sweetened the jetting on my current carb, and use Tompco inlet valves in place of standard N/S's. My head gasket thickness test shows mine as not meeting Duke's 2 gasket measurement - guess I just don't know how. The engine has never been apart unless there was a time I was in a coma and missed something.

Stu Fox

[Duke W.]

Re: Dieseling

Duke;


My concern was/is really the carb... She has a tendency to flood from time to time for no apparent reason... (i.e. run it, shut it off, go to start it and it is flooded... foot to the floor to get her going) so I was wondering if I might be getting post shut off fuel leakage which could be contributing to the dieseling problem as well..

Regards,

Joel

It could be needle/seat leakage or float level. If you observe the venturi discharge nozzles after shutdown, you should see no fuel seepage.

I think Stuart covered the AFB needle and seat issue not long ago in detail.

The B28 VAC should work okay with most aftermarket cams (other than very high overlap racing cams) , which usually produce at least as much manifold vacuum at 900 as the 30-30 - about 10".

Duke

[Joel T.]

Re: Dieseling

Joel;

I have covered in previous threads my experiences with the AFB's on my 63. I have long had a heat soak problem which I was able to limit only after I put a L-75 300 hp spacer under the carb w/2 L-76 gaskets. That lowered the heat at the carb body by some 35 to 45 degrees (checked w/ IFR gun). To do this, of course, requires a hose in place of the chrome fuel line from the filter as well as a longer fresh air heat tube to the choke. Otherwise, I think I tried every way possible to limit the problem w/o changing from stock appearance. I continue to use TEL additive with my fill ups of crappy (10%) premium gas. It's a fact of life in Florida so I've learned to live with it. I live near an international airport - but forget trying to get avgas there. I run an aggressive advance curve w/ a B-28 VAC as well and have no spark detonation or dieseling ever. I also have sweetened the jetting on my current carb, and use Tompco inlet valves in place of standard N/S's. My head gasket thickness test shows mine as not meeting Duke's 2 gasket measurement - guess I just don't know how. The engine has never been apart unless there was a time I was in a coma and missed something.

Stu Fox

Hey Stu,

Thanks for the "g2". I bought a couple of cases of octane (not the Jack Podell stuff the other one) booster which I have been using in the 1965 BB car, perhaps I will try a bit in the 1963 as well. The engine is basically new, just 1800 miles or so since a complete rebuild so I don't think carbon buildup is an issue. The car runs clean and is a beast (compared to the "barely ran" condition when I bought it). I have no heat riser valve in there, just the FI spacer... I don't trust those valve things worth darn.

All things being equal, I want to keep stock appearance (worked hard to get her there!), so I will focus on the idle speed and the octane booster.

As I recall, you had an issue with fuel entering the system after shut off.. how did you ever resolve that?

Hey, my kid brother lives in Tampa, R U near there?

Regards,

Joel

[Nick M.]

Re: Dieseling

ok, I give up. What is the 2" rule? and is there a concensus that the octane boosters are good for the high performance small block engines and if so, what is the reccomended brand?

[Duke W.]

Re: Dieseling

Let's see if anyone other than me can quote it correctly.

Duke

[Stuart F.]

Re: Dieseling

Joel;

One other point specific to the 63 L-76 (340 hp); the AFB application and 097 cam (with the exception of the 62's), are unique to our engines. I don't know much at all about the 62 340 hp, but understand it runs a different distributor and VAC (?) arrangement. The point is with the AFB, I have found there is a fine line for where you idle the primary butterflies. Any increase in idle speed above the recommended 750 - 800 rpm, or change made to compensate/maintain that speed with more vacuum advance at idle can disrupt the relationship of the butterflies to the off idle transfer slots. This can result in loading up at one end of the spectrum or hesitation at the other. I have to experiment further on this to determine exactly what is happening. I hate to say it, but I am not completely sold on the B-28 VAC for our unique application. None of the other C-2 years with their Holley's and different cams seem to be affected and most benefit/need the B-28. I love the AFB's, but they can be fussy, specially on our L-76 equipped 63's.

Stu Fox