Dieseling

[Michael H.]

Re: Dieseling

I agree with all of the above but I think it should be known that because you have this problem, that doesn't necessarily mean there is something wrong with your car. (other than the too high idle speed)
These cars did this when they were brand new and good fuel was available. Most of the owners (including me) used the method mentioned above to insure that the engine would stop. It only takes a slight drag with the clutch to prevent "run on".
When an engine does run on at shut down, it usually acts just as you described. Some difficulty restarting and a rough/rich idle for a few moments after a restart.

[Duke W.]

Re: Dieseling

Joel;

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

My '63 340 HP had chronic idle problems - stablity, loaded up, lost revs and died, in the first several years I owned it. Then as a 20 year old sophomore ME undergrad I figured out that a 15.5" VAC was an issue on an engine that only generated 12" idle vacuum.

I installed the ...236 VAC along with the '64-'65 SHP/FI weights and springs, which solved the idle problem, and the more aggressive centrifugal improved low end torque, which was sorely needed with a CR trans and 3.08 axle, and fuel economy improved to boot.

I never had any problem dialing in a solid, stable idle again. If the total idle timing isn't stable the engine will never achieve a stable idle. Thus, the "two-inch rule" to be sure the VAC stays locked at full advance at idle vacuum.

It's also best if the centrifugal starts above idle speed, but the above weights and springs start at about 800, and it's all in at 2350. Nevertheless, it would idle very nicely at 850-900 - just a bit "nervous", but rock solid stability within that range. I prefer a slightly high idle speed to minimize lope. I don't know how Chevrolet came up with their 700 RPM recommended idle speed for these engines - no way with the OE VAC, and even with the 236 700 it's way to low for reasonable idle quality.

Duke

[Joel T.]

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?

Nick;

Let me take a whack at this for you. Duke's Golden 2" rule says that the vacuum can on your distributor needs to achieve full advance at a level of vacuum (inches of Hg) which is two inches less than what the engine/cam produces at idle. So, for instance, if your measured idle vacuum is 10" Hg then then your vacuum can should achieve full advance at 8"Hg. This 2" gap ensures proper advance at idle with a margin of comfort.

It is important to know that not all vacuum cans are the same.... They are coded to let you know the vacuum level at which they achieve full advance. With this knowledge and your idle vacuum reading you can make sure you have the proper one installed.

My understanding is that when the 340 engine was introduced in 1963 GM had a mismatch in this space; i.e. the can did not achieve full advance at idle vacuum.. The result was an erratic/unstable idle. When I got my '63 back in 2005 I found it to be this way... drove me nuts until I learned of Duke's rule and figured out what was going on. I installed the B28 can and all has been well since.

Hope this helps..

Joel

[Stuart F.]

Re: Dieseling

Just as an after thought about the dieseling problem; I recall when my engine was new it was a lot more fussy, but it didn't actually diesel. Those that did (others I had known of) were advised that there was a possibility that the edge of the new head gaskets that were close to the combustion chamber take on a "hot knife" characteristic and act something like a glow plug. The same would be true for an over rich condition that would produce carbon build up quickly that too would glow hot. I don't know if that was an old wives tale, but thought I'd mention it any way being as how your engine is still fresh. Perhaps the more learned engine builders can comment on that - specifically; what is it that physically causes the dieseling.

About the 63 L-76 VAC situation, I believe Chevrolet tried to treat the L-76 as just a "minor" upgrade to the L-75, i.e. same carbs, same ignition, same pcv emissions stuff, etc. All that worked very well for the L-75, but with the Duntov in the equation, things changed a lot.

Stu Fox

[Duke W.]

Re: Dieseling

Nick;

Let me take a whack at this for you. Duke's Golden 2" rule says that the vacuum can on your distributor needs to achieve full advance at a level of vacuum (inches of Hg) which is two inches less than what the engine/cam produces at idle. So, for instance, if your measured idle vacuum is 10" Hg then then your vacuum can should achieve full advance at 8"Hg. This 2" gap ensures proper advance at idle with a margin of comfort.

It is important to know that not all vacuum cans are the same.... They are coded to let you know the vacuum level at which they achieve full advance. With this knowledge and your idle vacuum reading you can make sure you have the proper one installed.

My understanding is that when the 340 engine was introduced in 1963 GM had a mismatch in this space; i.e. the can did not achieve full advance at idle vacuum.. The result was an erratic/unstable idle. When I got my '63 back in 2005 I found it to be this way... drove me nuts until I learned of Duke's rule and figured out what was going on. I installed the B28 can and all has been well since.

Hope this helps..

Joel

A gold star for you, Joe! You stated the rule correctly. Also, you solved the idle stability problem using the rule.

I will add that for an auto trans the rule should be applied based on manifold vacuum with the engine idling in Drive and idling in neutral with a manual, and since idling in Drive typically shows less speed/vacuum than a manual idling in neutral, a more "aggressive" VAC might be needed, which is why I recommend a 12" VAC for a 300 HP/PG and 15" for a 300 HP/manual.

Isn't it nice when you can solve a vexing problem for ten bucks and an hour of labor.

Duke

[Richard F.]

Re: Dieseling

Can the vacuum canister be replaced on a TI distributor without taking the whole thing apart?

[Duke W.]

Re: Dieseling

Some say it can, but I prefer removing the distributor and disassembling it. That's also a good time to give everything a good inspection, fill the upper bushing grease well with fresh grease, and oil the felt washer.

You have to "tweak" the breaker plate/pole piece to get the VAC pin in the hole, and it's not worth taking the chance to damage them IMO.

Due

[Richard F.]

Re: Dieseling

As always, thanks much Duke

[Jim L.]

Re: Dieseling

..... the edge of the new head gaskets that were close to the combustion chamber take on a "hot knife" characteristic and act something like a glow plug. The same would be true for an over rich condition that would produce carbon build up quickly that too would glow hot.

An overly rich fuel mixture will definitely cause Dieseling. First hand experience with this one.

A '63 came to me with, I'm not making this up, a Holley from a Winston Cup car. No one had taken the time to dial this carburetor in (it had 80s in both primaries and secondaries) and the result was rough running and Dieseling.

My first step was to treat the symptom by buying some Chevron fuel additive to clean the combustion chamber. This worked and bought me some time to arrive at a more suitable jetting combination.

Once I had the jetting leaned out, the Dieseling stopped.

Jim

[Duke W.]

Re: Dieseling

I don't know if that assembly can be non-destructively disassembled, and it appears they used a correct technique for the measurement by not including the retainer thickness in the two distances.

My Cosworth Vega has dual springs, with different installed heights due to "steps" on the retainer. Of course, it's not an assembly, so the springs have to be checked at specified heights for each, and the total rate is the sum of the two.

Duke