Another "Dieseling" question... - Team Camaro Tech
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post #1 of 13 (permalink) Old May 17th, 14, 09:41 AM Thread Starter
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Jeff
 
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Another "Dieseling" question...

Hi there everyone, I have a question regarding dieseling and would like a couple pointers. I have been searching like crazy on these forums trying to learn as much as I can about this phenomenon so let me first tell you about my setup and then what I plan to do to fix the issue and if possible I would like some of you guru's guidance on the matter.

First off, I have a 383 with a healthy cam (not sure of specs, bought it with the car), edlebrock aluminum heads, demon 650 dp carb, an MSD dizzy (small body) and blaster coil. Until very recently I had a turbo 350 trans in the car but switched to a TKO 600 (so awesome I can hardly stand it!). The car has had an occasional problem with run on when I had the auto trans but usually it shut right off without a problem, now that the manual is in there it is dieseling much much worse and my small brain is thinking the heavier flywheel (compared to the flexplate) is creating more inertia which is making a small problem into a bigger one. I think the problem has been there all along but is now really showing up bad due to the heavier flywheel. The car starts great and really rips, no complaints.

Here is what I think is the problem and what I plan to do:

1. Throttle blade is open too far at idle. I have idled the car down as far as I can and this is still a problem. So from what I understand the timing might need a bit more advance which should increase idle RPM which should then be slowed down again by further backing out the idle screw which would close the blades even more. I need to buy a timing light and see where I am at because I really don't know at the moment. Long story short I was not using any vacuum advance as the guy who "tuned" the car (backyard mechanic...) said this car "doesn't need any vacuum advance". I think that may be wrong and could be part of the problem... but who knows. I hooked the vac advance back up but the problem has persisted. I really need to time it and see where it is at... I know you guys are going to ask I'll be doing that this weekend when I get some time.

2. Plugs. I have heard that too hot of a plug will create an ignition source. I pulled a plug yesterday and it was pretty black, not carmel colored at all but fairly clean and carbon free. This would lead me to believe that I am running a bit rich and/or cold... opposite of what I would think would cause the problem... thoughts? The plugs are Autolite AR3933's which look to be rather midrange in regards to heat.

I know there are more factors that this but I have already written a novel and I am sure there will be questions that I havn't thought of. Sorry I don't have the timing numbers yet, something tells me that is going to be the key to this story. I know timing itself means nothing in regards to dieseling but having that idle screw in too far due to slow timing could explain it right?

Thank you so much for any help, this site is just excellent!!

68 SS, 383, TKO 600/3.73's
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post #2 of 13 (permalink) Old May 17th, 14, 10:55 AM
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Re: Another "Dieseling" question...

Read the "Timing 101" thread on this site. Look up Barry Grant's Demon Carb Initial Timing Chart and dial in the base timing to what the chart says for the duration cam you have. Hook up the vacuum advance, especially for a street driven car! Get your timing light and tune your engine. You'll fix it.
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post #3 of 13 (permalink) Old May 19th, 14, 10:46 AM Thread Starter
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Re: Another "Dieseling" question...

I have been reading as much as possible about timing, starting with the timing 101 thread you suggested. Thank you!

Quick update: the problem is definetely with the throttle plates. Just to test the theory, I bumped up the timing and then lowered the rpm back down with the idle screw, dieseling is completely gone. Can't leave it this way though because the engine is pinging under load (4th and 5th gears, pings when you get on it a bit) so that is clearly not good. I retarded the timing back to where I had it and played with the idle mixture a bit and got the problem mostly solved. I bought a timing light yesterday so I will have some actual numbers to report back.

Seems to me a little more vacuum advance would be perfect... then I could lower the timing a little more to prevent pinging but still have enough timing to raise the idle and keep the idle screw out of the equation. Problem is my distributor does not seem to have an adjustable vac advance... great.

More work to do... but getting closer.

68 SS, 383, TKO 600/3.73's
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post #4 of 13 (permalink) Old May 19th, 14, 11:50 AM
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Re: Another "Dieseling" question...

Jeff I don't recall if you said anything about your vacuum advance but if it's on a ported source (no vaccum at idle) move it to a manifold source (vacuum at idle) and some of your vacuum timing will come into play at idle allowing you to lower you mechanical advance yet have enough at idle to keep your throttle blades closed enough so you don't have run on (dieseling) when you turn off the key.

The only other answer is to re-curve your distributor so you have less in the mechanical curve which will allow you to run more initial.

...Dennis

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post #5 of 13 (permalink) Old May 19th, 14, 12:09 PM Thread Starter
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Re: Another "Dieseling" question...

Good point, I never mentioned which source I was using. I have a T running from the big manifold vacuum source on the back of the carb that runs to my vac can. The other two ports (towards the radiator on the passanger side) are plugged. How much should I expect the idle to come up when I plug in the vac advance anyway? It didn't seem like it did a whole lot going from being unplugged to plugged in.

I took a peek under the distributor cap and it looks like it is a electronic dizzy, I didn't see any springs or weights like I have seen posted on here a bunch of times..

68 SS, 383, TKO 600/3.73's
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post #6 of 13 (permalink) Old May 19th, 14, 12:44 PM Thread Starter
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Re: Another "Dieseling" question...

Nevermind on the e-curve comment... I just wasn't looking at the right spot. Looks like it is just a regular mechanical unit. I will verify with a timing light, but it really seems like the curve is coming on too soon, or is just too big (meaning it is getting too much total timing). The car starts great, idles well, and has great response but to get it to stop dieseling it needs too much initial timing for what the curve is at. I'll probably be taking some pics and asking questions regarding changing the curve soon enough...

Thank you all for your help thus far, I'd be lost without these great forums!

68 SS, 383, TKO 600/3.73's
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post #7 of 13 (permalink) Old May 19th, 14, 10:24 PM
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Re: Another "Dieseling" question...

Make sure,..

1) You're running vacuum advance to a manifold (full time) source (I think someone suggested this above). This will raise your idle speed 2-400 RPM. Now you can reduce the curb idle at the carb which closes the primaries, which should prevent fuel from being pulled from the uncovered transition slots.

2) Make sure you're running around 34-36 degrees of total mechanical timing (VA disconnected), all in at 3000 RPM. Let the initial timing fall where it falls, probably around 16 degrees.

3) If she's still dieseling, you can crack open the secondaries. There's an adjustment that will prevent them from fully closing. There are no transition slots to worry about on the secondary side. Again, by opening the secondaries a bit, your idle will increase and again, you'll can close the primaries which will cover the transition slots.

One of those will work 95% of the time. If it doesn't, you might have to drill some holes in the throttle blades.

Good luck!
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post #8 of 13 (permalink) Old May 20th, 14, 10:49 AM Thread Starter
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Re: Another "Dieseling" question...

Straight line, thank you for the list, I love things organized in lists!

1) Check. Running full manifold vac, lowered the idle screw after plugging in vac advance. Definetely helped, but wasnt enough to completely fix the dieseling.

2) That is this weeks project. I ordered an MSD advanced timing kit so I will have the springs and bushings necessary to get it right. I don't think I will change the springs, looks like it is already set up with the light silver spring on one weight, and the blue spring on the other which should mean the total timing will be all in by 2800 rpms or so according to MSD.

3) I actually did play with this a bit last weekend. I opened the secondaries just a tiny bit then lowered the idle back down using the primary idle screw. This pretty much eliminated the dieseling and I barely adjusted the secondaries. I am sure if I opened them a touch more and lowered the main idle again the run on would dissapear. Problem is that I have too much timing in it right now so I need to get that right before resorting to this trick.

So is it just me... or is it a real pain in the rear to get the bushing off of a pro-billet dist? It is just in a horribly awkward spot to reach when the dist is in the car. If I can't reach it I might just pull the dist out of the car to make it easier. I can see myself dropping the bushing/washer/locknut while trying to install it and follow that up with a lot of swear word usage...

68 SS, 383, TKO 600/3.73's

Last edited by drmeff; May 20th, 14 at 10:56 AM. Reason: Additional info
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post #9 of 13 (permalink) Old May 21st, 14, 10:38 AM Thread Starter
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Re: Another "Dieseling" question...

Quick update:

I hooked up the timing light and checked what the engine was currently timed at. Turns out it was at about 40-42 degrees total (vac disconnected and plugged) which explains the pinging. I set the timing to 36 degrees total (all in by 2800-3000 rpm) and this brought the initial down to about 12. I plugged the vac advance back in but still had to mess with the secondary idle screw (since I took out some initial) to get it to idle at about 650-750 rpms. The dieseling seems to be gone (primary throttle screw is all the way out now), it idles great, has excellent throttle response and doesn't ping anymore, or is at least MUCH better. I can still get a tiny (REALLY TINY) bit of ping if I lay into it in high gear which I shouldn't do anyway, I just did it to see what would happen. I think to really get it perfect I need a bigger stop bushing so I can run more initial timing but keep total to 34-36, this being a stroker perhaps 34 is what I should be shooting for.

Anyway, thank you all for the help, this was my first time ever using a timing light or messing with a distributor in any way and this site made me feel confident attempting the job. I know there are many things above my knowledge level but a little bit at a time I am continuing to learn and it really is making the ownership of a classic car that much more rewarding.

Thank you all!

68 SS, 383, TKO 600/3.73's
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post #10 of 13 (permalink) Old May 21st, 14, 02:16 PM
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Re: Another "Dieseling" question...

Quote:
3) I actually did play with this a bit last weekend. I opened the secondaries just a tiny bit then lowered the idle back down using the primary idle screw. This pretty much eliminated the dieseling and I barely adjusted the secondaries. I am sure if I opened them a touch more and lowered the main idle again the run on would dissapear. Problem is that I have too much timing in it right now so I need to get that right before resorting to this trick.
u know u have the sec right when the idle and mixture screws are 1 1/2 turns out +/- 1/2 turn at a stable idle advance between 8 and 12/ 14 degs

Quote:
I set the timing to 36 degrees total (all in by 2800-3000 rpm)
Modern fuel and chamber/ cam designs have a faster burn across the chamber so 32/34 will be closer to ball park... and have it all in around the 3000 / 3200 rpms
With such light springs the curve comes off too steep for petrol... all in at 2800 is a LPG/ propane engine curve.

My Spelling is not incorrect...it is creative

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post #11 of 13 (permalink) Old May 21st, 14, 03:46 PM Thread Starter
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Re: Another "Dieseling" question...

Interesting about the idle screws. I think they are out about 1 turn max, any more and the exhaust started smelling gassy and the idle quality would break up. Would opening the secondaries more make it so that I can back those out to 1 or 1.5 turns or should I just leave it as is?

I think you are right on the money with the 32/34 comment. If I were to lower my timing any more I would have a tough time getting it to idle. Maybe with a larger stop bushing I could get it to work. My kit gets here tomorrow, I'll experiment this weekend!

Thank you again!

68 SS, 383, TKO 600/3.73's
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post #12 of 13 (permalink) Old May 21st, 14, 04:48 PM
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Re: Another "Dieseling" question...

Limit the mechanical advance by using the copper bushing that comes in the mr gasket 247 spring kit. Use your weights, install the bushing and use one light and one medium spring, then us know.

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post #13 of 13 (permalink) Old May 21st, 14, 10:33 PM
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Re: Another "Dieseling" question...

Quote:
Would opening the secondaries more make it so that I can back those out to 1 or 1.5 turns or should I just leave it as is?
u need to adjust adjust the secondaries while the car is running.. how to in old posts search for 90 deg rivet...
its a matter of adjust the sec , then adjust the mixtures and idle speed.. It these are going the right way for the 1 1/2 turns in idle speed and mixture screws.. keep going, if not go the other way....only 1/4 turn on the sec adjust max at a time til u get down to about 1/16th on secondaries
Caution DO NOT have the secondaries fully closed against the bores.. u wil wear butterfly bushes , possibly bend them, and damage the carb bores
If u cant get to the 1/2 turn tolerance then u have the wrong carb for the job , or most likely a vaccuum leak somewhere
Think about it, the gap in the secondaries is a built in vaccuum leak...

Quote:
If I were to lower my timing any more I would have a tough time getting it to idle.
bigger the cam the more inefficient the engine is at low rpms...because the inlet and exhaust are open too long and the engine cant devaple compression....but this can be over come by incresaing the idle advance.
on non lean burn egr engine one uses manifold vaccuum on the VA.
The idle is made up of the intial +VA so the VA must be all in at the idle vaccuum..
to dial in the idle .....
dial in means to establish what the idle advance should be in therory...
dump the VA and block off...
now adjust the intial , secondaries, mixtures and idle speed so everything is in spec with the [B]lowest intial for a reasonably stable idle.. if want a little lumpy a little less advance.. but stable...
Now what u have in the intial is what u NEED for your idle advance, which will be made up of the VA on manifold vacuum, and fully actuate at the idle engie speed plus the intial.

The dial in establish the total.. no VA start with a total of about 36/ 37 degs, do 3 runs over a measured distance and time them
knock the total back a couple degs and repeat, till withing acouple degree the time will suddenly increase.. car slower..
What the total was before the time increased ,, is now your dialed in total (intial+cent)
Since u now know
the required idle.advance and your idle vacuum
The total advance
it just a matter of juggling the amount of degs in the VA and the amount of degs in the cent and keep the all up.. intial+_cent +VA under 40 /42 degs.

BUT to do it right u also need the AFRs in the ball park with a ball park timing ready set up.
carb idle circuits around mid 14s to mid 15s
WoT around the 11.5 to 12.5 no higher
And cruise circuit about the same as the idle

My Spelling is not incorrect...it is creative

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