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post #1 of (permalink) Old 09-04-2002, 08:56 PM Thread Starter
 
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Dieseling-hard starting when hot-and carb tuning!

I have a 1981 scrambler with a 1984 I6 258 engine. The engine was rebuilt one year ago and a Crane RV cam was used. I run a Holley 2300 2 barrel 350 cfm carb with stock intake and exhaust manifolds. The carb has size 61 stock jets and a non-stock size 5 power valve.

I think the engine is dieseling. It doesn't happen all the time, but when it does happen the engine is usually hot (long periods of stop and go traffic, not highway). The run on has never lasted more than 2-3 seconds.

Sometimes, if I try to restart the engine after it diesels, it is hard to start and white smoke pours out of the tailpipe.

I think I am running rich based on exhaust smells and carbon covered plugs. What's the best way to tune the carb with a vacuum gauge? Do I use ported or manifold vacuum?

I have 17 mmHg of vacuum at idle and 15mmHg in drive. I cruise from 20-5 mmHg range.

If I fix the dieseling problem, do I have to worry about the carbon build up?

Sorry about all the questions, but any responses will be greatly appreciated!
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post #2 of (permalink) Old 09-04-2002, 09:58 PM
 
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Re: Dieseling-hard starting when hot-and carb tuning!

I think the engine is dieseling.
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Ummm... no offense, but it should be pretty obvious if it is dieseling. If it doesn't drop straight off and quit altogether as soon as you turn off the ignition, it's dieseling. [img]images/icons/tongue.gif[/img]
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It doesn't happen all the time, but when it does happen the engine is usually hot (long periods of stop and go traffic, not highway). The run on has never lasted more than 2-3 seconds.
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(To me) this sounds like typical behavior for an engine with either carbon buildup or pitted piston surfaces from detonation. Either way, more heat is getting trapped on top of the piston than can be pulled out by the cooling system. This latent heat is either causing glowing embers (in the case of carbon buildup) or getting trapped in the pits and miniature crevices (in the case of pitting from detonation) and subsequently acts as a replacement spark plug causing the engine to still fire.
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Sometimes, if I try to restart the engine after it diesels, it is hard to start and white smoke pours out of the tailpipe.
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White smoke? Hmmm... I always thought that white smoke was water, black was fuel (rich), and grey/blue was oil. Nothing else you have mentioned sounds like water getting in there though. (scratches head...) Sure hope you don't have head gasket issues. Has it ever been seriously overheated?
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I think I am running rich based on exhaust smells and carbon covered plugs.
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Those are two pretty reliable indicators.
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What's the best way to tune the carb with a vacuum gauge? Do I use ported or manifold vacuum?
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Manifold... although personally I am sold on using a digital volt meter hooked up to the O2 sensor. I was able to tune my Holley 390 really well with this technique and am convinced that I acheived much better results than with just a vacuum gauge and checking the plugs. I still used plug readings to verify things when I was done.
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I have 17 mmHg of vacuum at idle and 15mmHg in drive. I cruise from 20-5 mmHg range.
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Sounds good to me. [img]images/icons/smile.gif[/img] That's pretty much what I have.
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If I fix the dieseling problem, do I have to worry about the carbon build up?
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I would. I'd run some fuel system cleaner (take your pick - mine's Techron) through it every few tanks for a while. It will slowly soften up and eat away the carbon deposits.

Carbon deposits do all kinds of unpleasant things. The two biggest being absorbing fuel on the back side of intake valves (creating a hard start - but usually more when cold I think) and trapping heat on top of pistons (creating the glowing embers that can cause dieseling).
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Sorry about all the questions, but any responses will be greatly appreciated!
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Hope I was at least a little help. I'm not the most experienced mechanic here by any means. I just try to pass along what I think I've learned. Even if I'm not right, sometimes it triggers a thought for someone else that then leads to a solution.
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post #3 of (permalink) Old 09-06-2002, 02:31 PM Thread Starter
 
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Re: Dieseling-hard starting when hot-and carb tuning!

btt
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