Fresh 383 is Dieseling on shutdown? Help! - Team Camaro Tech
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post #1 of 17 (permalink) Old Oct 23rd, 02, 10:21 AM Thread Starter
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Ok, so here’s the problem. I’ve got the following fresh engine installed in my ‘69, which I just fired up for the first time this past weekend: 383 (stroked old-style 2pc rear main seal 350) with Eagle 5.7" rods, new stroker crank, and speed pro forged pistons. The heads are Trick Flow 23 degree, 2.02 intake/1.6 exhaust stainless valves, HD springs good to .540 lift. These heads and pistons = 9.6:1 compression ratio. The cam is a Crane powermax (119831 HR-284-2S-12 IG Hydraulic Roller Dual Pattern). The ignition is a Proform HEI distributor with the module removed and the magnetic pickup wired direct to a MSD-6A capacitive discharge ignition, per MSD instructions. Ignition wires are the gi-nourmous MSD "superconductor" type. The carb is an edelbrock 750 CFM vacuum secondary, the intake is a Performer RPM. Now the problem. The engine runs great, but is dieseling when the ignition is turned off. Not continuing to run-on smoothly, as in, the need for a diode on the alternator, but I have done that anyways as per MSD’s guidance. I also removed the bulb from the “GEN” light circuit for good measure. The timing is set to 6 degrees initial, no mods done to the Proform HEI as far as total advance. Plugs are Autolites (forget which, but as recommended by Trick Flow) gapped at .040” (I know I can open up gap w/the MSD..but would that do anything for the dieseling?)Tank is full of 91 octane with a can of octane booster. Edelbrock carb straight from the box, no adjustments. Fuel pressure (electric Holley Black, with BG 2-port regulator) is at a steady 7PSI at the regulator, and reading correct 13-14 at the pump. Idle on carb is set at around 800. I’m running out of things to check. The only thing I can think of, that I have yet to investigate, is wether or not a brief (less than a minute?) period of time, with fuel pressure at around 12 PSI, before the regulator was dialed in, could have damaged seats in the carb and is now causing an over-rich condition? But still, why the dieseling? There’s no carbon build-up in a new engine to get hot and stay hot to act as a source of pre-ignition…Hmmm…I haven’t yet pulled a plug wire to see if I still have spark occurring during the dieseling. Just thought of that! Anyone with any other ideas? Thanks! –Rob
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post #2 of 17 (permalink) Old Oct 23rd, 02, 10:58 AM
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Rob,

Run on is often caused by having the idle ckt improperly adjusted on the carb. The easiest way I can think to test this is to set the timing up from 6 btdc initial to about 12 btdc (this will raise the idle). Now bring the idle back down by adjusting the idle adjustment screw. My guess is now the engine will shut down without run on...

The only problem I forsee with leaving the engine setup this way if the problem is resolved is you just added 6 deg of advance to the equation. At idle you'll find it's good as the mill will actually run cooler. I don't know what you have built into that hei and 6 more degrees added to your total may be too much fro your stroker and it may ping. 383ci sbc's tend to like about 32 deg btdc total timing, not much more. With octane booster you may be ok. To run 91 pump gas and for optimal performance you may want a recurve on that hei...

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post #3 of 17 (permalink) Old Oct 23rd, 02, 11:03 AM
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Dieseling is usually caused by a lean idle mixture or a spark plug that's getting really hot. Make sure you don't have any vacuum leaks and recheck the idle mixture. Turn the idle down as far as you can when adjusting. If everything checks out you might want to try hooking the vacuum advance up to a timed port (only has vacuum when the throttle is opened) on the carb instead of a manifold vacuum port. If all else fails try running a cooler plug.

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post #4 of 17 (permalink) Old Oct 23rd, 02, 11:10 AM Thread Starter
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Thanks Guys. The vac advance can is currently connected to a timed port on edelbrock carb. I will try setting the initial timing to around 12 deg BTDC and then decreasing idle via the idle stop screw (not mixture screws) and report back tomorrow. I know that a recurve kit will be needed for this combo, and that will be done later on, after the engine is broken in a bit and the flogging of the motor begins...
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post #5 of 17 (permalink) Old Oct 23rd, 02, 11:25 AM
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I would also run the timing to full manifold vacuum. With an engine like that it will run better.

The other thing to watch out for is the Edelbrock carbs really don't like more than 5.5-6 psi at the carb. There have been numerous rich idle problems by people that were completely solved by lowering the fuel pressure to the above specs. This comes straight from Edelbrock.

Jody

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post #6 of 17 (permalink) Old Oct 23rd, 02, 12:03 PM Thread Starter
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Thanks Camco. I will re-set to 5 to 5.5 psi as well. Any reasoning behind why full manifold vac would be better than timed?
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post #7 of 17 (permalink) Old Oct 23rd, 02, 01:13 PM
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Call it an education from some guys who know a lot more than I do (Ignitionman for one). The extra vacuum at idle and cruise when under manifold vacuum makes the idle much cleaner, throttle response better, and definitely helps mileage. When you put your foot down it will pull out the vacuum advance which helps detonation resistance. You still have the initial plus centrifugal under full throttle which is all you need.

On a stock smog application it can help a little with emissions. On everything else, go with manifold vacuum. You will notice a difference.

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post #8 of 17 (permalink) Old Oct 23rd, 02, 05:44 PM
 
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Guys, I did the following when I got home: Re-adjusted fuel pressure to 5.5psi, set initial timing to 12 deg.BTDC with vacuum advance line removed from the advance can + plugged, (idle went high) then switched advance line to manifold vacuum port on carb, instead of timed port, re-set idle stop screw to get ~800 RPM (idling lower and smoother now). Turned key to off, and whaddaya know! She stops clean. Also runs cooler too, I can tell that already (no guages hooked up yet!) Did I mention to you guys that all of this experimentation to date was with open shorty headers? My spintech exhaust system arrives via UPS tomorrow, so a shakedown run on the street isn't far off! I can't thank you all enough, hope I can repay the favor sometime.
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post #9 of 17 (permalink) Old Oct 23rd, 02, 06:12 PM
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Excellent. It's always nice when someone who posts on a problem comes back to let everyone know the solution.

Jody

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post #10 of 17 (permalink) Old Oct 24th, 02, 05:39 AM
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I'm glad it was just that easy and must agree with Jody, it's great when folks post back with success stories...

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...Dennis
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post #11 of 17 (permalink) Old Oct 24th, 02, 08:59 AM
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I had a very similar problem when I rebuilt my engine. No matter how I changed my timing I would have dieseling. The problem was that when I put the car in gear, I would get a big drop in RPMs. So, if I turned the idle down, my car would nearly stall at a stoplight. I had installed a higher stall-speed torque converter, so I knew that wasn't it. I tested for a vaccum leak, and that wasn't it either. Finally I broke down and read the handbook for my Edelbrock Performer carburetor. What was happening was that when I put the car in gear, the non-stock vacuum levels were causing the off-idle system to screw up. After months of head scratching and Camaro Tech searching, I changed the step-up springs that control the off idle mixture and it ran so well I almost wept. I'm not very experienced with carbs, but it took five minutes. It was much faster off the line, too, because it had been bogging a bit at take-off. Both problems were solved by a set of $5 springs.

Anyway, it sounds like your problem is fixed, but FYI just in case.
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post #12 of 17 (permalink) Old Oct 24th, 02, 10:25 AM Thread Starter
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Luckily for me, I'm using a T56 and not an automatic, so it's a bit less complicated, as far as idle requirements...I added a link below to my Webshots photos.
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post #13 of 17 (permalink) Old Oct 24th, 02, 10:49 AM Thread Starter
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Let me try this again...having trouble adding a link to my webshots page
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post #14 of 17 (permalink) Old Oct 24th, 02, 10:52 AM Thread Starter
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Lets try it again:

[url=http://community.webshots.com/user/rswhite[/url]

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post #15 of 17 (permalink) Old Oct 24th, 02, 10:53 AM Thread Starter
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