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Old 05-09-2011, 02:04 AM
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New built 385 SBC Dieseling

I've got a 385 SBC I built at the local community college.. It's a Scat bottom end with Probe SRS flat tops. Dart Iron Eagle 230cc's ported topped with an air gap and Holley 850 dp. Cam is a Comp Cams 12-771 with 977 springs and 818 solid rollers. Everything is new top to bottom. I rebuilt the carb and top end is all new.. Engine did 489 on the dyno and air fuels were jetted to perfection.. It liked 38* total with a mech probillit MSD, 6al and blaster 3. It's got a 4.56 in the rear soon to be a 3.90 and I'm guessing a 3500 stall.. It runs great..

cam card
http://www.compcams.com/Company/CC/c...ookieSupport=1

My plan is to take some timing out of it and get down and dirty tuning the carb.. Any ideas on why I'm getting runon when I shut it down? I'm guessing it's the carb since everything is new and there's no carbon build up.. I run 92/93 octane on the street..

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Old 05-09-2011, 02:08 AM
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Oh I've not changed the springs in the Dist and Idle is just under a grand in drive..
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Old 05-09-2011, 02:16 AM
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Sounds like a classic case of too much compression not enough octane.
What size chambers? What head gaskets? How far down in the hole are the pistons at TDC? you have checked the distributor to make sure the mechanical advance is fully returning to it`s base position?
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Old 05-09-2011, 02:35 AM
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The pistons were not out of the hole but right there at the top.. When I decked the block, it measured 9.08 I believe.. 6 inch rods.. The combustion chambers were decked a second time (from blowing up the first 385 I built) and IIRC they were 62 63cc. When I rebuilt the carb I set the idle screws at 2.5 out per the holley book. I pulled a plug and it looked a tiny bit lean. Maybe I'll toss some bigger jets in there.
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Old 05-09-2011, 05:51 AM
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Run on is ALMOST always caused or complicated by too high of idle speed.
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Old 05-09-2011, 07:48 AM
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vacuum leak maybe? did you port the heads? you may have ported too much and the intake runner is leaking in from the lifter valley. happened to me once.
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Old 05-09-2011, 09:23 AM
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Run on, after the key is turned off is a sign of too much fuel in the chamber at idle (too rich).

This it typically caused by the carb throttle blades open too much at idle (blades past the idle transfer slots).

The cure is more ignition timing at idle which rises the idle rpm, which in turn allows the carb idle speed to be turned down to close the blades down. And more timing allows for a leaner idle mixture (idle mixture screw adjustment).

This is especially true if you are running a big cam, 25 or 30 degrees of ignition timing at idle would be good. This means you need to limit the amount of total timing in the distributor to get 36 degrees total. Do you have a stock distributor or one that can be adjusted?

You don't have a stock engine anymore, so stock ignition timing isn't going to work.
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Old 05-09-2011, 01:28 PM
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Like I said up top I've got a Probillet mech advance MSD with the 6al and blaster 3.. So I need more initial timing? I was gonna dumb down the timing to fight denotation and bring the combustion temps down.. I think I need to bring the idle screw down and find another way to get my idle up around a grand. I'm under the assumption that I need a bit higher idle due to my solid roller set up for cam oiling. I've not messed with the idle mix on the carb either so that could be another way to get more rev with less blade. I'll mess with it when I get done working and report back.
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Old 05-09-2011, 05:15 PM
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Timing doesn't matter when it's dieseling. It diesels, from my experience, cause the combustion chamber gets a hot point and ignites the fuel automatically. Deburring the combustion chamber with a dremel helps.

What was the temp of the motor? Could be running too hot.
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Old 05-09-2011, 05:39 PM
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It's been about 170* 180*
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Old 05-09-2011, 06:48 PM
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Quote:
Originally Posted by hotrod937
It's been about 170* 180*
That's a bit high, secondly bring in cool outside air if possible. Also 'till you get it sorted out you might want to ease out the clutch or shut it off in gear to prevent burning carb parts and making your manifold dirty. Try a carb spacer.
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Old 05-09-2011, 07:04 PM
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I just had a little bit of the same problem, def more advance timing helps and i been trying to figure out my electric choke and idle all those will factor into it. I would eliminate one variable at a time, turn that dist a little and see if that helps, if not check for other problems.

I also read a guy had an e fan wire directly into the ignition without a relay and he said that was causing some dieseling, once he put a relay in it stopped, not sure if you are running e fan or not. just another variable.
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Old 05-09-2011, 07:24 PM
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it is your 6AL box, did you install the diode that come with it in the alternator wire? if not it will cause run on.
http://www.msdignition.com/instructi...pdf?terms=6421
scroll down to page 8
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Old 05-09-2011, 07:51 PM
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how much initial timing are you using now?

a cam the size you are using will need at least 25 degrees at idle.

set the advance to 5 degrees in the distributor which will be 10 degrees on the crank (1:2 ratio).

This is what I think is happening in your case.
You don't have the timing up high enough, so the idle screw needs to be turned out too much to get the idle up. This puts the throttle blades past the idle circuit which makes the carb run way too rich at idle. When you turn off the engine, there is too much fuel in the chamber so it runs on.

You can test this by turning up the timing to 25-30 degrees at idle, then turn down the idle speed to 900 rpm, then adjust the idle mixture screws to get the highest idle (highest vacuum). Then shut it off.
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Old 05-09-2011, 08:14 PM
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read this post.

http://www.jalopyjournal.com/forum/s...d.php?t=280230

the guy had a dieseling issue, changed the timing from 6 to 16 and it went away. (solved on page 2)

he is running a lot smaller cam than you with too high cr, but higher timing still allows for less throttle blade opening which will stop run on.

On the post, there is also a few recommendations to drill the front throttle blades which also works, but is not for everyone. I have drilled my front blades in the past and that is a good fix for a small carb on a big engine.
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