Engine Run-On
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Tuned
Joined: Jul 2010
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From: West Fort Worth, TX
Engine Run-On
Need some help. My engine huffs and puffs a second or two after engine ignition is shut off. It even has a smoke puff back out carb. Any suggestions to look at to troubleshoot?
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Year make model engine series engine size stock or mods any recent work...….and correct engine forum.
Usually this is because of hot spots in the combustion chamber caused by poor design or carbon, and a throttle that does not close all the way. The engine continues to spin after you turn off the ignition. The hot spots ignite the air/fuel drawn in as the engine tries to spin down to stop.
Once we know what we're working with it can be run down further.
Usually this is because of hot spots in the combustion chamber caused by poor design or carbon, and a throttle that does not close all the way. The engine continues to spin after you turn off the ignition. The hot spots ignite the air/fuel drawn in as the engine tries to spin down to stop.
Once we know what we're working with it can be run down further.
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Tuned
Joined: Jul 2010
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From: West Fort Worth, TX
1985 year 351W
stock pistons
new after market cam (hydraulic roller)
new afr 185 heads 58 cc
new Holley ultra xp650 4150 double pumper
new long tube headers w/ 1 3/4” pri
only vacuum used off engine is brake booster and pcv
idle set to 800
new denso Iridium TT spark plugs gap at 35
new msd distro w/ 6L module
a/f mix set to 12.5 warm at idle
timing 35 adv wot (12 at idle)
new painless harness
did have 87 octane but change to 93
about 10-15 miles since assembly
stock pistons
new after market cam (hydraulic roller)
new afr 185 heads 58 cc
new Holley ultra xp650 4150 double pumper
new long tube headers w/ 1 3/4” pri
only vacuum used off engine is brake booster and pcv
idle set to 800
new denso Iridium TT spark plugs gap at 35
new msd distro w/ 6L module
a/f mix set to 12.5 warm at idle
timing 35 adv wot (12 at idle)
new painless harness
did have 87 octane but change to 93
about 10-15 miles since assembly
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Joined: Apr 2004
Posts: 34,519
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12.5 AFR seems rich, especially at idle. Are you idling off the idle circuits with the throttle blades closed down far enough?
The pistons old or new? If old, any carbon, nicks, etc?
What compression ratio did you end up with?
Light vehicle with short gears?
The pistons old or new? If old, any carbon, nicks, etc?
What compression ratio did you end up with?
Light vehicle with short gears?
Thread Starter
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Tuned
Joined: Jul 2010
Posts: 432
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From: West Fort Worth, TX
Not sure what u mean by idle circuits. Not sure I have those unless that is fancy talk for above idle. I got no circuit.
I moved the a/f mix up a bit to 13.5. I played with the mixture before and it didn’t seem to make a difference.
I looked down the carb and the butterflies r closed when off.
The pistons have a few thousand on them. I rebuilt the engine 1,500-2,000 miles ago. When I changed the heads this time I cleaned up the tops of the pistons so there wasn’t any carbon on the tops. No nicks.
I do not know my compression ratio.
I have not weighed the truck but I would say light. Maybe 3,200 lbs.
4:10 gears
I put 16oz of sea foam in the tank and 6oz in the oil. Ran it up to temp and rev’d it to 6k a couple times. Not Dieseling at the moment but I discovered that reving that high I have coolant coming out of the water pump to timing chain. Well that gives me something else to fix.
Once I take care of that leak I will put a few miles on it to see if the sea foam did the trick.
Also, I learned my spark plugs r in the cold side of the spectrum. 20s
I moved the a/f mix up a bit to 13.5. I played with the mixture before and it didn’t seem to make a difference.
I looked down the carb and the butterflies r closed when off.
The pistons have a few thousand on them. I rebuilt the engine 1,500-2,000 miles ago. When I changed the heads this time I cleaned up the tops of the pistons so there wasn’t any carbon on the tops. No nicks.
I do not know my compression ratio.
I have not weighed the truck but I would say light. Maybe 3,200 lbs.
4:10 gears
I put 16oz of sea foam in the tank and 6oz in the oil. Ran it up to temp and rev’d it to 6k a couple times. Not Dieseling at the moment but I discovered that reving that high I have coolant coming out of the water pump to timing chain. Well that gives me something else to fix.
Once I take care of that leak I will put a few miles on it to see if the sea foam did the trick.
Also, I learned my spark plugs r in the cold side of the spectrum. 20s
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Idle mixture, idle speed etc. Sounds like you have this info. Pages 4-6.
https://documents.holley.com/199r10565-1rev.pdf
https://documents.holley.com/199r10565-1rev.pdf
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From: south louisiana
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Tuned
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Hotshot
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From: south louisiana
Should idle at 600 rpm. 800's too much with a manual transmission. Close the idle adjustment screw, then just open it slightly and see if it'll idle there, if not open just a tad. Once it's idling, play with the mixture screws to get the best idle. Start with those open 1-1/2 turns. Once you get the mixture right, then you might need to fine tune the idle speed. What cam is in it ?
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Should idle at 600 rpm. 800's too much with a manual transmission. Close the idle adjustment screw, then just open it slightly and see if it'll idle there, if not open just a tad. Once it's idling, play with the mixture screws to get the best idle. Start with those open 1-1/2 turns. Once you get the mixture right, then you might need to fine tune the idle speed. What cam is in it ?
Hotshot
Joined: May 2003
Posts: 11,141
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From: south louisiana
So what's different about it that it doesn't have an idle adjustment screw ? Or Mixture screws ? I looked and they're still there. Not in the same location, but they're still there.
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Tuned
Joined: Jul 2010
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From: West Fort Worth, TX
I have only been adjusting the primary idle screws. I didn’t notice the secondary idle screws.
After reading the Holley document for idle bypass valve. I still don’t understand what that does that the idle screws don’t do. They both raise and lower idle.
After reading the Holley document for idle bypass valve. I still don’t understand what that does that the idle screws don’t do. They both raise and lower idle.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
Those are the mixture screws. Not to be used to set the idle speed. Once you set the idle speed, then you fine tune the mixture screws. My take is the idle bypass valve is to prevent people from opening the idle screw too much and exposing the transfer slots. But what they don't realize is, they don't tell you to set the initial advance timing with the distributor first before doing anything else. That has got everything to do with the idle speed as well. And how the carb responds to off idle response. If you don't have enough initial timing, then you'd need to open the idle screw so far that the transfer slots are exposed, thus rendering the mixture screw adjustment useless because the carb is then out of the idle circuit. I've never found the need to even use the secondary idle mixture screws. Maybe you'd need them on an engine with a huge cam. My 331 is pretty healthy as far as specs, but even it doesn't need anything added to the center carb's idle to get the job done. Both out board carb's screws are turned all the way in.