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  #1  
Old 07-20-2019, 11:31 PM
extralite extralite is offline
 
Join Date: May 2014
Location: nsw, aust
Posts: 2
Default No.4 Cylinder Shuts Down at Idle

Hi all. I have an IO 360 (350 hrs) that started backfiring at idle when warm. I went through those threads. i saw a little fuel/oil on the inlet manifold of number 2 cylinder so took it for an engine run to see if i could see anything about that cylinder.

Once warm at idle it began back firing again. But after about 30 seconds the backfiring stopped and the EGT and CHT of number 4 cylinder dropped away and the backfire stopped. So at least i think i can isolate the backfiring to number 4 cylinder because it stopped when the cylinder stopped.

Any thoughts on the most likely problem? Blocked injector? Stuck valves?

Help appreciated.
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  #2  
Old 07-21-2019, 05:47 AM
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scrollF4 scrollF4 is offline
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Location: Flower Mound, TX
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Default

Possible stuck valve. Consider doing a compression check. I had the same, and when Randy Richmond checked the compression, #4 showed 0 psi.
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  #3  
Old 07-21-2019, 09:39 PM
simpkinsona simpkinsona is offline
 
Join Date: Dec 2010
Location: Vacaville, CA
Posts: 211
Default Vacuum leak

Sounds like a vacuum leak to me. Lean at idle causing missfire/backfire and then so lean it dies.

-Andy
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  #4  
Old 07-21-2019, 10:53 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,710
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Quote:
Originally Posted by extralite View Post

Once warm at idle it began back firing again. But after about 30 seconds the backfiring stopped and the EGT and CHT of number 4 cylinder dropped away and the backfire stopped. So at least i think i can isolate the backfiring to number 4 cylinder because it stopped when the cylinder stopped.

Any thoughts on the most likely problem? Blocked injector? Stuck valves?

Help appreciated.
Look for an ignition problem on #4. The backfire is likely not a backfire, but an afterfire. When a plug intermittently doesn't fire, the raw air fuel mixture gets dumped into the exh pipe during the exhaust stroke. When that cylinder fires again, the hot exh gas lights off that raw mixture sitting in the pipe, creating the explosion you are calling a backfire (when the explosion is in the intake system and uncommon without valve timing or ignition timing issues as well as a variety of issues during the start phase).

Many ignition issues are exacerbated by heat and sometimes only appear once warm. As you get warm, the intermittent ignition failure / misfire starts and you get the afterfires. Eventually the issue becomes persistent and the afterfires stop and the temps fall like a rock because the plug isn't firing at all.

can't have a blocked injector that only causes issues when hot. Also, you would see real issues at higher power levels. Symptoms don't mimic typical stuck valve, if only noticeable at idle. Symptoms are typically reversed - problem appears when cold and goes away when warmed up. Hence the term Morning sickness.

Larry
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Last edited by lr172 : 07-21-2019 at 11:10 PM.
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  #5  
Old 07-22-2019, 08:13 AM
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Carl Froehlich Carl Froehlich is online now
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 3,266
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Are you running Lightspeed ignitions?

Carl
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  #6  
Old 07-22-2019, 07:17 PM
N220LS N220LS is offline
 
Join Date: Mar 2006
Location: Valley Center, Kansas
Posts: 20
Default Similar Problem on 0-360

Last year we had a similar problem on our carb 0-360. Actually, we had 2 problems. First, after pulling that cylinder and taking it to a shop they found we had one of the inner valve spring broken. Second, someone forgot to lock the primer with a twist. The engine was sucking in fuel through the primer all the time it was running. These may not be your issue but are both fairly easy to check. Had we examined the valve springs with a good light we may have seen the broken spring.
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