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Question For Experts

D

David Bell

Well Known Member
Slight Popping At Idle:
Superior O-360-B1A2
Aspirated
832 hours since new
Popping at cold start, #3 egt 50% of #1,2,4. Cht flat at idle. In flight running temps all normal. Lower plugs dry and normal. Leak down compression check 77/80, no intake or exhaust leaking under pressure. Reamed exhaust valve guide. Swapped egt probe. Replaced all intake manifold gaskets and hoses. Runup to 1600 rpm all normal. Gapped and rotated all plugs to different cylinders. Mag check at idle. None of this corrected my problem.
 
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David Bell said:
Slight Popping At Idle:
Superior O-360-B1A2
Aspirated
832 hours since new
Popping at cold start, #3 egt 50% of #1,2,4. Cht flat at idle. In flight running temps all normal. Lower plugs dry and normal. Leak down compression check 77/80, no intake or exhaust leaking under pressure. Reamed exhaust valve guide. Swapped egt probe. Replaced all intake manifold gaskets and hoses. Runup to 1600 rpm all normal. Gapped and rotated all plugs to different cylinders. Mag check at idle. None of this corrected my problem.
Click to expand...
potential blockage in #3 V slot in spider.
 
Is your carb float bowl tight and not drawing air? The FAB has loosened many a carbs. Plane will seemingly run normal, pop / lean backfire when throttle retarded in flight and poor idle characteristics.
 
Intake tube leak at sump? Did you happen to wiggle the #3 intake tube when you changed gaskets/hoses to see if it needs to be re-swaged? Similar to D-Dubya's comment about the MP hose loose/cracked, it sounds like something might be causing a lean condition at low MP in that cylinder.
 
krw5927 said:
Intake tube leak at sump? Did you happen to wiggle the #3 intake tube when you changed gaskets/hoses to see if it needs to be re-swaged? Similar to D-Dubya's comment about the MP hose loose/cracked, it sounds like something might be causing a lean condition at low MP in that cylinder.
Click to expand...
+1

This behaves like a moderate intake leak, given all the other remediation completed. Intake leaks are most noticeable at idle and unrecognizable at full throttle in carb applications. The 50% egt, along with the exhaust pops at idle only would indicate that this leak is not small. Start looking at everything from the sump to the intake chamber, including head cracking in that area.

You said you replaced intake gaskets. However, It is easy to install an intake tube incorrectly. The lip on the tube MUST rest in the machined relief on the ring. This is sometimes an issue when the rubber tubes are stiff or still clamped on one end. If it gets offset on that relief, it cannot seal correctly. You can look from belw as you install, looking for equal spacing between the tube and the ring.

If you can’t find anything, tape your shop vac hose to the carb intake and st on blow not vacuum. Then use soapy water to find the leak. Turn fuel selecor to off first.
 
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krw5927 said:
Intake tube leak at sump? Did you happen to wiggle the #3 intake tube when you changed gaskets/hoses to see if it needs to be re-swaged? Similar to D-Dubya's comment about the MP hose loose/cracked, it sounds like something might be causing a lean condition at low MP in that cylinder.
Click to expand...
D-Dubya said:
Any chance your MAP hose coming off the back of #3 is cracked, loose, or leaking?
Click to expand...
Bingo! Soft hose under instrument panel to manifold sensor was separated.
 

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krw5927 said:
Oh man, this was a fun thing to envision!
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Hey, I've used lr172's shop vacuum/blower trick before--it works! I used some soapy water in a spray bottle and it blew bubbles where the leaks were. When I used to actually work for a living in industry, we used some stuff called "Snoop" (no relation to Snoop Dogg) that we would spray around natural gas fittings that would indicate leaks with tiny bubbles. Same principle, just more expensive.
 
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