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Carb Ice?....Nope.......

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DaveG

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Joined
Mar 27, 2007
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The airplane:
Experimental Cub configured like a PA-18-95.
O-200A (That had spent the last 1200 hours in my Varieze, getting the bejeezus run out of it…. Still ran fine when removed…)
Mac 75-38 prop
1 mag and 1 Lightspeed EI

So….
About 20 hours ago, I started to notice subtle changes in RPM during cruise. Just enough to change the sound and get my attention. 25-50 rpm or so. Only in cruise flight.

All other operations were perfectly normal.

It continued to get worse as the hours built up. Eventually getting to the point where the engine would get noticeably rough. Not like it was dropping a cylinder, more like it was dropping 1 plug in one cylinder. I could pull the Carb heat on and it would stop missing and resume smooth operation….for a while. So I thought... aha… carb ice. (The weather was a little cool and damp so that made sense).

It continued to gradually get worse over the course of 10-15 hours…. So I started looking for possible ignition problems……

New plugs, new wires, new EI coils….Nothing made any difference and to make matters worse it wouldn’t do it every time I flew it…..And I could usually find a spot where it would run smoothly.

One day I had a really bad mag drop so I replaced the old Slick 4000 mag….Next flight, no miss… so I thought I’d found it….Then it did it again a couple flights later.

It gradually got worse over time…. Until, any time I pulled the power back it would start missing.
This was getting really annoying…..

I found that if I changed ANYTHING, it would stop missing. Pull the carb heat….smooth……run on either the mag alone or the EI alone….smooth….lean out the mixture…smooth. Back to normal….Missing.

Now I’m REALLY grasping at straws……

I thought ….maybe I have a sticking valve….. I’d had valves stick in the past, but that always happened right after starting on the ground….

So I Googled “Continental sticking valves” and started reading….

I ran across a post on the old rec.aviation list, where a lady with a 172 with an O-300, described MY symptoms EXACTLY! ! Right down to thinking it was carb ice and all. Hers degraded to the point where her 172 would start to miss on climb out and she could just change speed and it would stop.
After chasing it for over a year, she found an old timer that suggested that, maybe, she had a bad VALVE SPRING.
He did, in fact, find a bad spring, and installed a new set.
The lady said her 172 was transformed! It ran better then it had in quite a while AND the funny miss was gone.

So… I go to the airport and pull all the valve covers and rocker arms off and try to push each valve down with my thumbs…3 of 4 exhaust valves are noticeably softer the the rest of the valves.

So I buy a new set of springs from Pacific Continental and set about doing the old rope trick on each cylinder, and replacing all the valve springs. The 3 soft springs were clearly collapsed.

Reassembled last night, test run and leak checked… The engine ran real good and just “sounded” way better. Test flew it this afternoon, and it ran GREAT! I think it’s fixed…..

I was flying 1 to 2 hours a day when this started showing up….I could see where someone that flew less often could easily chock this up to carb ice, for quite awhile, until it got bad enough to force a real hard look.

One for the archive….

Dave Ganzer
NX118DG
_________________
DaveG
Exp Cub N118DG
 

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