65 HP engine issues

[dlafave]

On Jul 8, 2010, Dwayne.LaFave@k12.sd.us wrote:

I have had several mechanics trying to diagnose a problem with my 65HP that showed itself after not being run four months while the plane had some recovering done. The project began in Jan with me flying 300 miles to deliver the plane to have work done on it to repair some hail damage. Plane was annualled at the same time. I had never noticed any problem, but in the year since the last annual one of the cylinders had dropped to 30# so it was replaced. When all the work was done and the plane put back together in May, upon starting it between 1550-1800 RPM the engine runs very erratically, varying in RPM and popping. It starts just as good as it ever did and seems to run smoothly with full power at cruising RPM. In hope of alleviating the problem we have:
-retimed
-changed plugs
-torn down and cleaned the carb
-since another of the cylinders was in the low 60's and appeared to have the exhaust valve not closing, that cylinder was also replaced
-the mags check out OK

. I have pretty much always used mogas with an occasional shot of MMO (4 oz/10 gal). After changing the first cylinder I burned 25% LL to help with the break-in for 20 hours.

After all that and 25 hours of use, nothing we have done has changed what the engine does at that RPM range. There were absolutely NO SIGNS of any of this during the previous 300 hours I have owned/flown the plane. The first time it ever behaved this way is upon start-up after the work was completed.
Dwayne

[N86250]

A missing 1/8 pipe plug in a cylinder at the intake made an A65 act really weird. Only it was hard to start and only wanted to run at all at nearly full throttle. This is the plug that would be removed for the fuel injection or primer at the cylinder. jrh

[dlafave]

thx

[Roger Anderson]

Mine was, for the last few months, doing the same thing. I removed all the induction tubes and hoses and replaced carefully assuming an air leak. No change. Then had to remove a cylinder last week for an overhaul. While cowling was completely off, I gave the engine a real close inspection. I discovered one of the nuts on an intake elbow was missing. I could wiggle it enough to see the gasket move. Hopefully the now replaced nut will cure the popping and backfiring in the idle range. Sounds like you're getting an air leak somewhere.

[dlafave]

Turned out to be a leaking intake manifold gasket/missing nut on the bolt.
Dwayne