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Rough Engine with Conical Mounts

Ken Hoshowski, Salmon Arm, BC
RV-6 C-FKEH

I was the first one in our area to start building an RV (serial 20332—1988), the next closest being Sid Cope in Kamloops and Kurt Kaminski in Kelowna.   I believe they both had engines using dynafocal mounts.  When it came time to install my engine, I called our closest supplier and ordered conical mounts for an O-320 B3B engine.  I got some help in installing the engine from a local aircraft engineer.  The engine was installed and ran up fine on the ground.  I was never happy, though, with a roughness that appeared in cruise when I reduced power and reduced prop RPM (constant speed), and on final, when reducing power and going through 85 mph.  As soon as I had the runway made and I chopped the throttle the roughness disappeared.  There were no indications of roughness while on the ground while doing run ups or taxiing.  Full power at takeoff was nice and smooth.  On climb out—once I knew I had no usable runway ahead of me—I reduced power to 25” Sq. and this was also nice and smooth.  Once I reached cruise altitude and reduced prop RPM’s to 2400, and lower, the roughness started and got worse as RPM reduced.

I talked to number of people about this and did not resolve the problem.  I finally went to an old-time mechanic and we checked the differential, plugs, harness, mags, push rods, and timing, and changed one hydraulic lifter.  The engine seemed to check out.  We phoned Hartzell to see if one blade on the constant speed could be taking a bigger bite than the other.  At this point all things pointed to the prop.  Hartzell said to try flipping the prop 180 degrees, which we did.  I think it made a very small change.  A very slight roughness at 1200 RPM (ground run )seemed to disappear, but was it just my imagination?

Everyone looking at my engine rubber mounts said they looked okay.  Murphy’s Law says the last thing you check is usually the culprit.  I am on the RV list (internet) and asked the list (about 800 strong) if any one else had this problem.  Mike Seager and Mike Todd responded with comments about the rubber mounts.  I heeded their advice and ordered Lord J6230-1 engine mounts ($565 CDN).  Took me a couple of part days to install the mounts (torque bolts to 350 -450 inch lbs).  These mounts come with a metal sleeve between the rubbers, which align—with help—when the bolt goes through.  It must be tight enough to bottom out between the two end washers.  My original mounts were far too soft.  The new mounts were so hard I had trouble believing they would actually work,  but  they did.  What a difference!
My thanks to Mike and Todd for their help.  Now that I know what the problem was I know my flying hours will increase dramatically.


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