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OS FS-91SII: clunking sound when cab passes the push rod?

Old 12-28-2017, 08:57 PM
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bluesky123
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Unhappy OS FS-91SII: clunking sound when cam passes the push rod?

I have two OS FS-91SII and one of them makes loud noise, after push rod reaches the top position, i.e. cam passes under the push rod. I've opened the cam cover and saw plenty of lubrication there. The bearing in the cover seems ok (I haven't completely disassembled the engine and can't see the bearing on the opposite side of camshaft).
What can cause this clunking, axial movement of the camshaft?

I have never run 4-stroke engines, only 2-stroke ones, and this puzzles me...

Thank you for your advise,
Boris

Last edited by bluesky123; 12-28-2017 at 09:02 PM.
Old 12-29-2017, 04:18 AM
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flyinwalenda
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Are you hearing this noise while it's running or when rotating the prop by hand ?
4-stroke engines do make a different mechanical noise than 2-strokes .

A loud knocking when running could be pre-ignition caused by a too lean fuel mixture and/or loose valves.
Old 12-29-2017, 06:28 AM
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Loose valves would be my guess. A loud tappit sound is very typical on older VWs with air-cooled flat-4 engines. But like flyinwalenda said it depends on whether the sound is there when engine is running or not.
Old 12-31-2017, 01:48 PM
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bluesky123
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> Are you hearing this noise while it's running or when rotating the prop by hand ?
When rotating the shaft by hand, no how slowly.

> Loose valves would be my guess.
Seems unlikely: the sound happens when the valve is maximum open, just after the push rod starts receding from its top position (and the cam passes the cam follower of that push rod).

Some people suggested that excessive wear of cams or followers might cause this, so, I've disassembled the engine at took these pictures. All looks good to my inexperienced eye...

Thank you for all your suggestions!
Boris
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Old 12-31-2017, 04:32 PM
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bluesky123
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Good news: after I re-assembled the engine, the clunking disappeared. Must have been sticking cam followers.

Thank you all for your help!

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