Engine basics > Four strokes only

Stuck exhaust valve

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Robert Zambelli:
Dennis is right on.
When flying RC, people will often put a plane into a fast vertical dive.
The prop can windmill causing the engine to over-rev, allowing the valves to float.
At that point, there will be interference and the piston can hit a valve, bending it.
A long time ago, I bought a used SAITO 40 with a stuck valve.
After dissassembling it, I found a small crescent-shaped mark on the piston crown and a bent valve.
Fortunately, the valve guide was not damaged and installing a new valve solved the problem.
The engine ran perfectly.
When you reassemble, you need a special tool to properly set the cam/crankshaft relationship.
The tool is very easy to make if you have a small lathe.
Let me know if I can help.

Bob Z.

spare_parts:
Bob, I would guess the valve was stuck when someone tried to turn it over, or it was mis-timed previously. Few engines are interference by design. The RPM at which the valves float in RC engines is well above what you might observe in a dive. Doppler has an impact on what you hear. I've never seen or heard of an engine stopping due to a bent valve in flight.

Not my video, but if this didn't float and bend a valve at 15,500, what kind of RPM would it need? The FS-52S and old FA-40S was run out to 17k. I have no comment on the power figures.



Stuck valves are usually varnished. I strip engines like this unless it's obvious the crankcase isn't gummed up just as bad. The last thing you want is a plugged up big end oil feed hoping it clears up before failure.

Greg

Robert Zambelli:
Hi, Greg - good points.
The engine I was referring to was not the engine in the video but a very old SAITO 40.
I spent considerable time with one of the engine technicians at Horizon Hobby, the SAITO importers at the time, probably more than 20 years ago.
They also suggested that the issue might be due to over-revving, as they had a number of engines returned with similar issues.
Depending on the severity of the float, the momentum of the moving valve could carry it past the point of interference.
Regarding improper valve timing, that could well be the case. I just dissassembled the engine without checking.
In any case, with quite a bit of four cycle experience, I have never had a stuck valve due to varnish.
I have used straight castor, 50/50 castor-synthetic and straight synthetic.

Bob Z.

Dennis Toth:
Robert,
I saw an old Windy video when he started working with the four strokes and there was a short discussion about how the exhaust valves could stick if the engine was run with a lot of castor. They indicated that you need to check the valve stem for baked on castor and carefully polish it off with something like a Scotch brite like getting the baked on castor off a Fox head.

Best,    DennisT

Robert Zambelli:
 Hi, Dennis.
You are absolutely right.
Straight castor and even 50% castor will cause serious carbon accumulations on the exhaust valve.

My recommendation is 20% total oil, and no more than 5% of that castor oil.
Many very successful 4-cycle flyers go with 20% full synthetic and 20% nitro.

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