I have a Saito 91 on a Top Flight C182, did a test flight and about 7min into the flight the engine quit. Did a forced landing into the crop, minor damega to the plane, but nothing bad.
Checked the engine for dirt, found none and started it up. It ran backwards for a few seconds and stopped. At that point, when turning the prop by hand...no compression at all. checked glow plug, good...no signs of front bearing failure, no slop or grinding anywhwere. Put glow plug back in and..compression! started and ran fine, stopped and.. no compression.?? Compression slowly came back and has been good ever since.
Any ideas?
Dan

[marcham]

Overheated ???

[Cougar429]

Sounds like a sticky valve or ring.

Glad you got it back. I overheated mine last weekend trying to cowl in into a Skybolt. Back up and running, but not happy.

[VA6WGO]

I had a Saito 91 few years ago and had a similar problem once.

The little bolts where you ajust the valve's gap were completly loose. Check your valves ajustment bolts and gap.

Cheers,

Will

[Max]

+1 on the inspecting the valve clearances or obstuctions (carbon fouling) that may keep the valve from making a complete seal.


You could have a worse problem thogh, especially if your engine has overheated.
Take the valve head off and inspect the valve spring. It may be broken or weak.


Overheated 4 strokes can get hot enough to anneal the valve spring, in effect destroying its spring strength. So even though the spring looks like it is in one piece, it has no "spring" in it to close the valve.

This usually happens to the exhaust valve, because it is the one that is subjected to the hotter gasses venting from the overheated engine.

If unsure, remove the spring and see how much compression it can take. You may have to take out the other spring from the intake valve in order to make a direct comparison.

[AJCoholic]

Originally Posted by Scale Freak

I have a Saito 91 on a Top Flight C182, did a test flight and about 7min into the flight the engine quit. Did a forced landing into the crop, minor damega to the plane, but nothing bad.
Checked the engine for dirt, found none and started it up. It ran backwards for a few seconds and stopped. At that point, when turning the prop by hand...no compression at all. checked glow plug, good...no signs of front bearing failure, no slop or grinding anywhwere. Put glow plug back in and..compression! started and ran fine, stopped and.. no compression.?? Compression slowly came back and has been good ever since.
Any ideas?
Dan

It can only be one of two things...

#1 - a valve had some foreign material under it preventing it from closing completely. If compression came back and went away again, the clearances are OK. I have has a small chip of something sit under a valve and eventualy get washed away.

#2 - a ring is stuck in the groove, and worked itself loose.

The only way to know 100% is a dissassembly and inspection. But it sounds like #1 to me.

AJC

[AJCoholic]

Originally Posted by Max

You could have a worse problem thogh, especially if your engine has overheated.
Take the valve head off and inspect the valve spring. It may be broken or weak.


Overheated 4 strokes can get hot enough to anneal the valve spring, in effect destroying its spring strength. So even though the spring looks like it is in one piece, it has no "spring" in it to close the valve.

I have seen some very abused and overheated engines - never have I seen a spring loose that much springiness. Not saying it cant happen in a worse case scenario but I would say very unlikely. Especially with what was described. If it is a spring, you wont get it to fix itself.

AJC

[Guest]

i good way to test if its the valves is to take the carb and exhaust off and put your finger over both holes.now turn the engine over and if you get a suction out of the exhaust as your turning then you have some junk in the valve stoping it from seating.i had this happen to two of my engines.pilled the valve and cleaned her up.

Thanks for the replies! Will take a look at it this comming week.
Dan

[Max]

Originally Posted by AJCoholic

I have seen some very abused and overheated engines - never have I seen a spring loose that much springiness. Not saying it cant happen in a worse case scenario but I would say very unlikely. Especially with what was described. If it is a spring, you wont get it to fix itself.

AJC

Hey Andrew.

I mentioned it becuase it did happen to me on my Saito. It turns out the fuel I had was a bad batch and it caused my engine to run so hot that the spring softened. After that happened, I didn't clue into why my engine was running so poorly until I went to check my valve clearance for the N'th time and discovered the force to activate the vlave with my thumb was much higher on the intake spring when compared to the exhaust spring.


I also had an OS91 that had a spring break just as I was taxiing my float plane back to the beach from a flight. It happened "just like that", but finding the cause (broken spring) only took a second once I removed the valve covers.