Hello.

My 160 gemini starts and runs well. 18x8 prop 6500rpm, idle 2000. Thrust +6kg. But the engine is really easy to roll, VERY low compression in both cylinders. My friend's FS-91 is about 5 times harder to spin. Is this normal?

My OS four stroke engines have excellent to very good cold compression. Many people say that ringed engines have poor compression when turned over by hand. I don't believe that to be true for a good engine in good condition.

When turning the engine over listen to the exhaust pipes for leakage. This would indicate exhaust valve leak. Listen to the intake for intake valve leakage. Verify the tappet clearance is correct. If there is no leakage of the valves the only thing left is the ring. It could be stuck with old castor or just worn with a large ring gap. The piston could also be worn not fitting the bore properly, thus not allowing the ring to seal well. Also the cylinder could be scored. Inspect the cylinder visually.

A little leakage won't really reduce performance. If you can't start it by hand I would would do repairs. On the other hand you could just run it until running becomes an issue.

Ditto.
Careful running in with the proper valve lash should provide excellent compression. If not, a ring replacement or valve grinding job is required.
First of all, get that valve clearance right. With both rocker arms moving in one, you should FEEL some play in the arms of the other cylinder.

Henep,

hey, i have 2 of the FT1.60's and both are identical. serial numbers are 19 numbers apart. both have great compression, but they need to be flip briskly to feel the compression whereas my OS 91's have hard compression even when pulled through lazily.

i've only run one of my 1.60's so far, and broke it in by running it rich and bringing it up to peak for a second or two, then back to rich for 20-30 seconds and then back to peak and so on and so on for about 5-6 minutes, then i let it cool down completely before running again. the engine has great compression, but no more than when i started.

the 91's however became tighter as they broke in.

try fliiping the prop through and see how it feels to you. pulling through slowly will not give much resistence as you say.

Hello!

Both intake valves leaking. Clearances ok. Next job is valve grinding. Thanks for good advice!

Be carefull! Many model engines, especially the older ones leak by the valves. When runiing the castor oil seals this and the leak is not relavent. If it is running fine and producing the power it should leave it alone. You may gind and grind and never get the leak stopped and only end up with a sunken valve face that can not be sealed by the castor oil. Please see Bill Robinson's comments in the Saito Club regarding this topic.

I'll have to agree. I should have been more clear. Do not lap the valves unless you see that they need to be, maybe the seat or valve face shows a sign of damage.

This is a normal feeling for these motors. I currently have one in a 98" Mono Coup, and if the motor is turned over by hand it feels very soft on the compression. It has been this was since new but pulls like a mule and has never given one problem. I can't spin a 18x8 with mine, so I'd say your doing ok....

nah.......you guys are all wrong................."if it ain't broke, then by all means, tear it down and fix it"..........................

we had a guy in our Free Flight Gas club that insisted on reworking any engine that came into his possesion to the point where he'd increase nitro percentage to gain back what he reworked out of the motor..........1 ratail file, rubbing compound and a dremel was all he needed.

Mother's mag wheel polish will seat valves safely. Only takes a few minutes and a light touch. This is the recommendation for Enya heads from Tim at MRC. Works perfectly.