RC Fuels Nitromethane, Castor Oil, Synthetic, heli fuel, 4 stroke, etc...Fuel Q&A is here!

Fuel will not run engine.?????

Old 12-31-2003, 10:05 PM
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Default Fuel will not run engine.?????

I was told by several club members that to get great performance from a 91 4stroke to use 30% Heli fuel. After reading an article about Satio engines and they recommended the same thing. Well I bought a gallon of fuel, I used it first in a ST 90 2 st and performance was outstanding. Put it in my OS 91 4stroke, engine was running great at about half throttle when it stopped ( luckily on ground) and spun the prop off. Replaced prop and tried to restart engine. Only made feeble attempt at firing and running. Tore engine down expecting to find warped valve stem or broken springs. Everything was in top shape. Reassembled and tried it again with same results. I put the plane aside and more or less was ready to write the engine off. I pulled out another plane to get it ready for flight, fueled it with same fuel and it would not run, sputter at best, I removed fuel and put in 10% and engine ran perfect. Pulled out the 91 4stroke and fueled it with 10% engine runs great. Still in the 90 2 stroke the 30% heli fuel runs great ???
What am I missing, after 25 yr in this hobby I have never seen anything like this.
Old 12-31-2003, 10:56 PM
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Default RE: Fuel will not run engine.?????

Did you readjust your main needle and richen it up?

You will have to go out a good turn and sneak back up on the correct setting, and also readjust your low idle needle.
Old 12-31-2003, 11:13 PM
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Default RE: Fuel will not run engine.?????

What Warren said is correct. Nitromethane requires a different fuel/air ratio than does alcohol.
A four stroke backfiring and losing a prop is a sure sign of a too lean mixture.

By the way, I just bought my first Saito, a 91. Really nice motor; I had other 91s but I wanted the light weight of the Saito.

Old 01-01-2004, 11:56 PM
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Default RE: Fuel will not run engine.?????

Sounds like 30% is too much nitro for your OS .91. They have a bit higher compression ratio than a Saito. You may try a colder plug. May I suggest a K&B 4C plug? Colder on the top end but will still give a great idle.

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