Re: 912 detonation

[Kolb to Fox]

At full throttle on take-off there is a misfiring. My instructor says its a detonation. If I decrease the power setting even 200-400 RPM it clears up. I'm a newbe to the 912 and my fox, 4 hours. Drove a kolb FSII for the last ten years with my bulletproof 503. So this is all new.
Any ideas?

Thanks for all the info on this forum.

Kolbtofox

[DesertFox4]

Kolb to Fox, I've relocated your post with question to engines. Might get more visibility here.

[Dave S]

Kolbtofox,

It's pretty hard to diagnose a misfire over the internet; however, I'll throw out a few ideas & maybe that will help with your investigation.

On car engines, misfiring is not usually the first observed symptom for detonation - power loss and nasty metallic noises are.

For what it is worth - detonation can occur under certain circumstances WITHOUT any obvious observable symptoms and still wreck an engine.

1) Is your instructor a mechanic and is he familiar with the 912 series? I bring this up because an opinion that you have detonation going on, if it is not based on fact/knowledge, could misguide you away from the real problem. A rotax IS NOT a continental or lycoming and does not behave the same.

2) Which 912 do you have? 80 hp or 100 hp? This makes a difference since the 100 hp is a higher compression engine and requires a higher octane fuel. Running the 100hp on anything less than the recommended 91 octane (r+m/2) will successfully inspire detonation. The 80 HP, as I have been led to understand - should be fine with 87 octane. The 80 or 100 hp rotax will both be fine with 100 LL provided the oil & maintenance recommendations are followed.

3) If the fuel is the correct octane - is is fresh? Old gas can be a nightmare.

4) Full power misfires with max cylinder loading can be symptomatic of ignition problems - dirty and or worn plugs for instance (but can be other ignition things).

5) Full power misfires can also be a symptom of fuel starvation - inadequate fuel supply at higher flow rates can be caused by lots of things - not the least of which can be weak fuel pumps or fuel line restrictions resulting in an excessively lean mixture.

6) How is the prop pitched? Rotax recommends at least 5200 RPM at full power.

In any case, detonation/ignition/fuel supply or whatever is causing the misfire - keeping yourself in the gene pool is better supported by finding out what is wrong and getting it fixed before flying it again.

A rotax should hum smoothly & sweetly at full power.

Best of luck in figuring this out; and, if you can share what you find out it would be good others too. These things can be pretty challenging somettimes, but they do have a cause and can be corrected.

Sincerely,

Dave S
Kitfox Trigear
912ULS - Warp

[catz631]

I too doubt it is detonation if you are using fresh fuel of the proper octane . Give Lockwood a call over in Sebring Fl. They have very good techs that are most helpful.
Dick