When it was new, our 912S started as prescribed by Rotax when cold, i.e.,
full choke with throttle fully closed. Opening the throttle even slightly
would kill the engine, whenever the choke was applied. Over time (400
hours), it began to need just a bit of throttle along with the choke, in
order to start. Now it runs very roughly on a cold start with full choke,
and will die after a few seconds of this (if we can stand it) unless the
throttle is opened more than 1/4 inch. It then runs perfectly smoothly,
---From idle to full throttle, without the choke. Unfortunately, opening the
throttle this much causes the shaking to become so violent (momentarily,
until it becomes suddenly smooth) that it typically dislodges the muffler
---From exhaust header pipes. It starts just fine without the choke, once it
is warm.
Please don't bother to suggest balancing the carburetors, which we have done
very precisely (witness the very smooth running without the choke). We have
partially disassembled the carburetors and cleaned all of the jets with carb
cleaner (though there was no sign of any contamination or blockage). Mag
drop on both sides is fine, compression is like new, and we have never
experienced any kickback on starting. We have searched in vain for any
vacuum leaks, and the float needles seem to be keeping the float bowls
filled properly.
We feel that we have exhausted all of the reasonable possibilities, and so
are open to bizarre and unlikely suggestions as well at this point!
Dave DeFord
N135TD (monowheel XS, 912S, Airmaster)
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