912UL Engine vibration on throttle back

by Kevin Stewart in 912 / 914 Technical Questions 12 years ago

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Re: 912UL Engine vibration on throttle back

by Kevin Stewart » 12 years ago

Kevin Stewart

5 184 0

James,

Thanks for your info. I have checked the floats and as Bill says the horizontal pins on my floats are level with the surface of the fluid.

The fuel level in the float bowl appears the same in both carbs.

Kevin

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Re: 912UL Engine vibration on throttle back

by mike hallam » 12 years ago

mike hallam

3 40 0

Kevin,
Thanks for the link to what I posted elsewhere two years ago so FWIW it's copied here :

QUOTE.
I searched the web for Rotax 912 UL fuel mix gen., having suffered a persistent engine roughness in spasms for two Summers in my F/Wing Rans S6-116 & which even the best Rotax Gurus couldn't say what to do.

After two Summer's step by step efforts the 80 h.p Rotax S6-116 rough running episodes may be 'history'.
My thanks to Rans forum members putting up with my long saga, but here's another "reason" to think about next time someone has a mysterious rough running Rotax 912 twin carb. engine.

Starting with the engine losing power in flight run and an ensuing successful rough field forced landing at the beginning of 2009, my sequential attempts to clear the trouble by following all the usually offered 'answers' were a failure.

It gave me so many rough running episodes, which were 'cured' I discovered by chance, by WOT for ten seconds & keeping >4,800 rpm, that eventually I was almost conditioned to accept them. Even so I couldn't rest because at WOT on take off it also gave spasms which I couldn't overcome being already flat out !

The effect was lack of trust of this particular engine & curtailed any thought of longer outings. Thus I never flew enough in one decently long outing to make a realistic full tank to full tank measured petrol consumption check. Several times already I had checked if it was the correct mixture - either rich or weak, inspecting a sparking plug [not always the same cylinder or side] which appeared to say the mixture was "O.K."

By mid Summer 2010 simply by ignoring the rough running patches & on a longer flight 17 L/h was used - far too much compared with the Rotax power/rpm/fuel graphs !

As all else I could possibly think of had been done by then, I had to accept the plugs lied, and it was verging on too rich to run smoothly in patches at mid throttle.
New needles and needle jets were purchased, I have to say they look no different from the removed ones and the engine after all hasn't more that ~300 hrs ?

However over the last couple of weeks there have been no more 'episodes' of roughness even in high humidity air and I can return to run continuously at lower rpm regimes [4,400] without its onset. Result on the last tankfull was encouraging under 13 l/h & cruising at nearly 90 mph is certainly the way to get places compared with the delicate 56 mph of my Rans S4 ! UNQUOTE.

mike hallam

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Re: 912UL Engine vibration on throttle back

by Kevin Stewart » 12 years ago

Kevin Stewart

5 184 0

Today I fitted a new pair of main needles and their corresponding jets (the ones with 2.72 stamped on them) and there was absolutely no difference :( .

It has been suggested to me that the Rotax engine runs rich between 2500 & 3000rpm and this is the point at which the carb switches from the idle circuit to the main jet. Reducing the idle mixture screw from 2 turns to 1 turn may help smooth this transition. It always used to be smooth so I am still hoping that I can return the engine to its former condition.

Is there a method of setting the float level without using the gauge described in the Rotax manual?

Kevin

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Re: 912UL Engine vibration on throttle back

by bill martin » 12 years ago

bill martin

10 757 0

Kevin,

You can adjust the float level by bending the center part of the float arm that contacts the float valve. This is a last resort however. First carefully remove the float bowls without spilling any fuel. Then remove the floats. The fuel level should be about 1/2' from the top of the bowl. If that level is correct, your float arms and valve are OK. If the level is too high, make sure the float valve is sealing properly and the floats are floating with the pins in the floats just touching the fuel surface. Chances of the float arms being bent are remote. They can wear however, and should be inspected where they pivot on the pin in the carb body.

Bill.

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Re: 912UL Engine vibration on throttle back

by Roger Lee » 12 years ago

Roger Lee

127 5.21k 4.79k

Hi Kevin,

Just use a MM ruler. The carbs must be off and upside down. Set the ruler on the edge of the carb where the bowl sit. Then slide it over and up along side the float arm. It should be 10.5mm from the carb bowl edge. If it is too high or too low then slightly bend the float arm where the float needle attaches to raise or lower the float arms. Check both arm on each carb.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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912UL Engine vibration on throttle back

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