Flooding 912 and engine mounts.

[chrisag]

I have been working on a KF 4 with 912ul. Up to recently running well and still does over about 3000 rpm. At around 2000 engine becomes rough and then stops due to both carbys flooding. Both carbys have been overhauled so ok. It leads me to the engine mounts. They seem very soft and not that old. Is it possible to get a harder compound? They seem way too soft. What would happen if i shortened the bush on the bolt a bit to put a bit more compression on the rubber ? Any thoughts from anyone and is this a common event.

[jamesmil]

as crazy as this may sound a friend of mine had the same problem and after rebuilding the carbs, twice and then replacing the carbs, with new ones we found that the gear box was the problem.
his engin would shake so bad at idle the carbs, would fall out of the clamps holding them on. we removed the prop and ran the it and it ran great. got a rebuilt gear box and no problem sence. hope this helps

[chrisag]

Thanks for the reply jamesmil. I hadn't thought about the gear box. A very good point as the age of the engine is unknown. We will look into that. Do you know if your mates gear box had a slipper clutch or not?

[Av8r3400]

Good idea to check without the prop, James. A 912 will run without a prop, you you must be mindful of rpm, it will easily rev way up very quickly.

(Don't ask how I know... )

This is an old back-woods method for getting a 912S started with a bad sprag. Start and warm without prop, then reinstall prop and it will fire right up.


Chris - What prop are you running?

[HighWing]

What would happen if i shortened the bush on the bolt a bit to put a bit more compression on the rubber ? Any thoughts from anyone and is this a common event.

Don't know the vintage of your Model IV, but in the day, the bushings came from the factory a bit too long. When I was building - 1993 Model IV, I found that I could slip a credit card in between the rubber isolator donut and the steel disk. I shortened them just because. It was later that the factory advised that the bushing be shortened some - i don't recall how much.

Regarding the rebuilt carbs. I rebuilt mine after a bunch of hours and the new float needles wouldn't hold the fuel at idle. I could fly fine, but on taxi to the hangar, the engine would start running rough and pouring fuel ot of the vent lines. I went back to the old needles and everything was fine for another 400 hours.

I have heard about the gear box issue. How many hours do you have on the engine.
Lowell

[chrisag]

[quote=HighWing;24678]Don't know the vintage of your Model IV, but in the day, the bushings came from the factory a bit too long. When I was building - 1993 Model IV, I found that I could slip a credit card in between the rubber isolator donut and the steel disk. I shortened them just because. It was later that the factory advised that the bushing be shortened some - i don't recall how much.

Thanks for the input Highwing. On going through the paper work, the kit was purchased in 1991 so it is likely to have the long bushes in the mounts. Have just measured these and they are 1" and a 16th. About 27mm. .....Cheers.

[jamesmil]

chrisag, the engine we had with the bad gear box was an 80 hp. and as best we could tell it had about 1200 hr, on it as it was used when the plane was built. it did not have the slipper clutch it used a dog gear set up with one gear on a splined shaft with a spring washer behind it to allow the gear to slide on the spline to dampen the pulses at idle.
we found that gear scized on the shaft and so it would not move on the shaft.

[chrisag]

Thanks all for the replies and advice. I seem to have the problem sorted. The bushes were the early long ones and i turned them down to correct length. It has made a huge difference to the mounts.
AV8R, the prop is a Warp Drive and will keep an eye on the gear box.
Cheers from down under...

[Av8r3400]

Chris - Remember the heavier the prop the more torsional vibration you will get out of a geared motor.

If yours is a 3-blade, "square" tip, with nickel inlaid leading edge, any thing over 68" in diameter is considered "overweight" by Rotax and will void new motor warranties and potentially shorten the life of your gearbox. This is directly from LEAF and Lockwood (major Rotax dealers in the US).

I have a 68" three blade with nickel inlay, but the tips of the prop are tapered. This prop is what they consider to be the maximum allowable weight. It still vibrates much more than the same setup with an IVO or other lighter prop. Our EAA Chapter now has a Dyna-Vibe unit that I plan on using as soon as I can. It does make a difference, too.

The Warp props are great performers and very strong, but come at the penalty of weight.

[chrisag]

Thanks for that info Av8r. The prop is a 68 inch with a square tip, nickel inlay and 3 blade. It performs very well...consistantly 1000 fpm. Looks as if we will have to keep an eye on the gear box. Cheers....