rough 912ul Help

[egp8111]

This thing is making me crazy. My 912 ul will run very rough after start up from idle to about 3000rpm. Above 3000 is smooths out and runs fine at higher rpms. Here is wher it gets strange, after I fly for a while is will idle smoothly but still has some roughness about 2800 rpm but much better than at start up. I've had the cabs off several time checking and cleaning everything, float arm height, tips, needle jets etc. and balanced the carbs several times. They show even balance (with both a Carbmate and guages) at 2500 rpm but it stillruns rough. It's almost like there is a fouled plug that clears itself after it heats up but I can't detect anything when I pull the plugs. It runs the same on either ignition box. I'm thinking it is somewhere in the carburation due to it partially clearing itself up when warm, but I running out of ideas where to look next. Any suggestions ?
Thanks
Skip
NC

[bogmonster]

Mine was similar for a while. Biggest culprit was one blade of my WARP drive was 1.5 degrees different that the others. I also had wires on both my ignition modules which may have hit resonance at lower RPM.

Mike

[Av8r3400]

Again, sounds like carbs to me. If it smooths out at high rpm, that's the mid range or idle circuits input.

Check to see that the enrichener circuits are fully on or off. This has been a problem.

Check main needles for wear or broken clip.

Have you synced the carbs lately? Cables do stretch or slip...

Air filters cleaned and lightly oiled?

[WWhunter]

I have been having very similar problems with my 912. When I first got the plane I was VERY lucky, as I could have easily put it into the trees if it had not been some upstairs looking out for me. I did a run-up test and all of a sudden one of my ignition modules died, ended up being a broken wire within the insulation. Took a couple of days to track it down. I say I was lucky because as soon as I fixed the beak in the wire, the other ignition module went out. Again it was wires broken within the insulation. In researching I found others having this same problem in this year of engine (1996-7). I think I eventually fixed 3-4 broken wires. The cause seemed to be from the harness not being tied down well enough, vibrations got the best of it.

Even after this issue was fixed I still had/have the rough running similar to the problem you are mentioning. I have rebuilt both carbs, taken them apart and cleaned them more time then I care to count, and the problem seems to come and go. Generally it is mid-range where the engine runs rougher than I like. I am to the point of taking the engine off and using it for a bullet stop! I quit flying it last fall due to not trusting it. Haven't had the time to take the carbs off and go through them again. Hoping to delve into it within the next couple of weeks. Lots of guys are very happy with these engines but I sure am not one of them.

[kitfoxnick]

Check your coil pack wires.

[Av8r3400]

How many hours do you have on these rough running engines?

What props are you running now and in the past?

After a couple hundred hours of service, sometimes the grearbox needs to be serviced. This entails re shimming and usually replacing the spring washers.

IMO what can happen is he gearbox starts a slight vibration, then the carbs, due to the mounting style on the older foxes, pick up on this and make it worse.

[WWhunter]

Av8r3400,
Thanks for that bit of advice. Not sure why but that is an issue I have been thinking that might be part of my problem. I bought the plane with 140hrs TT and it had a 2 blade Warp Drive on it. When I first bought it, the vibrations above, say 5,000 rpm were to the point that looking through the windshield looked like a grease covered windshield. Also high frequency buzz felt through the floor. I swapped out the prop for a 3-blade Whirlwind which made a huge difference.
This engine also had the 95 HP pistons and it was using oil at a very high rate so I installed a Zipper kit. Only have ablout 10 hours on the Zipper kit since the voltage rectifier went tits-up before winter.
In my continual research on trying to get this engine running better, the mention of gearbox repair was stated a few times. I am thinking this may be the next step in my quest to get this motor to run smoothly.

I have since purchased a 75" 3-blade Whirlwind STOL prop and had read a post that the gearbox needs to be 'reset' (?) when running this bigger prop. Not sure what the exact reason for this was but I never installed the prop due to this posting.

Any suggestions on who to sent the gearbox to?

[avidflyer]

I would send it to Leading Edge Airfoils in Wisconsin. Jim Chuk

[WWhunter]

Thanks Jim! I had also been told that Ronnie Smith (?) is another good Rotax guy. Don't know him but one guy in particular highly reccomends him.

[Danzer1]

Have any of you guys with this type of issue tried dynamic prop balance - particularly with a Dynavibe II - as it can pinpoint other sources of vibration (other than at the prop)?