912ul woes

[egp8111]

Well I got the carbs back from Lockwood. They did a two hundred hour inspection on them and sent them back despite my specific instructions to run them on their test engine first.... Not that mattered, it runs just the same as before. Rough between idle and 3000 rpm,smooths out above. I thought the A box was running rougher than the B box so I swapped the plugs. The roughness seemed to move to the B box. Again it is only a small difference between the two ing. circuits and the rpm drop is the same. I've checked all the wiring best I know how and cannot find any breaks. New plugs and fuel.
I'm really getting tired of this motor.
EG

[DesertFox4]

EG, did you post what kind of fuel you are running? I went back and reread quickly but did not see if it is auto fuel or 100LL.

We had a gentleman on the site some years ago that was having lots of difficulties getting his 912ul to run correctly. After months of trying every thing suggested, we found out he had been running 100LL Avgas and not using the Decalin fuel additive at all. Had so much lead deposited on the inside of the engine it needed tearing down and removing all the lead. Worked fine after as I recall.

[egp8111]

I use premium auto fuel 90% of the time, when I have to use 100LL I add decalin. I don't have a record off what the previous owner used. Is there a simple check for lead deposits without tearing down the engine?

[Av8r3400]

You could do an oil analysis. What oil are you using? Mineral or synthetic?

[gregsgt]

OK I'm going to toss something out there that may be a solution because I ran into something similar that I solved.

The older Kitfoxes use a setup where you have a throttle cable that pushes on a bellcrank. The bellcrank then pushes on two individual throttle cables that go to the carbs.

What I found is that there were a whole bunch of small tolerances in this setup that would add up to throw the carbs out of synchronization when the throttle was manipulated.

I had my carbs balanced at idle and midrange perfect on the carbmate however as soon as I started moving the throttle the carbs went back out of balance and it would run rough due to tolerance differences in the two sides.

I'm going to install this new throttle cable and bypass the bellcrank which will make the setup much tighter.

http://mcfarlaneaviation.com/Product...r=MCTV0005D048&

[Dave S]

Greg's post got me thinking......about devices on older 912 powered Kitfoxes.

Not saying this is it, but possibly something to verify. Do a search on "ignition Failures"

Due to reversal of the carbs and modification of placement of the ignition parts on some early kitfoxes, some kitfoxes ended up with the two ignition ground wires (not the P leads - but the module grounds) NOT running back to the manifold but being attached to the top of a bracket that went between the manifold and whatever was above. Some of these brackets have cracked resulting in anything from an intermittent miss under a variety of conditions to a complete shutdown due to a crack developing on the bracket.....behaves much like a fatigueg/cracked wire.

Might check to assure the module grounds are secure and intact, including anything the two grounds are attached to.

Again...no clue if this is helpful or not.....just an idea to throw into mix.

Sincerely,

Dave S

[happiestflying]

I finally found the problem with my own ignition rpm drop and rough running on one side, and you may find it helpful. All along I had thought it was ignition modules or carb sync.

Pulled the plug wires, tested each pair (top 1&2, bottom 1&2 etc) and found high resistance (greater than 15,000 ohms plus or minus 800) on one pair (top 3&4). Pulled the caps, found a bit of corrosion on the plug wire. Trimmed the wire 1/4" back to shiny copper, put a drop of electrical anti-corrosion spray on it, replaced the plug caps with new caps, tested the resistance (now in spec) and presto, the problem disappeared.

Sorry if this isn't quite on the thread track, but sometimes problems seem to be inter-related.

[HighWing]

Jeremy,
Thanks for the post. It is definitely one more check in the diagnostics tool bag. I don't remember anyone posting this solution before.

[egp8111]

Hopefully some progress on my rough running motor (or another dead end)
After doing all I could think to do for the carbs and gearbox I ordered some new spark plug caps for lack of any better ideas. I've always had some plugs that were black and sooty that I attributed to carb issues. It's always run slightly better on the B ignition system than the A but only slightly. Mag drop same on both systems. I ran it today same roughness between idle and about 3000rpm. I shut it down and replaced the plug caps on the A box plugs. (only had 4, I was too cheap to buy all 8). Fired it back up and at first I had the same rough running but suddenly as I was calling it bad names, it smoothed out and idled like a 912 should! My working theory(wishful thinking) is that there was a bad plug cap fouling the plug and with the new ones the plug cleared itself after a few minutes. This sound plausible to you guys?
I'm not totally out of the woods yet, it's still a little rough around 2500 rpm but hopefully that really may be cured with some more carb syncing. Or maybe another plug will clear or a unicorn will kiss it, who knows. I'll probably invest in some more plug caps irregardless.
Thanks to everyone for the tips and suggestions.
Skip

[Av8r3400]

Don't tell Rotax, but those same NGK plug boots are available at any snowmobile or motorcycle parts outlet.

I got mine from Fleet Farm (their aviation department).