Hard starting Rotax 912 ULS

[rcpilot]

I have a Zenith 601 XLB E-AB with a 100hp Rotax 912. My plane is tied down in the NE. Lately it's been hard/impossible to start. About 6 weeks ago I went to fly. The plane gave a hard time starting(upper 40's) and I eventually killed the battery. Jumped it with my car and it started. After warming it up I went flying for about an hour, no issue flying or restarting it. Since it had been 5 weeks since last I flew it I figured maybe I need to fly or at least run the engine more often. About a week later I go out, temp was 40 degrees. It's hard to start but starts without a jump. Flew again and no issues. Another week goes by and I go out just to start and run it being it was too windy to fly. This time it was 50 degrees. Again a hard start. Ran it till all the temps came up to normal(probably about 10-15 minutes). So next trip to the field was again low 50's. Plane absolutely refused to start even when I jump it. I research the issue and apparently a narrower plug gap is recommended for cold weather. So I pull all the plugs. All but 2 are fouled. Their gap was >.030. The recommended gap was .025. I clean and regap them. Take my battery home to charge. Go back this past Saturday and re-install the battery. Again too windy to fly but it basically fires right up. Everything looking good and I again run it till the temps are in the normal range. But, right before I shut it down it's starts running rough and nothing I do seems to rectify the situation. This past Monday I go out to fly and again won't start. Fairly pissed I pull the plugs and they are all fouled. I'm going to assume whatever suddenly caused it to start running rough is now what's causing the no start. There appears to be many potential issues. At this point I'm leaning towards a carb issue(they were rebuilt at Lockwood 4 years ago) but willing to listen to any other suggestions. My current A&P is not a Rotax guy so I'm inclined to troubleshoot as much as I can myself.

[Type47]

My shoot from the hip guess is ignition modules.
This is a well known problem that causes hard starting when cold.
I believe that warming the modules with a hair dryer and then trying to start is one test that can be done.
This happened to mine a couple years ago and was solved with $1900 worth of the newest iteration of the modules.
Mine is a 2006, so the jump to the latest modules with different ignition timing also improved the way it runs both warm and cold.
Search Rotax 912 Modules for many other peoples experiences.

[3Dreaming]

I'm not going to say it can't be the ignition modules, but from your description it doesn't sound likely. For it to start running rough while it is running, and finding fouled plugs I would also guess carbs. I would check the floats first. My guess is heavy floats, allowing to much fuel. A carb overhaul 4 years ago would put the floats in the suspicious range.

The pair of floats should weigh less than 7 grams, but the quick easy check is to make sure that the pins that ride on the float lever are at the fuel level in the bowl. If they are below the fuel level the float is to heavy.

[drseti]

First off, you were right to regap the plugs - I use .024" in the winter.

As for the plug fouling, can you be more specific? Wet, oily or dry, ashy? If the latter, your engine may have been running too rich, which would point to carb floats. Especially suspicious would be only two fouled plug sets, both on the same side of the engine - that would tell you which carb.

Tom's technique of observing the floating pin level is an excellent trick. If you actually weigh the floats, use a calibrated tenth-gram resolution electronic scale (borrow one from your stoner neighbor!) After removing the float bowls, I put each individual float on a paper towel to air-dry for one minute, and weigh each one separately. A good float should weigh 2.8 grams, and I would not use one that weighs anything over 3.5 grams. (Weighing the pair is consistent with the Rotax service bulletin, but one light and one heavy float could pass that test and still cause problems when banking or in steep turns).

BTW, the Rotax condition inspection checklist includes weighing floats, which would have them checked not less than once per year.

[3Dreaming]

Roger's

Tom's

[drseti]

Oops... sorry, Tom! Post corrected.

[rcpilot]

All but 2 of the plugs(left? lower)had black soot. I also posted on the Rotax forum and after answering a few of his questions, Rodger suggested I weigh the floats. I have a digital reloading scale which should do the job. I'm also going to swap out the plugs and fuel filters.

[drseti]

All but 2 of the plugs(left? lower)had black soot.

Black soot generally suggests mixture too rich, which could indeed point to carb floats. Weighing them will probably prove informative.

[FastEddieB]

(Weighing the pair is consistent with the Rotax service bulletin, but one light and one heavy float could pass that test and still cause problems when banking or in steep turns).

I check mine annually, weighing each float individually using a reloading scale. To date, I’ve never found an out-of-spec one.

One tiny correction - mere banking or even steep turns shouldn’t matter, as long as the turns were coordinated. Right? Slipping or skidding turns certainly could, though.

[drseti]

One tiny correction - mere banking or even steep turns shouldn’t matter, as long as the turns were coordinated. Right? Slipping or skidding turns certainly could, though.

You're absolutely right, Eddie.

[3Dreaming]

Black soot generally suggests mixture too rich, which could indeed point to carb floats. Weighing them will probably prove informative.

With automotive fuels sometimes the black soot is from the additives, and not a rich mixture. Also the idle mixture can cause soot, when the running mixture is fine.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8153779/

[rcpilot]

So the results are in. As a pair the floats from both carbs weigh 6.6 grams. Each one was about 3.3 grams. The float bowls were clean. So it's looking like new floats and of course new float bowl gaskets.

[3Dreaming]

6.6 grams for the pair should not be enough to cause a problem, Rotax allows 7 grams for the pair. Also it you have the black rubber bowl gaskets they can be reused and will seal up fine. The composite gaskets might be okay. If they are cork, replace them for sure.

[rcpilot]

Ok the gremlins seem to have gone. My mechanic finally had time to work on the plane with me(he had chastised me when I told him about my earlier troubleshooting). First he verified that there was spark from both ignition modules. Then he checked to make sure I had properly reseated the carb bowls. he had to fix one. I had bought new plugs and gapped them to .025(looks like out of the box they are about .029). We swapped out all the plugs. The only other thing he noticed was that the top plug wires for cylinders 2 and 4 were swapped. So he fixed that. Before all this I had done some online research on the proper starting technique. What I found was throttle out, choke on and when it seems like it's going to fire, choke off and advance the throttle just a tad. I did this and it fired right up. He had me warm it up and then we checked the carb sync. Surprisingly it was all good. I ran it a bit more including up to 5000 RPM(it was chocked and tied down). All good. Too late in the day for a flight but I guess the real test will be when I can go fly it if it starts right up. No real clue what the original problem was.

[drseti]

Having plug wires for two adjacent cylinders reversed could be sufficient to prevent it from starting.