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Rotax 447 rough idle

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crkckr

Well-Known Member
Joined
Apr 26, 2016
Messages
58
Location
Warrenton, MO
I am, it would seem, the creator of all of my own problems. This will be another long post so again, I beg you to bear with me.

A few weeks ago, when I was having all of the problems with my Rotax 447 ignition module and was unable to get the engine started, I began to worry about the bearings in my engine. Everything I've read points to corrosion with high alcohol content fuels and mine are running in the 7 - 8% range. I had visions of my engine's internals turning into piles of useless rust... so I squirted some fogging oil into the carb and spark plug inlets. Just a bit, no where near as much as called for in the Rotax preservation instructions. Just enough, I thought, to maybe coat the bearings and rings and prevent them from rusting away to nothing.

Just a week later, I was able to get the engine started. Unfortunately, the idle was nothing short of horrible! The entire airplane shook and vibrated as if it were trying to tear itself apart. Since it didn't have the prop on at the time, I didn't think much about it, figuring it just didn't have prop to dampen the roughness out. Oh boy, was I wrong about that! I should also mention that at first, it was nearly impossible to even turn the engine over due to the compression being high enough that I thought I was going to break the rope! Ultimately, I ended up putting fuel directly into the cylinders thru the spark plug holes and pulling the engine thru... enough so that my arms ached! I also, after my arms gave out, took the exhaust manifold off and drained out about a pint of fuel. I'm glad I didn't try and start it without doing that, I had visions of gouts of flame coming out of the exhaust, much like a flame thrower! Certainly not a good thing while in the hangar with a bunch of other airplanes... Only then was I able to get the engine started. During that short run in the hangar, I noticed that the CHT and EGT were really low on #1, compared to #2, but I didn't think much of it. Besides, I didn't run it that long, since we were in the hanger. A total of maybe 10 minutes or so, just long enough for it to stop smoking like a chimney stack.

Once I was able to get the airplane out of the hangar and unfolded, I put the prop back on and tried to start it, just as I had previously: Squeeze the primer bulb 3 times with the bypass line pinched, squeeze some more until there are no bubbles visible in the bypass line, open the enrichment lever all the way (from now on referred to as the 'choke' - them Germans are sooo formal!), switch the ignition on and pull for all you're worth. No go. Now what?! Except for the idle, it had run very well in the hangar without the prop and now nothing? After remembering that we had primed the engine by drizzling fuel directly on the air filter (something I truly dislike doing!), I resorted to putting a bit of fuel into the carb via the "primer port" (I'm not at all sure if that's what it really is, it's just a brass nipple covered with a little rubber bulb on the aft side of the Bing 54 carb. As it shoots fuel directly into the carb and I can't find any reference to it in the manual, I'm calling it a primer port!). Whatever. The engine started right up. However, just as before, the idle was horrible and in fact, if left there too long, the RPM would drop below 2000 and the engine would die.

RATS! What have I done?

I should note that prior to all of the problems I had getting the engine started, when it idled, it was as smooth as any other 2 stroke I've ever seen and would idle for several minutes without loading up. So there are only a few possibilities here, things that have been changed or touched in some way: The ignition module; the trigger (currently set at .020: air gap per the manual; however, I've been told by some that know about these things that it should be at .018". I expect that the next time I get a chance, I will set it to .018); the spark plugs (however, I have tried both the old, original plugs, gapped at .020", which had around 3 hours on them, and a new set, gapped at .015"; the rough idle happens with both of them and I have also swapped the plugs between #1 and #2 cylinders with absolutely no noticeable change) and, most likely in my mind, the fogging oil. I also took the carb off of the intake manifold and removed the exhaust manifold to drain the fuel out. Thing is, I don't see how any one of those things could affect just one cylinder, unless (again, #1 thought in my mind) the #1 cylinder rings are glued in by the fogging oil some how.

After a bit of mucking around with this and that, I eventually tied the airplane down to a big post and ran it for about 30 minutes at various power levels, including full power of 6000 RPM. The Rotax manual says my full power should be between 6000 and 6200 RPM, so everything was good there. I did notice some things when it came to the EGT and CHT's, however. At idle (such as it was) the #1 CHT (the forward cylinder) would barely get up to 200, and at that only after 5+ plus minutes of running, while #2 was up to 225, after just 3 - 4 minutes. These numbers are what was normal in the past - BTF, or Before the Fogging. The #1 EGT was sitting on 500 while #2 was at a normal 1000. This is, by the way, at something above 2000 RPM, usually right at 3000. Clearly the 'problem,' whatever it is, it with #1 cylinder. Also at this time I started having trouble with the CHT temps. Number 1 kept dying completely, which finally lead me to remove the indicator and repair the frayed/broken wires. However, during that run, the EGT's on #1 went much higher at 4000 RPM, to 1150, nearly up to the redline of 1200, while #2 sat on 1100, the normal max temp. Thing is, once I ran the RPM's up to 5000 or more, the EGT's on both cylinders went down to around 1000/1050. I think that's normal but I had never run the engine above 4000 RPM before.

I will admit to also going thru a bit of trouble with very high idle speeds (3000) but that ended up being a bit of mis-routing that some idiot did, by running the throttle and choke cables under the carb boot instead of over it. Opps. As usual, I create my own problems!

At any rate, when I was done with the run, the idle was a bit better but still nothing like it was before I had all the problems with the ignition module and, of course, prior to the fogging. A week or so later, I ran the engine again for nearly an hour. The CHT at anything under 3000 RPM was always lower on the #1 cylinder and higher, around 50 degrees or so, at the higher RPM settings. The #1 EGT was also lower at 3000 RPM or less, and 50 - 100 degrees higher (than #2) at anything above 4000 RPM. However they did even out at the higher RPM settings towards the end of the hour. The idle also got a little better and when the throttle was pulled back to idle, it would actually idle ok for maybe 10 - 15 seconds and then drop down to 1800 RPM and start the rough idling. I believe it's what is called "4 stroking" where the cylinder only fires every other stroke, instead of every stroke as it's supposed to. I also had the engine do something I've never had happen on this airplane, in that the engine kicked back and jerked the rope handle out of my hand and slam it against it's stop behind the seat! That kind of made my fingers numb for a couple of minutes before I could continue. It had never done that before, nor since, thankfully. I've never heard of this before and wonder if it's something that happens often. I'll have to google it and see.

There hasn't been much done to make this problem go away, mainly because I don't have a clue what might be wrong! I did take the fuel pump off and cleaned it out. There was a bit of white gunk in one chamber but it was by no mean clogged up in any way. I also drilled out the little hole that Rotax is so worried about and my Mikuni didn't have. Didn't change a thing, of course. I've swapped, then changed, the spark plugs (all of which look a perfect tan color, by the way. There is no clear difference between the #1 & #2 cylinder plugs). I've run the engine for nearly and hour and half, mostly at higher RPM's. The idle has improved, but not enough that it's any where near reliable enough to fly it this way. In fact, while doing a couple of fast taxi's Saturday, I had the engine die at the end of a run when I let it sit on idle too long. And just as I was trying to get up enough nerve to do the test flight with no one else around... (yes, I know, a foolhardy idea but I've been trying to get this airplane in the sky since June and I'm starting to get mighty frustrated!) I've also checked the spark plug wires, which all look and test just fine with the VOM (I'm still going to replace them soon anyway, along with all of the fuel lines that have not already been replaced).

One other problem did make an appearance Saturday, one that I had hoped would cure everything, but of course, did not. While priming the engine the bulb sort of blew up in my hand and sprayed fuel all over the back of the airplane. Looking it over, it's cracked a split in several places. I took the bulb out completely and just ran a clear line from the tank to the fuel filter. Once I primed it at the carb, it started just fine. It did not, unfortunately, have any effect on the poor idle. Another fine idea shot down in flames!

So, what can I try next? I'm up for just about any idea at this point, short of ripping the engine off and sending it out for an overhaul (actually, this engine is a fresh overhaul from a guy named Danny Day, of whom I know nothing. I'm not even sure when it was overhauled as I have not been able to contact the original builder to get this kind of information). The airport was totally empty Saturday, otherwise I'd of asked the other guys for advice. Only one other guy finally showed up to work on his Quick but unfortunately, he doesn't know any more about the Rotax engines than I do.
Thanks for reading thru all this stuff and TIA for any advice. Right now I'm wishing I'd of gotten that Firefly with the MZ engine on it but it was bought out from under my nose! Rats again!
Cheers,
Mike
 
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