rough mid-range and carb questions

I cleaned the carbs and and installed the LEAF level 4 rebuild kits.  Of course nothing in the carb looked at all suspicious, but I'll have 5 years now, and I'll probably move many of the hard parts to my other engine which is really ancient and in need of some TLC.  The only thing I didn't replace was the shaft o-ring, because it appeared to be a solution in search of a problem.  Whoever peened the screws before, also peened the screw slot partially shut, so you can't get a normal size screwdriver in there now.  Trying to fix that slot seemed like a good way to screw up an otherwise working assembly, so I decided to leave the shaft o-ring.  It can be tested later if needed.

After re-assembly, and yet another carb sync, the results are exactly as expected.  In other words, no change in the problem.  I also removed those drip trays in the hope that maybe they weren't sealing properly, but they were likely fine.

From the very beginning I've thought this was ignition related, and I finally found something that's not right.  I checked the trigger coil gaps, which are supposed to be 0.016"-0.020", and they're all about half of that . The smallest was 0.006", and the largest gap was 0.010".  I've never had that plastic cover off, and the screws looks like they haven't been touched in years, or maybe decades.  I checked my other engine, and they're all 0.016", so no doubt this needs to be corrected.

My only question for the day is whether there's any known issue with running the gap too small?  I have to believe they give a range for a reason, so maybe the pulse that's generated is too large, or distorted so that it's not being reliably recognized by the ignition module?   I suppose it's possible that a misfire feels worse now with the large cylinders. 

Thanks,

Rusty (DIY Rotax heavy maintenance trainee)

I find it difficult to advise/guess when the engine has been modified so will fall back on the basics:

Spark + fuel + oxygen = burn However the burnt gas must escape (exhaust) for the next cycle to occur.

Obviously it is a little more complicated - the spark must occur at just the right time and valves must open & close (seal) also at the right time but whenever I am stuck on a problem, I go back to the above.

Could you have an exhaust problem?

"I've verified that the balance stays pretty stable across the normal RPM range. "

Carb balance is best done from idle (about 1800)  - to fast idle (2500)  - the vacuum generated is greatest at this stage and any variation between the two carbs will be highlighted

"...engine almost won't run with the balance tube disconnected..." 

If by this you are saying that the engine wont run with the CarbMate connected in leu of the balance tube, there must be an air leak somewhere. If there is an air leak the CarbMate (or any vacuum gauge) will not allow for correct balancing of the carbs.

This may be where your problem is - an air leak in the CarbMate or its connections to the carburettors. This could result in unbalanced carburettors, that in tern results in uneven power delivery, felt at certain rpm as vibration.

I can't see any chance it's an exhaust problem.  It's a Toucan system that's been there all along.  Those mufflers are flying on plenty of 100+ HP 912s. 

I did the carb sync at 2500, but checked it up to about 4000 to make sure it didn't change in the area where the main problem is.  I'm almost 100% certain there's no leak in any of the external tubing for the CarbMate, but I did wonder if it could be leaking internally.  That would just be evil since it would appear to be working, but would cause you to set the sync wrong.  I guess I could swap the connections on the CarbMate to make sure the centered indication stays there when swapped.  I'd bet that's not likely the problem, but nothing is above suspicion.  

This has felt like an ignition misfire from the beginning, and finding the air gaps at half the spec gives me hope.  I'd feel better if I could find anyone who ever had that be a problem, but most gap issues are when it's too big, not too small.  Sadly, I probably won't get a chance to try to adjust those until Wed, and there's so much rain in the forecast that I'm not sure when I'll be able to test it.

Thanks,

Rusty

try syncing with something else, like clocks.  Or by eyeballing and see if the vibration is reduced.   It should run fine without the balancing tube and if it doesn't hen that does not sound right.

I was out today and with the cold (about 28F) the lean condition is worse. Need to pull choke at cruise to get AFR to below 15. Have 13 at WOTclimb.   The lean misfire I get from above 15 is subtle and if I did not see the AFR meter it could be misread as prop/gearbox vibration. But pulling the choke makes it run better and AFR drops by about 0.5.  Also it runs better at lower RPM like 4500 where AFR drops to around 14.5.  Larger needle jets in transit and hopefully that will do the trick

So maybe SEVERE vibration could be a sync issue or even vacuum leak ?   As when one bank drops out altogether ? That would be severe

I'm 99% sure this is not a sync issue.  I also don't see it being a vacuum leak with new o-rings literally everywhere. 

This just feels like ignition, so I'm betting it's going to be related to the air gap on the pickup coils.  I can't understand why anyone would have set them to half the min spec unless it was a metric conversion gone wrong, or they were trying to compensate for some other ignition issue.  I found a number of reports of the effect of a too large gap.  They say it retards the timing, and reduces the voltage to the ignition.  It would seem that if the gap is too small, it would do the opposite- advanced timing and higher voltage.  I could easily see this causing erratic ignition operation, but that might be wishful thinking.

Rusty