Trouble shooting 600rpm CDI mag drop.

[C ICEY]

Hi Guys I just returned from getting the sprag clutch replaced, (trailered the fuselage to the shop) also changed the flywheel to a 'softer' timing start model. I have the soft start CDI, but not sure if they are bullyhawk or Rotax. Anyways.... got the wings back on and started things up, easy start again, everything seems fine, but now there is a 600rpm drop on the right switch, accompanied by very rough running. EGT on #2 cylinder drops when running on the rough mag. Checked spark plug connectors, and indeed, one connector was very loose, so I replaced that, but the problem persisted. Very rough running on right mag, smooth running on left, and smooth running on both mags, with even EGTs. I reversed the connectors from the CDIs (red/blue, and green/none) and experienced the same roughness, but on the Left switch position, with the same # 2 cylinder experiencing EGT drop on rough running. That was as brave as I got. any suggestions for the next step of trouble shooting? It appears to be a bit of a bear to get to the trigger coils, does the engine have to be pulled forward to access these? or is there another way to test them? Mike

[Roger Lee]

Since the problem did not stay on the same side then the problem is before the ignition modules. You can check the electrical wire coming out of the fly wheel area and make sure it is plugged in tightly. If this is not it and since they were back there working the problem most likely lies there. You can look in the Heavy maint. Manual and it will have some ohm values to check at the wire/plug coming out of the area to the ignition modules.

If the problem had not changed sides then the issue would have been after the ignition modules. Sorry, but it may come down to pulling the engine to get back in there.

[C ICEY]

Thanks. I suspected that it wouldn't be far from where the tinkering happened. the downside of such a tightly cowled installation. Now a silly question. if I accept the risk of no redundancy, is it unreasonable to operate the hour I need to get to the shop? Everything is smooth running otherwise. I will also consult with the shop on that opinion, too!.....

[Roger Lee]

You could fly to the shop, but I wouldn't just leave it for good. At a 600 rpm drop you may be loosing 2 plugs. About 300 for one. If they used the new style fly wheel then they should have used the Rotax modules. You can tell because the modules have yellow tape on them.

[sandpiper]

I personally know a guy flying a twin who lost one over a perfectly good airport but decided to go for home plate since it was "only" another hour. You guessed it. he lived but the plane didn't.

 

Only you can decide if the risk is worth it.

[C ICEY]

Does this sound right?? 6 pin connectors to the soft start. Previous test of swapping leads only shifted the mag position for rough running to left mag because the leads include the red stator leads...so that was inconclusive test. Swapped out only the red stator leads, roughness stayed on the #2 cylinder, so the stators are OK. Returned the stator pins, and swapped out the trigger coil lead pairs (red for black, and blue for green) and the roughness stayed on the same right mag, so the trigger coils are apparently OK. Stripped off the CDI boxes and will send them in for testing..... no flying involved, just patience as the perfect summer flying days go on by.

[FastEddieB]

There probably does not exist a pilot that has not operated with a known deficiency. And most get away with it.

 

But some don't.

 

Choosing to fly without redundant ignition could be the first link in a causal chain that leads to a chute pull or worse. Unlikely, but possible.

 

Imagine that your real problem turns out to be a bad batch of spark plugs. So far, only two have failed. However, on your flight to the mechanic another two drop offline. Well, there you go - what now?

 

Like I said, highly unlikely that on any given flight your "good" ignition will drop offline - unless something else is going on that could compound what appears to be an isolated problem.

 

Anyway, the "Most Conservative Action" would be to not fly until the problem is resolved.

 

But we're all adults here (I hope), and if you choose to accept the increased risk for a short flight, then there you have it!

 

Good luck, either way.

 

edited to add - Just saw, "..... no flying involved, just patience as the perfect summer flying days go on by."

 

I think that's a wise choice.

 

BTW, have you tried new plugs all around? At about $20, that would eliminate one set of possibilities.

 

 

[Jim Meade]

Just curious, how many EGT probes do you have and where is the other located if only two?

At what RPM did you check mag drop and did you check it at several or other rpm?

[Roger Lee]

A 600 rpm drop isn't an entire ignition module. That should be only two plugs. If you have done all that additional testing then you may be right about the trigger coils being okay. Having the ignition modules tested is a good idea. They can only be tested by 1-2 places, nothing an individual can do. I know Lockwood and Rotech in BC Canada can do it. I'm always suspicious of a problem if maint. was done in that area prior to the problem.

Why do you say it's on #2 cyl. only? Have you checked the firing on the plug wires for #2? Both #2 plugs fire off a different coil from on top of the engine. Just trying to get my head wrapped around this better.

 

You must have a Flydat (all 4 EGT's)? Most of us only have an EGT on cyl. 3-4 with Dynon's.

 

Have you unscrewed the plug boots and trimmed the plug wire back about 1/4" then re-install the boot? Changed both of those plugs? Double checked the gaps? Checked for thermal paste on the plug electrode? Make sure both cyl. #2 plugs aren't firing? Checked the plug wires where they screw into the coils for that cyl?

Just trying to rule out the simple and quick items.

[C ICEY]

@4000rpm, drop varies but settles into about 3400-3500, and the EGT drops almost immediately. Eliminated the carbs with choke tests. I have the Flydat, with 4 EGT. just put new plugs and battery in prior to my sprag clutch work. gapped to .630mm. changed out the connector boot that seemed loose, trimmed 1/8" and used dielectric on the boot screw. Used a small amount of white wacker paste on the plugs, applied carefully with a small brush... no paste on the electrode. certain that it is # 2 , as my number 4 EGT probe can be disconnected: flydat reads -42 that way for #4. I will recheck both #2 plugs but am sending in the CDI modules to Rotech as all that work has been done, but maybe it is the top plug. The bugger is that this popped up (almost) immediately after getting things back together from the sprag. Two short test flights ( I keep close to the airport for the first few hours after service... it only took one post service failure to learn that lesson :huh: ), the first flight without any indication of mag drop (to my recollection) the second with just a bit of roughness that seemed to go back to normal. third start three weeks later (had to leave on family matters) and the drop was undeniable. Kudos to the guys at Rotech, as they have been talking me though the details fo the pin swaps. In addition I value (most of ) the posts and input that I read here, so thanks for the help! The biggest temptation was to fly the 46 minutes over to Rotech, but its all pretty rugged country the reminders of this are all on http://www.youtube.com/user/ICEYtheCT. Lost 2 friends this summer already to separate tragic aviation accidents, no interest in being third time lucky with a cascade as Ed suggests. ICEY is just too much fun to risk that. mike

[Roger Lee]

Rotech has some pretty nice guys to work with. I'm not 100% sure what's going on with your system. I was hoping it was a loose wire. I hope it isn't something back in the stater housing and could could still be in the ignition modules.

[C ICEY]

School of hard knocks this trouble shooting process :wacko: ....... must say I have learned a lot!!! lesson of the day is one change at a time. Plus, not all problems are directly related to the latest wrench bending. that really threw me off. Sent my ignition modules off after demonstrating that the stators and trigger coils were OK. CDI were tested OK> so back to the drawing board. I re checked my notes for trouble shooting and realized that when trouble started, I checked the plugs and noticed that the #2 plug boot was very loose, so I changed that boot out. Unfortunately, at the same time, I decided to replace the plug as well, just to make sure. The problem remained! how can that be? Turns out that the plug that I pulled out of my spares as replacement was a bad plug. The original plug that I had was OK, but the boot was the problem.. feeling pretty sheepish, :blush: but the entire process introduced me to the electrical system with a new appreciation. less mystery, far greater appreciation. Time to get back to flying again! thanks again for help along the way.