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Intermittent mis on ignition B

todehnal

Well Known Member
I have an intermittent mis on ignition B. I've done quite a bit of troubleshooting, new plugs, plug cap test, loose are damaged wires etc. I finally swapped the 2 top, horizontal connectors for A and B ignition modules input. Well the condition moved from B to A. If I am reading the prints correctly, that proves that everything from the spark plugs, through the ignition module is good. The problem must be between the ignition horizontal plug to the trigger coil for B. Wiring looks good so I assume that I either have a bad trigger coil, or the air gap is out of specs. Has anyone successfully tested and adjusted trigger coil air gap with an installed engine? Thanks for you help..........Tom
 
Well ****, I think my theory was wrong. I forgot that when I swapped the top horizontal plugs, that the ignition switch moved with it. So, if I have it right this time, since the mis moved from B to A, that proves that the trigger coils are working and the the problem is further down stream. So I guess it's back to the drawing board. My next task will be to swap the lower vertical plugs and see if it is the B module. Then keep moving down stream until it quits swapping from B to A.. Can you swap those vertical plugs without removing the intake manifold? Thanks.............Tom
 
At what RPM do you have the miss? I have same problem but only at lower engine speed. At 4000 RPM ignition test... engine is smooth on both A & B.
I changed plugs and caps with no improvement.
 
The mis is not rpm sensitive. However it feels worse at higher rpms. I?m getting about a 300rpm drop and it is as rough as a cob.
 
Don't overlook carburetion. Check weight of floats and also synchronization. If floats are close to 7 gram limit you can set floats out to ?dry? for a few days. Reinstall ?lighter? floats and see if it makes a difference. Check EGT?s to see how closely they match throughout entire speed range. Maybe switch floats L-R. Possibly running lean if circlip failed and allowed needle to fall. Needle could also be worn causing rich mixture. Check for vacuum leak - inspect rubber flange that mounts carb to intake manifold for hidden crack. Might be time for carb rebuild and thorough cleaning.
 
Hi Jim, Don't think it's vacuum since it runs so good on A ignition. I saw a blurb about pulling the enrichener on temporarily to see if it smooth out while on the questionable ignition side. That did not help. My first attempt to solve the roughness was to comply with the circlip bulletin. Also these are brand new floats (new style) and the weight is good. I had a float issue sometime back and it was easy to detect with a pool of gas on the drip tray. the drip trays are now nice and dry. Everything points to ignition. I have eliminated the trigger coils and ignition switches, so it must be further down the line. So I started with the easy stuff, new spark plugs, checked spark plug cap pull off and metered all of the plug wire sets, which verifies the secondary of the ignition coils. No joy. So, it must be somewhere between the vertical ignition connectors and the primary side of the ignition coils.
 
The plug caps are soft rubber compound. They are resistor caps and I feel that maybe with soft rubber the resistor gets damaged when pulling off spark plug - just a theory. I replaced my caps with hard plastic resistor caps. NGK - Spark Plug Resistor Cover - 90deg. Elbow Type (LB05F) 8051. I bought qty (8) on eBay for $50.

Also check spark plug wires for abrasion. There is several spots where wires touch brackets, etc.
 
Made a tool for disconnecting the vertical ignition connectors and it worked. Swapped connectors and the roughness moved from B to A.
In studying the print, that says that the Ignition modules are good. I have already changed the plugs, tested the spark plug caps for pull off grip, and ohms tested the plug wire pairs, which also verifies that the coil secondaries and plug wires are good. So the problem must be between the ignition modules output to the coil primaries. Still not sure which one, but it is either the 3 & 4 top, or 1 & 2 bottom coil circuit. So what to do next.....Tom
 
I have already changed the plugs, tested the spark plug caps for pull off grip, and ohms tested the plug wire pairs, which also verifies that the coil secondaries and plug wires are good.

If the high voltage wires and caps check good for ohms there is still possibility that high voltage is "leaking" to ground during engine run. I remember seeing old car engines running at night with hood open and sparks imitating from the wires along their length. Capacitive discharge ignition used on Rotax is very high voltage with solid wires. Maybe look to see if wires have breakdown in insulation allowing high voltage jumping to ground. At one point I was going to change high voltage wires. Pretty sure they "unscrew" from coils just like caps "unscrew" at spark plug ends. High voltage solid wire should be available at auto store, racing shop, or eBay.

I'll keep following your thread. Please post solution when it is available...
 
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Well, it's with my hat in my hand and egg on my face that I write this. I wanted everything back to original after swapping the vertical ignition cables before moving on to examine the high voltage coils. Somehow I got the connectors mixed up and had a devil of a time getting that straightened out, but I finally got it right. Of course I test ran it to verify. Someone posted a daunting Rotax ignition problem quite awhile back. After many hours of troubleshooting, the end results were that he had replaced a bad spark plug with a new defective one, and it took them down the garden path. Well, I guess I did the same thing. I had another brand new full set that I was saving to install when I had it running properly, but I decided that it would be well worth $25 to just do it, even though I had already replaced all of the B side spark plugs once. Well it ran like a top, and I actually was able to fly it today. It was a lot of work and I learned a hard lesson, but I did become quite familiar with the Rotax ignition system.

Sorry for raising the help flag and I appreciate all of the suggestions, It was a hard lesson, but I did learn a lot about the Rotax ignition system. I was finally able to fly today and I am a happy camper......Tom
 
Out of curiosity, did one of your troubleshooting efforts involve checking the resistance of all of the spark plugs -- both the old set and the new set? The NGK spark plugs should be 3 - 5 k ohms. What reading did you get on the faulty plug?
 
Out of curiosity, did one of your troubleshooting efforts involve checking the resistance of all of the spark plugs -- both the old set and the new set? The NGK spark plugs should be 3 - 5 k ohms. What reading did you get on the faulty plug?

I have no idea. I was so PO?d after several days of chafing my tail that I just tossed them. However I appreciate the expected ohms reading and will make a habit of testing for that in the future. Thanks. Tom
 
Been there - done that but wait!

I've been there and done that with car replacement parts. Also OEM high tension plug wires. So the good news is that your lifetime quota of one of these occurrences is now used up!!! But I plan to carry a spare plug and ht wire when I get my AC flying just in case.
 
HT wire? I think the ignition coil and plug leads are one assembly that doesn?t lend itself to replacing a plug lead.
 
When I was having this problem I was getting ready to replace the high-tension leads. My research showed that the wires "unscrew" from the coils. Pretty sure the coils have a threaded barb just like the removable caps.
 
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