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Rpm drop at ignition check

D

dick seiders

Well Known Member
First flight scheduled SAturday last did not occur as ignition check at 4000rpm showed a significant drop on the B side. A side was 115 drop, B was 460 or so. Switched module wiring around all possible ways today with no change in B performance. All indications point to stator/trigger coil problems based on input I rec'd from Dean at Lockwood. (Larry's suggestion, thanks, Larry). Has anyone else experienced this difficulty, and if so what were the remedies? I have also been advised by Rotax owners to make certain the carbs are in sync before pulling the engine. This of course makes sense if indeed that is a possible contributor, but I am not confident about that as a solution. I would of course be estatic if it was. Not looking fwd to yanking a brand new engine out out of my 12. Any input greatly appreciated.
Dick Seiders
 
Hey Dick

Just a quick check. I helped another guy with a similar problem. Be sure all the plug wires go to the correct location. If you mix up a couple, you might be shutting off an entire cyl if wires in wrong place. Easiest first check.

John Bender
 
2nd Hey Dick

John is right! This exact problem was solved for a friend of our by John's suggestion to make sure the plug wires were plugged into the correct plugs. Without both ignition switches on it was always running on less than 4 cylinders. The wires have little yellow numbered tags on em for the cylinder they belong to.
 
JBPILOT said:
Just a quick check. I helped another guy with a similar problem. Be sure all the plug wires go to the correct location. If you mix up a couple, you might be shutting off an entire cyl if wires in wrong place. Easiest first check.

John Bender
Click to expand...

Peterk said:
John is right! This exact problem was solved for a friend of our by John's suggestion to make sure the plug wires were plugged into the correct plugs. Without both ignition switches on it was always running on less than 4 cylinders. The wires have little yellow numbered tags on em for the cylinder they belong to.
Click to expand...

I agree.
Dropping one cyl. will cause a 400-500 RPM drop.
It is not likely caused by out of sync carbs but that should have been the first thing you did after the first engine start so you might as well do it to rule it out.
 
Dick, several people I have talked to have had the same problem, and like other have said here it was a spark plug wire switched. Double check that and see what you find. Double check the yellow number tags on the plug wires and make sure the top plug wires match the bottom plug wires.

Let us know what you find.
 
Last edited:
Checked all plug wires, and they are correctly routed. Found a suspect plug on number 1 top and replaced it with no change in performance. Checked plug wires back to the coils and all appear correct as well. Several 4000 rpm checks revaled same. The A ignition is losing 450 or so, the B only 115. Note I changed the ignition designation as when B is off with a drop it is the A side that's having a problem. I will research further with Rotax tomorrow. Thanks to all for the interest. Can't believe this is happening on a new engine.
Dick Seiders
 
Have discussed with Rotax tech and a cert. repairman. Def. a trigger coil problem. Need to reset coils gap and spacing. Did the deed while building as per plans , but what went wrong I don't know. Will try to do on the airplane, but suspect engine may have to be pulled to gain access. Will follow thru with results report later. (probably MUCH later).
Dick Seiders
 
Dick,

That is a worrisome report. I also adjusted pickup gaps per the plans. Sure hope I don't have the same problem when I start her up.

John
 
John, didn't mean to imply that setting the trigger coil gap created the problem. At this point just don't know. It's possible the trigger coil is defective. I have isolated it to the A trigger coil which services the No 1 and No. 2 top plugs. Will learn more tomorrow, and will keep the thread up to date with findings.
Dick Seiders
 
dick seiders said:
John, didn't mean to imply that setting the trigger coil gap created the problem. At this point just don't know. It's possible the trigger coil is defective. I have isolated it to the A trigger coil which services the No 1 and No. 2 top plugs. Will learn more tomorrow, and will keep the thread up to date with findings.
Dick Seiders
Click to expand...

Dick, The good news is you are going to become the local Rotax ignition expert! ;) Good luck, and keep us posted!
 
Conducted further testing and determined that the offending trigger coil is the A 3&4 bottom. I had not tested it before and when the the other A (1&2 top) t.coil dropped 1200 rpm on the 4000 run up ignition check felt I had the one. Wrong! The A 3/4 dropped 1700 rpm on the test. Now need to do a resistance test on both coils to determine if either one or both are defective. If not then they need to be re-gapped. Either way the engine has to be pulled to access the t.coils. Not a happy event. The question to answer is did I mess up the gap by doing the adjustment check requested in the plans or were they wrong as set at the factory. I suspect the former rather than the latter. Will continue to keep post updated with results.
Dick Seiders
 
Removed engine, checked trigger coils. Found the two B coils close to spec at .010, and .014'. Found the two A coils out of spec at .024" and .050". Don't know how that happened, but feel that checking gaps with coil cover in place played a role. That has been changed in the plans as the cover is now to be removed. Setting the gap is quite a bit easier with cover off as there is no chance you'll measure using cover versus trigger coil cam. At any rate earlier testing indicated what I found in the course of this effort. I suspect next test will be successful now that all gaps have been set at .012-.014. ( Rotax recommends setting tighter rather than on the high side for better operation. They also wonder why we are checking gaps as they are set at factory). Hopefully the work I just outlined will produce the desired result. Will update post after next test.
Dick Seiders
 
My plans still had me setting the gap with the cover in place. It only took a moment to decide that it would be faster and more accurate to pull the cover. I'm glad Vans changed the plans.
 
Conflicting information.

dick seiders said:
Removed engine, checked trigger coils. Found the two B coils close to spec at .010, and .014'. Found the two A coils out of spec at .024" and .050". Don't know how that happened, but feel that checking gaps with coil cover in place played a role. That has been changed in the plans as the cover is now to be removed. Setting the gap is quite a bit easier with cover off as there is no chance you'll measure using cover versus trigger coil cam. At any rate earlier testing indicated what I found in the course of this effort. I suspect next test will be successful now that all gaps have been set at .012-.014. ( Rotax recommends setting tighter rather than on the high side for better operation. They also wonder why we are checking gaps as they are set at factory). Hopefully the work I just outlined will produce the desired result. Will update post after next test.
Dick Seiders
Click to expand...

Dick,

This thread is really worrisome to me, because I followed the instructions precisely, as we are always admonished to do. Didn't say anything about pulling the cover, so I struggled through checking the gaps behind the cover. Found one out of spec (too tight) so called Van's and they told me to change the gap. I sure hope I didn't change it wrong, because I won't find the problem until the first mag check. You say the Rotax people wonder why we are checking gaps. Well, that ought to be fairly obvious to them, since the plans tell us to do it! Also - where does it say tighter is better? The one that was out of spec on my engine was .010, and Van's told me to change it.

It appears there is different info coming from Van's and Rotax - not a good situation. If I have to pull the engine because of this I am going to be pissed, to say the least.

How much stuff did you have to undo to pull the engine sufficiently to get at the trigger coils?

John
 
John, I just spent 30 min writing you a response and when I tried to submit it the site told me I wasn't logged in. Not so, but lost all I wrote anyway. Happening a lot lately. Give me a call and I'll be happy to tell you what I've learned.
Dick Seiders 770 377 8342
 
Aren't they run at the factory?

I did a quick check of mine and did remove the cover when I did so. However, I did not check them all that carefully as I was also unsure why we were doing this. I could not find anything in the Rotax installation manual that suggested doing so and I thought all of the engines were bench tested anyway just like a factory new Lycoming or good overhaul would be. Mine sure looked like it had been run given the oil and coolant in the lines. Hard to imagine they would shift during shipping- should be less vibration and jolting during shipping that will occur during operation on the plane.

Any other RV-12 builders with prior 912 experience had to do this with prior installs?

Jeff
 
jte65 said:
However, I did not check them all that carefully as I was also unsure why we were doing this.
Click to expand...

The primary reason is because the manual directs the builder to remove one of the mounting screws for an upper trigger coil. It is done as a preventative check just in case it moved while the screw was loose.
The secondary reason is because I have experience with a brand new engine that from the factory had one of the trigger coil gaps set right at the maximum limit. This caused intermittent firing problems that were corrected by readjusting that trigger coil gap near the minimum end of the range.
 
Scott,

Thanks for weighing in on this. Do you think it's possible to check the gaps while the engine is mounted? I just got past that step and the fuel and electrical hookups, but not the exhaust and other systems. After all this discussion I sure would like to re-check before I proceed with installing all the rest of the accessories.

John
 
Scott, that is the first good reason I have heard for at least checking the top coil. The others? Well while you're there I guess it makes sense with the cover removed of course.

Big John, I have input for you if you call as I suggested yesterday. Checking coil gaps with engine mounted is not a possibility. Either gamble you did it right or if you are going to lay awake nites worrying (like I did) then pull the engine and check. It's your call.
Dick Seiders
 
BigJohn said:
Scott,
Do you think it's possible to check the gaps while the engine is mounted?

John
Click to expand...

Yes it is possible, but it is much more difficult.
All of the work done on the one engine that was running poorly was done with it on the airplane.
Considering the errors that some builders have had measuring with the engine removed, I would suggest great caution if checking with it installed. At this point it might be better to just run it and see how it performs. If it does not run well, then decide what how to approach checking the gaps.
 
A decision - pull the engine now and check gaps

Thanks to Dick, Scott, and all the others for advice and comment, and to Dick for the phone call.

Since I have only made the electrical and fuel connections, it will be fairly easy to pull the engine back off at this point, and for my peace of mind that is what I am going to do. Much easier to do now, before exhaust system, cooler, radiator, coolant, and oil is added to the assembly. This time I will pull the cover, per the new instructions, and check all 5 gaps. Dick says the Rotax tech told him to aim for .012", and even as close as .010 is OK, so that is what I am going to do.

I'll post a report after all this is completed.
 
Clipped all the cable ties, disconnected fuel and electrical, except for the big generator cable, had enough slack there, and pulled the engine. Removed cover and found 4 out of 5 coil pickups at .012, but one gap was wide at .016 plus. Now they are ALL at .012 and the mounting screws re-torqued. Cover back on and engine back on the mounts. Still have to re-connect fuel and electric. Two hours total. This time I'm waiting until the bitter end to tidy everything up with cable ties!

As it turns out, I probably didn't need to do this, but now I have peace of mind.
 
Two hours was well worth it John if only for the peace of mind. Glad it went well. Took me 6-8 hrs to pull a fully completed installation and after 10-12 more am ready to purge the oil system again. Good luck with your project.
Dick Seiders
 
This site is getting to be a pain . About half the time one attempts to submit a thread or response the site drops your log- in losing the message.
I wanted to update the rpm drop case on my 12. Since I am concerned it will happen again I will abbreviate the rewsponse.
Test was satisfactory with consistent drops from 4000 to 3850 on both A and B.
Be very particular in setting the gaps as ir is critical Take cover off to do it.
Dick Seiders
 
dick seiders said:
This site is getting to be a pain . About half the time one attempts to submit a thread or response the site drops your log- in losing the message.
I wanted to update the rpm drop case on my 12. Since I am concerned it will happen again I will abbreviate the rewsponse.
Test was satisfactory with consistent drops from 4000 to 3850 on both A and B.
Be very particular in setting the gaps as ir is critical Take cover off to do it.
Dick Seiders
Click to expand...

Good news Dick! Now get that baby in the air!
 
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