keithhickling(at)clear.ne
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 Posted: Mon Mar 02, 2009 12:18 am Post subject: mag drop on both mags, 912 uls |
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Hello all,
Many thanks for all the really helpful suggestions and information. First, to answer some questions:
The carbs are statically balanced as well as I can judge. I haven't done pneumatic balancing yet. I am using BP 98 octane mogas. Trying the mags with the airbox off gave the same result. Haven't checked the timing, but I understood it is not adjustable on that engine.
Today I worked through the Rotax carburettor troubleshooting guidelines. Running at 2200 rpm and gradually opening the choke gave a continuous increase in rpm up to 3000 with progressive increase of choke, and progressive increase in EGT. No decrease in rpm as the choke was fully opened, (as the guidelines say I should see after 400-600 rpm increase initially). But doing the same with the airbox off I did get a decrease in rpm after an initial increase. The air filter is new and clean, no obstruction in the box.
The guidelines say to try the mag drop at 4000 rpm with the choke half out - this made no difference to the mag drop, suggesting they say that it is not due to a carb problem. But I did try raising the needle 1 notch in both carbs, which also made no difference, so I put them back in the original position (position 3). So we concluded it is probably not a carb problem.
Next I checked the trigger coil gaps - 2 were 8 - 10 thou, and 2 were 12 - 15 thou (it is difficult to get the feeler gauge in completely square because they are not too easy to get at with the engine installed, but I was fairly happy they were in those ranges).. The manual gives a range of 12 - 16 thou. Anyone know whether 8 or 10 thou instead of 12 - 16 would be enough to account for the problem ?
Much of the rest of the troubleshooting guidelines consists of switching various leads between the 2 ignition modules to see if the mag drop moves to the other mag, but with a similar drop on each mag this will not help. Tomorrow I will check out some of the resistance values for the coils, leads etc and see if anything else comes to light.
One of our builders here suggested, as some of you did, that it might simply be a stiff engine that needs a bit more running in, so if nothing else turns up I might just run it a bit more and do some taxiing.
Many thanks for all the help. Any more suggestions very welcome.
Regards,
Keith.
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