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JohnF
Joined: 13 May 2010
Posts: 124
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 Posted: Thu Jun 07, 2012 11:03 am Post subject: Rotax 912 ULS Strange Problem |
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Taking Roger Lee's advise I removed the air filters, removed the oil with detergent, dried them, put a shop vac on them and paper held nearby was snapped on the outside and held tightly, so I know they are moving air freely.
Re-gapped spark plugs to 0.027" VERY carefully. Reinstalled with correct torque and heat transfer paste.
No help; still way too much mag drop and can't get even close to static rpm I want. Mag drop 330 and 360 rpm ..static rpm WAY too low. RPM decreases if throttle increased past 4000 rpm.
The new Rotax fuel pump shows 1.75 psi more fuel pressure than the original pump did, so I reinstalled to old pump...ran the engine only to find there was a serious fuel leak at one of the stems..(a new event) ..reinstalled the new fuel pump.
Ran the engine...mag drop 100rpm on both sides, static rpm 5050
This happened once before; that is, everything looks well, but next morning it is same old problem.
Non-contact laser thermometer shows heads all within 2-degrees F after engine was run. Differential pressure check like new on all cylinders, each within a pound of each other.
CarbMate carb balancer show carbs balanced as near as that instrument can measure...really close.
Adjustable prop blades set exactly at same on both sides...each reads the same to 1/10th of a degree.
Cylinder head temps on Dynon D-180 show normal and about the same - typically around 175-degrees F
EGT on Dynon also normal...didn't run engine long enough to see final temp, but they are generally just fine...1200 or so - both about the same even though not fully warmed up
All throttle and 'enrichment' cables, and settings and stops are normal; springs OK, nothing has moved.
Added 8-gallons of fresh fuel just in case that was somehow involved (though remote)
I gave up working on it for today...will re-cowl the engine tomorrow and see if it stayed "OK"
John
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Thu Jun 07, 2012 3:05 pm Post subject: Re: Rotax 912 ULS Strange Problem |
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Damn good report, John.
First most static rpms are usually around the 4850-5000 for a WOT rpm flat and level rpm around 5500-5600 in flight. This varies a little from prop to prop. So the static is fine. I would fly it.
Is the 300 rpm drop only on one mag or both? Are the mag drops on each equal at 300 rpm? Is the engine smooth on the mag drop check? If it is smooth and they both drop 300 rpm I would suspect something with the read out and not the engine itself. If one is 60 rpm drop at 3500 rpm and the other is a 300 rpm drop at 3500 then there is a loss of one plug.
Where are the carb vent lines attached?
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Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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Dick Maddux
Joined: 24 Jun 2008
Posts: 516
Location: Milton, Fl
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| Posted: Sat Jun 09, 2012 4:45 am Post subject: Rotax 912 ULS Strange Problem |
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Say, Roger brings up a good point.
I once had a problem with a mag drop .I checked the gap on the plugs and they were OK. The plugs were brand new but I decided to "bomb " check them with a friend of mine's plug checker. Sure enough,one of the plugs would not fire when the air pressure was increased. I reinstalled another plug and that solved the problem.
You might give that a try.
Dick Maddux
Milton,Fl
912UL
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Roger Lee
Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.
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| Posted: Sat Jun 09, 2012 5:12 am Post subject: Re: Rotax 912 ULS Strange Problem |
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Hi Guys,
This can't be happening on just one side like a choke open, out of sync carb, one bad plug or one bad plug wire. What ever is going on affects both sides equally so the problem has to be common to both sides.
What has me going a little is usually you see only one mag drop at 300 rpm then it is one plug.
These mags for the most part are equal, so it can't be just one plug or one mag drop would be okay and the other 300. It could be a coil next to the ignition modules because a coil fires two plugs. That would make the engine rough and make the drop equal at 300-360 on both sides.
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_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056 |
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