912UL Engine vibration on throttle back

by Kevin Stewart in 912 / 914 Technical Questions 12 years ago

Page 11 of 39

Start
Prev
6
7
8
9
10
11
12
13
14
15
Next
End

Re: 912UL Engine vibration on throttle back

by Kevin Stewart » 12 years ago

Kevin Stewart

5 184 0

Hi Paul,

I feel your frustration, it sounds like your issue may be very similar to mine. There is another Eurostar in the hangar at Bodmin which has the same problem. I have flown in it and the symptoms are identical to mine. Fine under high cruise power but vibration when the throttle is reduced.

Here are some thoughts that might help you out.

Always suspect what you last changed, so I would look very carefully at your newly installed drip trays. I used to check for air leaks on cars using a can of volatile carb cleaner. If you sprayed over the area of an air leak the revs would noticeably increase. That said the symptoms were normally present at idle.

If your fault only manifests itself when airborne then what is different compared to being on the ground: air density, heat, air flow through the engine bay?

Could heat be causing something to expand and cause problems? I am thinking of things like the throttle cables or the large piston body in the carb.

Are your float chamber overflow pipes positioned correctly and not into the prevailing air flow? Mine are about 100mm long and the open ends are tucked under the float bowl retaining clip. See the attached photo.

With regard to the throttle springs cutting through the mounting bushes, that really doesn't sound right. My Rotax has 700 hrs and there is no sign of the bushes being worn let alone split. Does yours have the correct springs fitted? If they were too strong that might lead to premature wear. Can you compare the spring force with someone else's springs? If I had mine here at home I would do a quick Hooke's Law experiment for you.

The other thought about your perishing bushes is: have they been contaminated with anything that could degrade the material?

With regard to the main fault there are basic diagnostic parameters that you could do with knowing when the fault occurs. Manifold pressure, throttle angle and exhaust CO content would be useful. I can imagine connecting a couple of throttle potentiometers to the carbs so that you could monitor their position in flight. I am not sure whether you could determine the individual manifold pressures with the balance pipe installed. As for the CO you would need 4 independent lambda sensors in the gas pipes in order to see what was going on. It may be possible to monitor the exhaust temperatures by attaching some thermocouples.

Kevin (England)

IMG_2348.JPG (You do not have access to download this file.)

Report
Re: 912UL Engine vibration on throttle back

by mike hallam » 12 years ago

mike hallam

3 40 0

Kevin,

FWIW a) My carbs (912 80 hp) are Pull On against the spring "just like a real aeroplane" !

b) On my other 'plane with simpler Bing 447 Rotax 2-stroke in winter cold I use the "choke" enricher sparingly to keep EGT down to the usual figure, as it bleeds in a tiny bit more fuel & the EGT is a good device, extremely sensitive & fast to react - seconds in fact.

Perhaps you too as a diagnostic tool, could - in the air (when it's not pouring with rain) use the 912 Bing choke circuit pulling just a fraction to change the mixture to a little bit richer. If it gets better - or worse - you at least have another pointer on carburation. It won't lean it off so is relatively harmless to the engine.
I know it's daft but as you are currently without any EGT monitor it might help beat your conundrum.

mike hallam

Report
Re: 912UL Engine vibration on throttle back

by Roger Lee » 12 years ago

Roger Lee

127 5.21k 4.79k

Hi Mike,

The 912 enricher won't affect anything over 3500 rpm. For those that have never tried it use the enricher on the ground after your engine is warmed up. There will be a significant change. Go fly and while at cruise over 5000 rpm put the enricher on again. Nothing will happen. It can be a diagnostic tool on the ground sometimes depending on your symptoms.

Paul, it still looks like you need to check for air leaks?

Just a helpful note:
All fuel and oil lines should be in fire sleeve. A fuel fed fire will burn at 2000F and consume anything in your plane and maybe before you get to the ground. It will burn much faster being fuel fed than just an electrical type fire. The fire sleeve will also help protect against abrasion or mechanical damage.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Report
Re: 912UL Engine vibration on throttle back

by Paul Brydon » 12 years ago

Paul Brydon

3 34 0

will look into it more after xmas, bit too peeved to be funtional at the moment. it's a shame i can't rememnber exactly when i first noticed it so could then tie it into what change (if any) i made. the drip trays seem the most recent addition.

is there meant to be some gasket seal compound between rubber carb mount and drip tray? on the intake manifold/drip tray interface there is the O ring that was originally between the carb rubber mount and manifold.

regards the throttle springs - the one that cut thru the bush/bracket after 60hrs was the original that came with the motor at new. the replacement parts came from the rotax dealer and i noticed that the new spring had even more pull. the new nylon bush on the new bracket already has an indentation from where the spring rests against it. the right carb now has the tighter spring (swapped over by me in case it would make a difference to the vibration issue, it didn't) and the nylon bush has now failed/split so have spring on bracket. i presume that means the bracket will now fail in time :-(
a friend with a 912uls parked next to me in the hangar checked his (about 300hrs) and the nylon bush crumbled in his fingers so he also had metal on metal. he has since replaced the bush at unbelievable cost (i think was over $15 nzd!). there must be a better way surely. i have thought about putting a plastic bead (like what kids always stick in their ears/noses and end up in the emergency dept) as then at least there will be the curve of the spring end against the curve of the bead, having the bead sit under/up against the bracket hole. i also wonder if rotating the spring 180 deg on it's vertical axis so that the upper attachment to the throttle lever has entered the hole from the outside rather than inside. harder to get on/off without taking carb out of drip tray but seems to align the pull of the spring at the lower point in a direction less likely to cut the bracket out. hope that makes sense. softer springs would be nice but would probably not open the throttle fully i imagine.

so in the new year i will ;

remove drip trays
check float bowel in left carb for debris/crud
check all engine mount bolts for tightness (they are safety wired so unlikely to have moved)
put lighter cables in throttle outers

and if still vibrating :

either sell my soul to the :evil: but if he doesn't want it, sell my left kidney for enough to buy a new plane

merry xmas and happy new year to you all B)

Report
Re: 912UL Engine vibration on throttle back

by bill martin » 12 years ago

bill martin

10 757 0

Paul,

Check the Iglide plastic bushing website. You can buy those $10.00 bushings for about one dollar each. They have every size from 1mm up and they will sell individually or in bulk.

Bill.

Thank you said by: Paul Brydon

Report

Page 11 of 39

Start
Prev
6
7
8
9
10
11
12
13
14
15
Next
End

You do not have permissions to reply to this topic.

Title	Category	Replies
912 Engine propeller gearbox - Interval Operating hours	912 / 914 Technical Questions	6
Weird Engine Roughness 912ULS	912 / 914 Technical Questions	3
582 Engine Block Bolt Torque	2 Stroke Technical Questions	3
912ULS 5800rpm in WOT?	912 / 914 Technical Questions	6
9912ULS Ignition Module Issue, maybe	912 / 914 Technical Questions	12

912UL Engine vibration on throttle back

Recent Videos

Safety First!

ROTALK News

Prize Giveaway!

Log In/Out

Login