912UL Engine vibration on throttle back

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

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Re: 912UL Engine vibration on throttle back

by Paul Brydon » 12 years ago

Paul Brydon

3 34 0

I am starting to think my throttle back vibration is something to do with the cables. A couple of months or so back I had a throttle spring go west on the left carb forcing me to land with the engine at idle. When I replaced the bracket and spring I noticed that the spring whilst looking identical to the right carb one actually had palpably higher tension in it. It seems since then that the vibration issues have started. My vibration can occur at different rpms but usually somewhere in the range of 4000-4800. WOT for me is 5750 straight/level, 5500 in climb. It mostly happens during throttle closing from a climb but has also happened with throttle opening. It lasts seconds (a few to 10 or so) and usually goes with closing throttle completely and re-opening. Idles fine in flight at about 2300rpm, 1800 on ground. My throttle cables are probably a little heavy at 2mm with only gentle curves to the outers and kept as short as possible. They are however not the same length, the right carb being slightly longer. Interestingly, the weaker spring is on the longer cable. I can reach under my dashboard and close the carbs manually individually (have to undo the seatbelt though!) and seems that I can reproduce the vibration, especially if I close the right carb a little. An interesting observation I have made whilst fiddling on the ground - if I close the right carb whilst running the engine at 4000rpm there is more vibration then if I do the same to the left carb. When my spring went west in flight, it resulted in the left carb closing to idle when I worked the throttle (not knowing what had happened) and even whilst there was not enough power to maintain flight at full throttle, the amount of vibration was not as much as I expected there to be. I suspect it would have been a lot more if the right carb spring had gone instead. Any easy explanation for this observation?

I flew today after torquing prop bolts, cleaning prop in attempt to diagnose problem but made no difference. The next time I go I will swap the springs over, maybe a tighter spring on the longer cable and the right carb may change the picture. Failing that, in the new year I will make up new throttle cables with the lightest weight cable I can get and will report back. Comments appreciated.

Paul

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Re: 912UL Engine vibration on throttle back

by mike hallam » 12 years ago

mike hallam

3 40 0

Kevin et Al,

I think one should consider the differences mentioned above between simple rough running & seriously rough whole engine shaking the airframe & virtually no power.
My own case 912 80 hp took a long time or doing things before it was 'cured' since then it's remained 'good' to me. I didn't fiddle after that !

Kevin has bought & fitted new neeedles & needle jets, like I did, but unfortunately no improvement. Did you try the WOT effect when it runs rough, as this was my only in flight palliative till the needle/jet 'cure' came along ?

I have to say without any of the 'standard solutions' working for me I'd begun to think due to possible over rich mixture - perhaps ever since new - there'd been an accumulation of carbon in the worst (lowest compression when prop turned) cylinder. Front left cylinder was either self-firing maybe soft carbon deposits in or around the exhaust valve stem or a valve sticking open again either carbon on the stem when less loaded ??
Nothing made sense but the next possible on my mental list was to get this weakest cylinder removed to check that out. Luckily it wasn't necessary.

I'm now anxiously awaiting the next mails from readers in case my own solution is on a knife edge !

Hapy Christmas,

mike hallam, England.

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Re: 912UL Engine vibration on throttle back

by Kevin Stewart » 12 years ago

Kevin Stewart

5 184 0

I would say that my rough running problem was marginal to start with and has gradually got worse over the period of about 12 months. It has become far worse since I have been investigating the problem by stripping the carbs.

When I flew a week ago it was not so bad that I was happy to be back on the ground. That said, when I was experimenting yesterday with the gauges on and the balance pipe disconnected it was so rough that I would not have flown it in that condition. Refitting the pipe did return the engine to its previous state of roughness.

In answer to Mike's question, if I get the roughness in flight, opening the throttle fully clears the problem but as soon as you throttle back to less than 4000rpm it starts again.

What I did neglect to report is that I pulled the top plugs from the rear cylinders and the ceramic insulator was white. The inside of the metal threaded section had a light covering of carbon. I expected the central plug nose to be at least light brown if not black. This suggests that my mixture is lean and not rich.

I have some measurements that I previously made from the float bowl mounting flange to the float lever arms and I think they were both in the region of 10.5mm+/-0.2mm so I guess the levels are OK.

My main needles had the circlip in the second groove up (position 3 as per the Rotax manual).

It would be handy if you could insert a 4 gas analyser in the exhaust from each pair of cylinders to see what was going on.

Kevin

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Re: 912UL Engine vibration on throttle back

by Roger Lee » 12 years ago

Roger Lee

127 5.21k 4.79k

Hi Kevin,

You could still have a bad cable. With that I mean it may have a little friction at just a certain area or a little slop in the cable. Something in the throttle system that does not allow the cables or throttle system to stay equal through out its range. Is the rough engine problem repeatable on the ground? If it is then start it up and take it up above the rough part. Then reduce throttle until it runs its roughest. Then put your finger on the carb throttle arms one at a time and pull it back. If you can smooth the roughness out with one or the other carb then you have either found your problem or at the very least eliminated it as the problem.
What are the chances for an air leak? I forgot, have you ruled that out?

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

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Re: 912UL Engine vibration on throttle back

by Kevin Stewart » 12 years ago

Kevin Stewart

5 184 0

The cables seem to move freely without friction and there is no kinking. The rough running is repeatable on the ground so I can do as you suggest with the throttle arms, that is an excellent idea.

I have visually inspected the various component interfaces for signs of an air leak. The main carb rubber mounts are both in good order, I fully removed them for a thorough inspection.

I will take a can of carb cleaner with me next time and test for leaks with it running.

I think that I will be printing out this thread to take with me to the plane ;)

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912UL Engine vibration on throttle back

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