Rotax 912 ul, severe vibration

by Jorge in 912 / 914 Technical Questions 8 years ago

Page 3 of 6

Start
Prev
1
2
3
4
5
6
Next
End

Re: Rotax 912 ul, severe vibration

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 240

Just to be clear; the current Kitfox ownership did not have anything to do with the design of the model 4.
But if someone came out with a new engine mount with standard intake manifolds, airbox and throttle cable system that would be great!
hint, hint.

Report
Re: Rotax 912 ul, severe vibration

by Jorge » 8 years ago

Jorge

1 10 0

Hi,

The rubbers are new ...

For sure, the engine setup in this models is far from ideal but it will require major engineering to correct all these problems related to the inverted manifold ... engine mount, oil tank etc. Way to complex for me !

I could make a plan with the throttle and go for something with less slack.

However the engine was running jet-like smooth with this same configuration at the beginning until something triggered this vibration... I agree that the installation is not helping, but I don't think is would be the cause.

Many thanks,
Jorge

Report
Re: Rotax 912 ul, severe vibration

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 240

Not sure I understand you: the throttle cables need more slack, not less; a nice "S" curve so they flex instead of pulling on the carb. Remember this engine mount allows the engine to move/shake, a lot.
You can get very good high-quality bicycle teflon control cable that is very light and flexible.
Some guys have crossed the cables from the kitfox johnson bar: The left cable goes to the right carb and the right cable goes to the left carb. This gives them nice flexibility. Do not tie them to anything that may interfere with slack.
You probably don't have a single big issue, more common is stacking of small issues like carb sync, worn/cracked carb sockets, loose engine mount rubbers, out of balance prop......
You need to address them all to get anywhere; it's pretty hard to sync the carbs when they are shaking so bad fuel is pouring out of the float bowl vent.

Report
Re: Rotax 912 ul, severe vibration

by Jorge » 8 years ago

Jorge

1 10 0

Hi Rob,

my bad, with slack in the throttle system I meant that the bellcranck mechanism is not that precise and adds up too much variables or "play" for a perfect fine tuning when synchronizing the carbs.

My throttle cables have enough slack so to allow the free movement of the engine running without pulling or interfering as you say. Good to know about the crossing of the cables.

Carbs sockets are new, even when the previous ones were inspected and free of cracks.
Prop was checked for pitch and statically balanced. I tried with a different one and the vibration persisted.
Mount rubbers are new ... however I still have that SB to be performed to stiffen the rubbers.

The carburetors synchronization was done several times without any improvement. The last time we tried to sync them, when the vibration got worse, it was impossible to get a proper reading in the gauges.

Forgot to mention before, I observed that sometimes on the first start of the day the engine will run smoother for the first 7-10 seconds until the real shake kicks in.

Report
Re: Rotax 912 ul, severe vibration

by Rob Seaton » 8 years ago

Rob Seaton

3 1.32k 240

A cold engine prefers a rich mixture, that may explain why it is smoother at start up.
It is probably the shaking that is putting raw fuel into the intake but maybe consider:
-are the idle and mainjet atomized ports clear? (too much airfilter oil can plug them up)
-Are the float valves sealing well? do they pass the bench test as shown in HMM chapter 73-00-00 page 24?
-What are the needle clips positioned at?

Report

Page 3 of 6

Start
Prev
1
2
3
4
5
6
Next
End

You do not have permissions to reply to this topic.

Title	Category	Replies
912iS Stator Failures	912iS Technical Questions	0
Rotax 916	General Tech Discussion	2
912 ULS Crankshaft Nut Torque Question	912 / 914 Technical Questions	10
Roger Lee 912uls	912 / 914 Technical Questions	17
912 S2 Maintenance Manual Line	912 / 914 Technical Questions	3

Rotax 912 ul, severe vibration

Recent Videos

Safety First!

ROTALK News

Prize Giveaway!

Log In/Out

Login