914 vibrations

[rhtrudder]

Noticed Engine vibes  between 4500 and 5000 , above and below, all good, Carbys just done , gearbox recently done, could it be prop balance 

[facthunter]

Worth checking. Nev

[IBob]

I would be doing a simple check on the balancing of the carbs: someone (here?) recently said that the main problem they had come across with balancing Rotax carbs was poorly installed or maintained throttle cable/lever systems.

 

I do a static setup (engine stopped) where I push to full throttle, and check that both throttle levers have gone to the full throttle stops on the carb bodies.

I then very gradually close the throttle, watching to see both throttle levers come off the stops at exactly the same time: I do this several times, as it's hard to look two places at once.

I then check the same for throttles fully closed, but it is the fully open I am focused on, and any adjustment of the cables is done to get simultaneous action there.

 

I have done this from new, and have very smooth running: several minor adjustments were required in the first 40 or 50 hrs, but none since then.
I also keep all throttle linkages and cables clean and lubricated to maintain free movement.

 

[rhtrudder]

Have replaced the throttle cables with good quality ones, smooth operation, carby balance looks good, at 5000 revs the butterflies would be nearly fully open , engine revs to redline on takeoff, has a inflight adjustable, if I run it at 5300 it’s good but when I pull it back to 5000 it’s very noticeable , I thought if the prop was out it would vibrate through the whole range, I’ll get it checked anyway 

[IBob]

It was just a suggestion for a very simple check that has worked well on my 912ULS.
If it was vibration free before (?) I would be looking at what has been altered/replaced in the work you mention.

[skippydiesel]

3 hours ago, IBob said:

I would be doing a simple check on the balancing of the carbs: someone (here?) recently said that the main problem they had come across with balancing Rotax carbs was poorly installed or maintained throttle cable/lever systems.

 

I do a static setup (engine stopped) where I push to full throttle, and check that both throttle levers have gone to the full throttle stops on the carb bodies.

I then very gradually close the throttle, watching to see both throttle levers come off the stops at exactly the same time: I do this several times, as it's hard to look two places at once.

I then check the same for throttles fully closed, but it is the fully open I am focused on, and any adjustment of the cables is done to get simultaneous action there.

 

I have done this from new, and have very smooth running: several minor adjustments were required in the first 40 or 50 hrs, but none since then.
I also keep all throttle linkages and cables clean and lubricated to maintain free movement.

 

No argument with checking the throttle linkage movement through its full range IBob but remember,  Rotax carburettors use a "balance tube" specifically to smooth out/accommodate any variation in throttle opening AND (no expert) but variations are most likely to impact on low power/idle engine smoothness.

Lubricating modern Bowden type cables, is likely to lead to sticking cables ie in most instances (there can be exceptions) they are constructed/designed not to require lubrication 

 

 

[IBob]

Good points, Skippy.
Regarding the balance tube, I'd be curious to know the specific function it was designed for. It is a skinny little tube and I find it hard to imagine much mixture travelling through it: I don't think I would be relying on it to compensate for mismatched throttle openings.

 

As for the Bowden cables, I'm sure you're right, though my kit came with DIY cables you cut and thread yourself, and I doubt they are very high tech. I should probably have added that I am looking for smooth free operation of the levers and cables when checking them, and especially as the standard setup is only positive when closing the throttles: on opening, we are relying on the carb throttle arm springs to pull the cables.

[skippydiesel]

The balance tube does not transfer significant fuel between inlet manifolds - it balance's induction pressure (vacuum). This is why the carburettors must be isolated from each other when doing a vacuum balance ie its not possible to do an effective  balance with the tube connected.

 

Your throttle set up (opening depends on return spring on each carburettor) is the same, in principal, to my last & current aircraft. Anecdotally (myth?) I have heard of pull/push type throttle cable's.