912 missing carb?

[mcomeaux53]

Having a mystery with running.
No problem with run-up and takeoff.
After approx 30-40 minutes into flight feels like vibration/miss.
Pull throttle back to 3500-4000 rpm for brief moment then advance to 5200
And it returns to smooth operation for short period then again stumbles.
Any ideas? New plane and not much experience with the 912.
Motor has 360 hrs

Mike

[avidflyer]

my first thought is you are burning more gas than it's getting... Just a guess and first thought though so take it with a grain of salt. Also, it's 11:30 PM Jim Chuk

[kmach]

Any work done before the problem ?

I think the carbs should be checked for debris contamination in the float bowl, proper float level, cleaned and ....

after , synchronize the carbs well .

Check the whole fuel system from tanks to carbs, partially blocked filter somewhere? fuel line "flap" ?

Do you have a fuel pressure gauge , if so what is it showing ?

If you have a backup electric fuel pump , does turning it on help ?

Just some things that come to mind

[mcomeaux53]

I just purchased this aircraft recently. We synced the carbs today and I will
Fly tomorrow and see if any changes. Just weird how it runs so smoothly first 30-40 minutes before she acts up. I'll post my results as I work through it.
Thanks for the input. Great site here.

Mike C.

[Danzer1]

Mike,

Have you considered doing further testing static?

Syncing the carbs likely will not produce a positive result unless they were way way off.

Have you inspected the plugs - this is where you can tell what your fuel delivery condition is. White, sooty, dry, wet, black and on which plugs and where on the plug. It's the 1st thing I'd do before flying it!

Do you know what the CHT's and EGT's and RPM were just prior to the problem and what they were after the problem develops?

A rich condition can take some time to show as the engine heats up. I suspect that might be the case here.

Greg

[Eric]

Hello

Maybe a problem into the ignitionsystem? failing sensor / coil or something

[Av8r3400]

How old is your engine? Have the carb boots ever been replaced?

This was one of the pieces that helped sort my vibration issues. I got mine from JBM Industeies. Superior in build to the oem Rotax units at about half the price!


http://jbmindustries.com/912FLANGE.html

[Danzer1]

I agree the carb flanges should be checked - I never understood why Rotax made them the way they did.

If the flanges were bad, as the engine heats up, it could cause expansion and air leakage around the flange causing more airflow than desired. This would cause a lean condition.

If that were the case, it would be nearly impossible to pneumatically sync the carbs and rough running would be noticeable anytime the engine is warmed up.

While I'm thinking about synchronization - I have always disagreed with the Rotax manual in this regard. They state to do mechanical sync 1st then pneumatic "if required". I believe pneumatic is ALWAYS necessary for a few reasons.

1. Mechanical only sets the idle stops.

2. I've never yet seen a multiple carb set up really synced unless done pneumatically. It can be dramatic!

3. You need to check the sync throughout the throttle range and that can't be accomplished mechanically.

A few of the most common pneumatic sync mis-steps are:

Not doing it with the engine FULLY warmed up to operating temperature.

Not checking the sync through the full throttle range.

Not checking the sync after carb work. It should always be done anytime a carb is removed and reinstalled.

Not checking the sync after carb adjustments. Any adjustment can (and usually will) affect the sync.

Greg

[mcomeaux53]

I synced carbs today motor ran much better and no stumbling.
After digging in a bit prior owner did very little in regards to
Maintenance. I'm going with all the considerations. My engine has
A aluminum tank mounted to carbs and air filter attached to this.
There is two ports on under side with nothing connecting to them.
Is there a Rotax site that would help with service & identify items?
Thanks for all the suggestions. The motor is 2003 912S build.

Mike

[kmach]

I use Rotax-Owner.com , it has helped me with the 912 learning curve.

They have free downloads of maintenance and repair manuals , parts list . I think the parts list would help you out with identifying items.