912 missing carb?

[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.

[Av8r3400]

Sounds like the aluminum "tank" is the OEM intake plenum.

[Slyfox]

arm yourself with some carb or brake cleaner. remove the bowls and blow everything out good. If you don't know the carb than take everything that has a hole and shoot into it. reinstall the bowl. you generally don't need to worry about the gasket it's pretty tough. I would consider replacing all the black hose in the fuel system. seems it don't last long. yesterday morning after an hour flight when I was coming in to land I smelled the familiar gas smell. I got her back in the hangar and looked around the plane, than all the sudden there it was some fuel dripping and doing it good. I removed the cowl and right under the right carb was the leak. I put those lines on about 3 years ago ,but they needed new again. last night I flew it after putting new on and it ran real nice, wondering if the inside of the hose had an issue also. thought it might have been a fluke. went out again this morning, wow, better than it has run in a while. I think the fuel line had a problem. so change them, just do it and blow out the carbs. good luck.

[mcomeaux53]

I subscribed to the Rotax owner web site best 29.95 I've spent.
I've learned a lot in the last 24hrs this site thanks to all.

Mike