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.

[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.

[Dave S]

Mike,

I am basing this on your explanation of the airbox on the back of the engine....most people do not have it so it is a little new to most.

1) There is a photo of a Series 7 with 912ULS and the Rotax airbox installed.

2) If you have the airbox, and there is nothing connected to the two ports on the bottom of the airbox, the engine is running completely on UNFILTERED air......chunks as big as the openings of the ports can get sucked in - rags, bird parts, anything.....

One port is supposed to be connected with SCAT tubing (the red stuff) and would normally run to the right side of the NACA cooling duct to connect to an air filter inside the cooling duct just in front of the Radiator on the starboard side - this would be your normal cold induction air. The other port is for alternate air or carb beat - if alternate air a simple screen over the port would be a normal option to keep the lumps and chunks out of the engine. If carb heat...which I doubt you would have on an S7 - the second port would hose to the carb heat pickup. There is a device on the back of the airbox to switch from normal to alternate air function which would normally be connected to a cable going to the cockpit so you can switch between normal and alternate. The second photo of the panel shows where the actuator knob (silver thing) is located to the right of the throttle. The last photo shows where the hole in the NACA cowl is for the aluminum fitting that connects the air filter to the SCAT hose. This uses a single K & N filter of a different configuration and a larger capacity than the ones used on the individual carbs.

If the plane has been running on unfiltered air for who knows how long.....there is a whole bunch of possible things that could be going on starting at the airbox through the carbs, etc.

Most people do not use the airbox....cost mostly....but, it has a couple advantages....1) the carbs can't fall off and the flange rubber is pretty well protected from unusual forces that could damage it. 2) Rotax claims the 100 HP of a 912ULS cannot be realized without the induction airbox.....without it it's suposed to be limited to 95 hp...

Hope that helps some

Sincerely,

Dave S
KF7 Trigear, Flying
912ULS Warp Drive

[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

[Rowdy1]

I am also new to the 912. I bought a kitfox 3 about two years ago and had a roughness in the mid range. I checked syn mechanically and it is good. I found an article on the importance of the carb vent tubes being 120mm and routed between the carb bowl and the retainer spring. This significantly helped it above 2800rpm. Then I sitll had a roughness between 1900-2600rpm. I checked the idle mixture screws and they were 1 1/2 turns out. I adjusted the idle mixture and it smoothed out the rest of the range. Vacuum syn is the best but the vent tube and the idle mixture adjustment made a huge difference. Hope this helps.

[Paul Z]

I concur with Mike join the Rotax web site a lot of good maintenance and installation how to videos.