Rotax 377 strange throttle response

[j_omega]

I recently finished assembling a Quicksilver MXL with a Rotax 377. The airframe is ready to go, but the engine is not responding properly. I will slowly apply power and the RPM climbs steadily as expected. The tach reads ~4k RPM just under half throttle. Then right around half throttle, the engine hits a point where it suddenly jumps to redline levels (8k+). If I push the throttle past the half point, it will come back down into the 4k RPM range. No matter what I do, I can't get it to the full power level (6k RPM).

Any ideas of what is happening?

[Infidel]

Call Lockwood. I don't recall what time of the day they do it, but there is a designated time you can speak with a tech/mechanic. It's been awhile since I contacted Lockwood for anything but it should be listed on their website.

Hope this helps as I don't have much to offer on 2-strokes.

[dusterpilot]

Sounds like a Carb issue. I assume you're using the Bing 54? Do a YouTube search for "bing 54 carb." There's a real good series of videos on it that show you step by step how to clean, adjust, and rebuild your carb. A good cleaning and adjustment should do it.

[1600vw]

To me it sounds like an induction leak somewhere. There is no way an engine can go to red-line rpm and not move the throttle or keeping the throttle at partial opening. The only way this can happen is from a leak. Then when you open the throttle more the jetting takes over. I bet if you look at the jetting and needle jet they are not what the factory calls for.

Sounds like an induction leak, failed seal somewhere or cracked carb boot. Rotax recommends every 5 years to have all seals and rubber parts replaced. A new engine on a shelf after 5 years goes back to the factory for all new seals and rubber parts like carb boots and such.

On a two stroke a failure like this will cause you to ruin an engine if it happens in flight. On a 4 stroke you do not have this happen. One reason I like 4 strokes over 2 strokes.

Tony

[j_omega]

I've been looking into what it will take to rebuild the carb. In the process I noticed that my carb is missing the tubing shown in the picture below. Could this be my issue?

[dusterpilot]

Yes, you need those vent tubes and be sure they have holes in the bottom of them.
Here's another good source of info: http://www.ultralightnews.ca/bing/bingservice.html

The vent tube can be made from a piece of primer line and putting two side by side holes in the BOTTOM of the line in the center is important! The holes are about 30% of the diameter of the primer line. Too small and they will not allow the carb to breath properly.

[j_omega]

I opened up the carb today and found the main jet in poor condition. It wasn't completely blocked, but there was noticeable build-up inside.

Everything is soaking in a bucket of carb cleaner...

[j_omega]

So I got everything cleaned up and it doesn't seem to fix much. I took a short video today illustrating the problem

[1600vw]

Sure sounds like an induction leak. In watching the video what I would have liked to see was the throttle. Looking at the gauges and listening to the engine we just hear the engine. I would like to see the amount you are moving the throttle as this happens. But it sounds like an induction leak. Remember between the two cylinders on the crank is a seal. If this seal fails it will cause problems. I mention this so if you replace all seals and still have this problem it could be the seal on the crank that is bad.

You have an induction leak I believe anyway.

Tony

[j_omega]

Alright, I figured it out. The belt between the drive shaft and prop was loose. Thus, it was slipping under high enough torque. After tightening the belt, the problem went away.