zenoah 26 problem

[Craneman]

I have a Bergen EB inttepid that i just finished breaking in (two and a half gallons) it has flown great until now.

I am now testing it with an underslug mount for AP it weighs 6Lb, the problem is after flying for maybe a two minutes it begins to loose rpms until it just wont fly!

During the first two minutes it flies great! climb outs are ok and rpms sound fine!

Here's a video of the problem
http://www.youtube.com/watch?v=1oo3LMKo8xA

Here's what I have done to try and fix it, checked the plastic isolator between the carb and engine for airleaks (Chris bergen suggested I get an aluminium isolator, I probably will) but it seems ok not warped or cracked, checked the muffler, its nice and tight.
I opened both needles two two turns and went down from there until i got to 1 3/8 turns on both needles. the problem persists no matter what the needle settings are.
Also tried playing with the pitch curve... nothing...

Head speed is 1,600
Ohh, and I'm flying at 8600 feet above sea level.

The heli flies great if not loaded, even if being pushed quite hard.

I'm probably doing something wrong as I new to gassers any ideas?





Please any suggestions would be great.

[Craneman]

Just re-installed the engines stock muffler.... same thing.

[KarbonBird]

I have a GSR 260 with a PUH26 motor so there may be some similarities.

The motor may be running a bit rich - pull the spark plug out and post a pic here. That will tell a lot.

My guess is that your pitch curve is too high. Have you started with a mechanical setup on -6, +6 and +12 for bottom, mid and top stick?

Once that's done, work on your throttle curve and tach according to the required RMP and it should be good to go.

It sounds like it's nearly there so its only fine tuning required now.

[crashnwreck]

I have a gs voyager running the puh26, I was running 40:1 fuel oil mixture, great if your less than 100 metres above sea level, I flew it on a mates farm just under the mountain at 1500 metres above sea level, it seized up on me, I now run 30:1 and seems fine at all levels.