Lack of Full Power Rotax 912 - Carburetor repairs.

[CrayonBox]

I have made a blue for sure... I am not as good as I thought I was (5 year rubber replacement).

 

I have replaced the diaphragms in both carburetors and repaired a throttle mount (thread worn and unreliable). Everything was just dandy last week before the dismantle.

 

I notice when I first ran it up, the idle was too low, and full power now only offers 4800rpm, which I believe should be more like 5500rpm.

 

I can only think that the diaphragms are in the wrong way - even though I am sure I aligned the notches.

 

I just watched the Rotax videos - no ideas there either. Looks like the diaphragms can only go one way.

 

I am wondering if I might have switched the sliders between carbs. There are holes at the bottom which connect the chamber to the airflow (vacuum) in the carb throat. I am sure these go towards the throttle butterfly end of the carb don't they?

 

Any other ideas?

 

 

[fly_tornado]

air leak?

 

 

[facthunter]

There is not much to tell you which way the diaphragms go in. Just a little projection on the rubber. Without it in front of me I hesitate to say much except the diaphragm may be restricting the travel of the piston. This will reduce airflow and make it run richer. Check for anything like that happening. see if there is any impediment to the piston going all the way up. If it is backwards the piston will not lift normally as it is only ported one side, and the reduced pressure lifts it, when the butterfly opens and the engines suction operates on the topside of the dashpot diaphragm. Nev

 

 

[ruffasguts]

check distribution spider the leg to the fuel pressure gauge is resticted i have seen this spider fitted incorectly with restriction supplying a carby cusing lack of flow hence lack of power easy to check remove hoses from carbies run aux fuel pump both flows should be the same

 

Mick W

 

 

[CrayonBox]

Ok all fixed. Yes it was my fault for two reasons...

 

1. Don't fix it if it aint broken.

 

2. I didn't take the care and time to clean the slider diaphragm sealing surfaces.

 

I really felt the need for maintenance because I wanted to check the health of the diaphragms, and after all, it is time for the 5 year mandatory rubber replacement.

 

So, the verdict seems to be that there was a hardened sealing material (grease?) on the surfaces where the diaphragms (both carbs) seal onto the slider and the compressing ring above it. Removing the old diaphragms broke the seals.

 

What I did...

 

Today i removed the sliders, everything had been assembled correctly. So i decided to investigate the sliders in more detail at home.

 

After dismantling the sliders, seeing the dirty surfaces, decided to clean and polish them. Which i did. I reassembled carefully after applying a very thin coat of grease to ensure a good seal.

 

After reassembly and resyncing the carbs, i took the trike out into a clear area for a good run up test. The dual rear brakes couldn't stop it sliding across the grass at a successful 5600+ rpm!

 

After a few more tests and holding 4000 revs, watching the temps etc, all is now fantastic.

 

Ready to fly again... Now where did i put that instructor.......

 

 

[daza]

Great to hear!

 

And on the bright side, you know your bird that much better now.