Weak ULS

by Marton Os in 912 / 914 Technical Questions 7 years ago

Hy guys!
I bought a used 912 ULS engine recently. It has very good mechanical conditions, good compressios, low hours, but it has power as a 80hp 912.
The sparks are a bit withey-gray, and if we use bigger main nozzle nothing change.
Any idea? Thanks!

Re: Weak ULS

by Roger Lee » 7 years ago

Roger Lee

125 5.12k 4.53k

Hi Marton,

How do you know and how are you measuring a 912ULS to think it is running down at 80 hp like the 912UL?

If your mag drop is normal and the engine can achieve its rpm then it should be fine and messing a round with the carb main jets may be self defeating and or dangerous if you don't understand what's going on. The clip on the main jet needle should be in the third notch down from the top. Your wide open throttle at your normal cruise altitude should be close to 5600-5650 if you are using a ground adjustable prop and of course that number will be different if you are using an in flight adjustable prop. If you have this prop over pitched then yes its performance will be terrible. Static rpm on the ground is usually a round 4750 RPM +/- depending on the prop. All props should be flown after a static set for a final adjustment. The carbs should be pneumatically synced. Compression test should be done and all okay.

Are all these things done?

p.s.
After the engine is hot and you shut it down is the prop hard to turn by hand?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Report
Re: Weak ULS

by Marton Os » 7 years ago

Marton Os

1 2 0

Hi! Thanks your answere!
At the airport there is 3 same fox aircraft and same fix props, with 912 ul and uls engines. But mine (with 100hp engine) is flying like a 80hp. The engine is running nice, 5100rpm at the ground, temperatures and pressures are good, and compression in cold and hot is good.

Suspicious the white - gray spark plugs.
We changed the inside of the carbs for new and syncronised but nothing change.
After we take the main jet needle from the third to the fourt position, but sparks are always light grey....
After we changed the main jet bigger (155-165-175) but the sparks are grey, and no more power...
My mechanic is experienced, but he does not understand.

Report
Re: Weak ULS

by Roger Lee » 7 years ago

Roger Lee

125 5.12k 4.53k

Hi Marton,

Don't worry about the plugs right now. You're chasing your tail worrying about best performance just looking at plugs alone. So long as they are not black or white you should look at other things. They are not all telling. It is just one item to look at for the entire picture.
You should put the carbs back to original. These were set up and tested at the factory during their research and is the best all around performance setup. (Unless you have special circumstances)
If you are getting 5100 rpm static on the ground you are probably getting around 5750+ rpm in flat and level WOT. This means you have a better climb prop, but are suffering a in cruise speed and fuel economy.
The one major factor in aircraft performance is hull design. Not all things are created equal here. Some sleeker aircraft may fly 140 mph and some may only get 90 mph with the same engine and prop. It's all hull design. Even if you have more HP your hull design will limit your flight envelope.

Take a sailboat. If it has an 8 knot hull it will not go any faster when pushed. It will not do 12-15 knots.

First you need to see if all 3 of the same aircraft have the same prop and get the EXACT same rpm at WOT in level flight. If you have too much of a climb prop and they are more set up for cruise (speed) then they may be faster even with less HP.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Marton Os

Report
Re: Weak ULS

by Marton Os » 7 years ago

Marton Os

1 2 0

What I can't understand is my climbing rate 4 - 4.5 m/s, my friends same kind fox climbs 5.5 - 6 m/s.
(with same static rpm, prop, weight etc). This is why i think some engine problem.
I will put back my carbs original, and accept the situation.

Report
Re: Weak ULS

by Roger Lee » 7 years ago

Roger Lee

125 5.12k 4.53k

You're getting 800-875 ft per minute climb. There is nothing wrong with that and it's normal. If they are getting better it could easily be due to pilot performance and flaps, drag and speed in the climb and ESPECIALLY the prop pitch..

Your friends are 1000 to 1180 ft. per minute. That's normal to.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Marton Os

Report

You do not have permissions to reply to this topic.

Title	Category	Replies
912 ULS setup in a KitFox II	912 / 914 Technical Questions	2
912 ULS Crankshaft Nut Torque Question	912 / 914 Technical Questions	10
Roger Lee 912uls	912 / 914 Technical Questions	17
Selling my Rotax 912ULS - Price?	General Tech Discussion	3
NEED 912 ULS CYL Head Assy #2 CYL	Parts	1

Weak ULS

Recent Videos

Safety First!

ROTALK News

Prize Giveaway!

Log In/Out

Login