unable to reach full power

[jabkwab]

Hello all, after a shamefully long period of quietly keeping up with all the messages on this great forum the time has finally come to formally introduce myself (the reason being that i have a problem with the aircraft of course).

My name is Jan, i am a 29 years old aircraft engineer in the Netherlands and recently I had the chance to purchase a Kitfox 4 speedster.
The aircraft is a great flyer, pretty basic equipped thus light. It is something of a flying project however.

A problem did arise with the airplane. On a flight a couple of weeks back I noticed that when pulling the carb heat (exhaust heater type) the rpm of the Rotax 912ul increases instead of decreasing, what you would expect.
The difference in engine power is quite notable. During full power cruise the rpm rises from 5500 to slightly above 5800, and the max. speed increases from 90 mph to around 98 (the airplane does not have any fairing or other fancy streamlining stuff).

First I checked the airfilter, no problem there.
Then I pulled the carbs apart. I did find the little dampener o-rings on top of the needles where not installed. The needles where pretty worn out so I changed those, as well the needle jets and the float valves and brackets.
The needles where installed in the #3 position.
After adjustment and synchronisation I did a little test flight....no difference.

To me it seems there is something wrong with the mixture, but I cannot figure out what anymore.

Any help would be greatly appreciated!

[jiott]

If your carb heat pulls in air from a different place (many systems bypass the air filter) it would indicate that there is a significant air restriction in the normal (non-carb heat) air flow path. Probably the air filter or some part of the air intake system. Try making a short flight with the air filters removed.

[jabkwab]

Thanks for the tip Jim, today I flew a circuit without the airfilter.
No change unfortunately.
Funny thing is that the problem only appears in flight. Pulling carb heat on the ground does not cause any rpm change whatoever, at any powersetting.

I also checked for air leaks in the induction system by spraying starting pilot on all the joints with the engine running idle. No rise in rpm so no induction leaks.

Then I read that the needles in the carbs only regulate intermediate mixture? That means that any full power mixture issue is not going to be solved by raising or lowering them a notch. Let alone that to further enrichen the mixture i would be moving the needles to the lowest, no4 position, something that appearantly nobody else does.

I'm probably pulling the carbs apart again this weekend, although i don't have a clue anymore on what could be wrong.

Is there anybody out there who can help me with a hint?

Jan

[n85ae]

Many times weird power problems proceed loss of power while test flying and
trouble shooting ... Take care testing it. Having flown a glider with an IO-240B
up front previously I can say, it's startling to have the engine go silent.

Jeff

[gregsgt]

Do you have an EGT gauge and if so what are the readings with carb heat off vs on?

[Dave S]

Jan,

Not saying that individual suggestions would necessarily be on target, or that you have not considered them (maybe you have already) I'll pass along a few thoughts. It's pretty hard to diagnose an engine a person does not have his hands on. Realize I don't know anything about the age, hours or general condition of the engine from this far away.

Don't know what to say about the observation that the issue shows up in flight and not on the ground (really good reason to be very cautious)

On the fuel system:
1) Has a flow test of the fuel system established that fuel flow meets/exceeds the engine's burn rate?
2) Has the fuel pump condition been determined to be within spec: a) is the pump pressure within spec? b) Some 912's (not sure if this includes your specific engine) had wear problems on the fuel pump cam/fuel pump push rod.
3) Are the carb floats OK and holding the correct fuel level in the bowl.
4) Maybe a bit of a WAG - but some kitfoxes had a brass T between the fuel pump and the carbs - a possible restriction point where particles can get caught restricting the fuel.
5) Are all of the SCAT/SCEET tubes on the induction side solid and free from restrictions?
6) Are the throttle cables hooked up so the throttle valve is fully open at "full throttle"?
7) Any restrictions from the enricher (choke) not fully opening after start?

I'd also consider possible effects with the ignition system - plug gap and plug condition

Keep us informed on how the resolution of this problem works out.

Wishing you good luck,

[Dutch]

I know this is going to come across sounding pretty dumb.
Have you considered that the linkage action is what you assume it to be?
On is on & off is off.
Dutch

[DesertFox4]

I know this is going to come across sounding pretty dumb

Dutch, your post will be pure genious if it turns out that his carb heat cable operates in reverse to every other instalation in a Kitfox.

Push carb heat On, Pull carb heat Off would really mess a guy up.
Hey, it could happen.

No suggestion is dumb to a guy fighting through a problem. It is one more thing to check in the stream of possible solutions.

[jabkwab]

That carb heat linkage question popped up in my head as well, in fact it made me drive to the airfield on Sunday night last week.... unfortunately it was hooked correctly so that was a trip for nothing

After troubleshooting in accordance with Dave's list yesterday I actually found the vent hole in the membrane cover of the left carb partially blocked. I also found a big crack in the rh carb rubber socket. Not sure if this crack is old or new but two new carb sockets are on order.
Hopefully fixing this will solve the problem!

It's a steep learning curve, working on a Rotax for the first time

Jan

[DesertFox4]

Jan, hope those discoveries solved your problem. Let us know if you solved the problem with those fixes.