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Zorro
09-09-2010, 08:29 AM
need to fly my Fox, but

Everything looks good right through run up, then when I'm all lined up on the runway and add full power the power comes up then drops off, rpm just drops 600 or so If I power back it's fine then power up it drops.
I bought the electronic balancing tool and it balances fine.
this is a standard Rotax 912 with bing and all.

any suggestions? Also dose any know of any good Rotax Mechanics in Washington state?

akarmy
09-09-2010, 09:32 AM
Also dose any know of any good Rotax Mechanics in Washington state?

Jim Scott at Aircore Aviation in Arlington really knows Rotax. He runs a Light Sport focused A&P shop and can fix any problem you might have.

http://www.aircoreav.com/

Dave S
09-09-2010, 05:22 PM
Hannibal,

E-mail diagnosis is pretty tough; but, I'll try a few traditional troubleshooting suggestions.

It would be interesting to know if this came on all at once or if it came on gradually.

The condition you describe potentially sounds like a shortage of fuel at what should be max power. (speculation #1) I don;t know about ignition on this one.

Shortage of fuel means either a shortage of fuel, as in lack of supply; or, excess of air, as in an induction system leak.

If you have or can set up a temporary fuel pressure gauge - that should help a person determine if the fuel pressure drops off at max power, possibly indicating a weak fuel pump or fuel line restriction the pump cannot overcome. If you gravity tested your fuel system during testing, you should be able to do it again and if there is no change there should be no line restriction and rule that out.

Carbureted autos with a mechanical fuel pump often display this exact set of symptoms when the fuel pump gets weak; but this almost always comes on slowly, not all at once. There is a service bulletin on certain rotax fuel pumps - the affected pumps may get weak because the plunger wears down faster than it should.

If you find a flow restriction - that should be able to be traced back through the tubing and fittings to get corrected. I know of one case where a chunk of gunk lodged in a fuel T fitting and restricted the flow sufficiently to reduce fuel flow. Flow restrictions can be just about anything from crud in a filter (especially water in a paper filter which tends to seal the paper) to a torn off pieced of tubing inside where you can't see it or a fuel hose that has swollen to the inside where it can't be seen due to solvent contamination of the fuel. Lots of possibilities anywhere from the tank finger strainers to the carb needle valves.

On the induiction side - it has happened where the rubber connector between a 912 carb and intake manifold has cracked or broken and admitted air to mess up the mixture - a small leak might not make for a rough engine but something to check. A big leak will be impossible to ignore.

How about the carb throttle plate? I do not know if it is possible, but if a throttle plate goes over center (too far) it essentially has the same effect as closing the throttle a bit.

Don't know if this helps any; or if my speculation on theory has any reflection in what you are seeing but these things do have a cause and it is a matter of finding the cause - logic will prevail in finding the problem.

One other thing I think about when a piece of machinery messes up is to see if anything was repaired or adjusted at the same time the problem showed up - I'd always do back the exact item that was worked on - for instance, if a fuel filter was changed or hose taken off and put back on - first place to look to see if something happened when that was done.

Good luck & let us know what you find out.


Sincerely

Dave S

HighWing
09-10-2010, 09:12 AM
One thought here. I suspect as well that is fuel flow, but I had an issue once when I changed out my fuel lines under the cowl and misplaced one of the firesleeve clamps and tightening it also closed off much of the fuel flow to the carbutretors. The issue with me was that the engine ran fine until the float bowls emptied and that was after I got to about 300 ft. altitude. I doubt a fuel flow issue would show up right away as it takes a bit of time to drain the bowls. I would look at the jets to check for debris.

One more thought and this flies in the face of my first comment, but some time ago there was a guy that had some fiberglass fibers find their way into the gascollator. On start up they rested nicely on the bottom of the gas collator bowl, then in time, with motion of the fuel, they would stir up and collect on the screen and then impede fuel flow. I happened over and over again.

Lowell

Zorro
09-10-2010, 10:47 AM
Thanks, I suspect an air leak , or fuel pump. Originally I thought it might be gunk in the fuel system as I do get some tank crud when I sample. I put a new inline filter and the gascolator filter was clean. I just cleaned the carburettors 20 hours ago. I'll have to get a mechanic with a fuel pressure gauge to test my fuel pump.

BTW Jim Scott did the annual on this KFIV 1050. I'm at Peason field (KVUO) cant fly to him (well I know she will fly with partial power just fine but I am a cautious guy)

Zorro
09-10-2010, 10:48 AM
Whats the static line? I have checked the balance tube?

Dave S
09-12-2010, 05:42 AM
Regarding the " Static Line", I believe Rotax calls this an Air Vent Line - it is a small plastic tube (about 1/4" & 120 mm long) hooked to a fitting on the engine side of each 912 carburetor - near the rubber manifold connector. If a person has the optional airbox - the two air vent lines are connected to nipples on the airbox. If you don't have the airbox - I believe they are left to hang near the carburetor - I am not a rotax engineer; however, what I have been told these are for is to balance the air pressure beteen the inside and outside of the carburetor. If they don't work (plugged) - it messes up the air fuel mixture.

Sincerely,

Dave S

Zorro
09-12-2010, 08:19 AM
Ok yes on mine they go back into the k&N filter, when they come out it runs rough and I get a strong fuel smell in the cockpit.

napierm
09-12-2012, 01:28 PM
I know this is reviving an old thread but I have a similar problem. I picked up a CH601HDS from upstate NY last weekend and flew it home. Checked it out on Sunday and it flew great. Good climb, handles well, faster than what I'm used to but fun.

On the way home Monday the engine lost some power. Like 20 minutes out from Saranac Lake west of the mountains this stupid thing didn't want to climb well and I don't know why. No where to land.... Crap.

So landed when able a few miles south of active MOA. Checked everything I could with limited tools. Engine seems smooth and reliable just gutless at full power.

Compression test consisted of pulling prop and feeling quality of compression on each cylinder and listening to exhaust and intake for leaky valve. Far as I could tell fine.

Checked float bowls - floats riding high and level looks right. No sign of debris or anything wrong in jets. Don't know about carb sync but that shouldn't affect full throttle much. Again, everything seems smooth so sync doesn't seem likely.

Engine seems to be getting enough fuel. If anything the fuel consumption is a bit on the high side. Air filters are supposed to be fairly new. Dont' know what the static RPM's are because the @!@#$!! tach has gone out.

Engine start running some better and I basically nursed it all the way home to 6ga4. My route was over a few foot hills but no mountains - kind of a trick from NY and avoiding the 60nm ring around DC so went through Shenandoah Valley. Very long day and a half flying because of fuel stops and reduced cruise speed.

Was well worth getting as the plane flies very nice, not a Kitfox, but nice.

So I've got a tiny tach coming and will start debugging when I can make some reasonable comparisons.

Any words of wisdom on similar problems?

Thanks in advance,

Mark

HighWing
09-12-2012, 02:54 PM
I recently checked over the theory behind the Bing carburetor and this is what I think I learned. Idle fuel flow is set with the idle jets. Mid range is set by needle position and is affected by the diaphragm, spring and balance tube. High RPMs are set by the venturi effect through the open pathway when the piston is fully up - powert is determined by the butterfly and the suction of fuel through the jet. With that in mind, if it is carburetor related, My guess is that it is a fuel flow issue. Has the airplane been sitting for any length of time before you bought it, or has it been flown regularly? Has the throttle linkage somehow been compromized. I once had an issue with the engine faltering at high RPMs and I traced it to a firesleeve clamp that was pinching the fuel line. Keep in mind that if it is fuel flow, by the time you taxi and check the bowls at idle, there would be sufficient fuel to fill the bowls. The same is true with the plugs. Lean running at full throttle would be followed by normal ratio at taxi and the actual plug appearance - lean at full - could be masked by several minutes run at idle while taxiing.

If it runs smoothly at all throttle settings except high RPM I think you could discount compression issues, though that is easily checked with a differential compression tester. With the tester in place on each cylinder, If it is the intake valve that is leaking, you would hear the leak hiss through the carburetor; if it is an exhaust valve leaking, you would hear the hiss at the exhaust.
Lowell

napierm
09-13-2012, 05:55 AM
Idiot question: what do you expect out of a mag test on a 912? RPM's?

ClickClickBoom
09-17-2012, 07:52 AM
Idiot question: what do you expect out of a mag test on a 912? RPM's?

912 operators manual
10.3.5

4000RPM

300RPM drop max

120RPM difference between A&B circuits max.

napierm
09-17-2012, 08:05 AM
Dug in last weekend with "help" and found a few problems. 2 wires burned on exhaust. Exhaust badly rusted - could have bad baffle. 200 rpm rise on full power static w/o aircleaners (not cleaned properly). Installing new exhaust, plugs, wires, etc. Reading installation and maintenance manuals, learning about new engine (to me).

Darned GSC prop has 4 out of six outer bolts dead loose. They were torqued up in NY. I flew nearly 900 miles on it!! The prop just became a wall ornament. Debating either an IVO or Warp....