912s Engine Problem Help

[brannan78]

We are having a curious issue with our 912s (Speedster IV) and was hoping you might be able to lend an idea.

On a three occasions after takeoff during or soon after initial climb-out the engine acts as though it’s failing (decreasing in power and RPM) with almost a sputtering sound. At first we thought it might be too lean but the plugs look okay. Thought about maybe bad diaphragms in the carbs but would like to know if others have found this.

In two cases powering back to less than 4,000 RPM helped the issue. It does it for about 30 seconds soon after it runs fine for the remainder of the day, and the issue doesn’t arise for weeks and multiple flights after.

On two of the occasions it happened in the first minute of flight and not again. On the third occasion it happened in the first minute on the first three takeoffs and then not again.

Any ideas?
http://www.teamkitfox.com/Forums/picture.php?albumid=288&pictureid=3394

[ClickClickBoom]

Carb ice?
Not a popular concept, but after prolonged idling(like waiting for engines to come up to temp), carb ice is a very real potential. All it takes is for the right combination of temp and humidity, no matter how improbable, the possibility is there.

eric

[Av8r3400]

Hot or cold? If hot, What fuel? If auto gas do you have a return line to prevent vapor lock?

[brannan78]

Hot or cold? If hot, What fuel? If auto gas do you have a return line to prevent vapor lock?

You mean outside temp? If so outside temp I'd guess to be between 70F and 90F.

If you mean use, three occasions were from cold starts and small amount of run-up time.

Using 100LL at present time.

[ClickClickBoom]

You mean outside temp? If so outside temp I'd guess to be between 70F and 90F.

If you mean use, three occasions were from cold starts and small amount of run-up time.

Using 100LL at present time.

The question is what was the relative humidity?
The common consensus seems to be that the 912 is immune to ice since they draw air from deep in the cowl, but the reality is the location may delay/lessen the possibility, but it cannot prevent it entirely. .
eric

[Dave S]

Brannan,

I'll throw this out in the blind - obviously a person cannot truly troubleshoot a situation they do not have their hands on directly so take it as just an idea.

Do you know what your fuel flow is? i.e. the amount of fuel the system is capable of delivering to the carburetors. Fuel flow potential/delivery rate must be more than what the engine actually needs/burns

Do you have a fuel pressure gauge?

The reason I ask is two of the symptoms you describe may or can be indicative of an intermittent fuel delivery issue.

1) Decreasing power and RPM on takeoff following maximum throttle
2) The fact that reducing the throttle permits the engine to run smoother.......throttle reduction could be allowing the fuel delivery to catch up with the demand.

If you have a fuel pressure gauge - this possibility could easily be confirmed or refuted.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp

[ClickClickBoom]

You mean outside temp? If so outside temp I'd guess to be between 70F and 90F.

If you mean use, three occasions were from cold starts and small amount of run-up time.

Using 100LL at present time.
Cold engine, OAT 70-90F, depending on the relative humidity, that is the sweet zone for carb ice. The cold engine equation means the intake runners had not had time to have heat transfer from the cylinder heads. Fuel starvation could mimic these symptoms, but would likely reoccur at times regardless of the situation.
Interesting at the very least! Occam's Razor.....
eric

[ClickClickBoom]

the issue doesn’t arise for weeks and multiple flights after.

On two of the occasions it happened in the first minute of flight and not again. On the third occasion it happened in the first minute on the first three takeoffs and then not again.

Any ideas?
http://www.teamkitfox.com/Forums/picture.php?albumid=288&pictureid=3394

If you were flying from or near a powered airport the temp/dewpoint spread might be available online.

[Av8r3400]

You mean outside temp? If so outside temp I'd guess to be between 70F and 90F.

If you mean use, three occasions were from cold starts and small amount of run-up time.

Using 100LL at present time.


Yes, I'm talking OAT. (My first reply was from my phone so typing was not easy...)

Heat in the cowl can be an issue. The design of the fuel pump and lines often leads to vapor lock or partial lock. This is one of the reasons that Rotax is pushing the return line very hard. The return line starts from the pressure line, through a .014" orifice back to the (header) tank. This return line offers enough flow to keep the fuel from boiling (locking) in the line.

The second place I would look to is fuel flow. There are some known issues with the mechanical pumps on the 912 motors (and the can be intermittent). There have been several updates in recent years. If your fuel pump is weak, this could be your problem. Do you have a supplemental (electric) pump? Would you consider adding one? Example (http://www.aircraftspruce.com/catalog/eppages/facetpumps.php)

I can't agree on the immediate jump to carb ice. A Model IV, round cowl with a 912S is almost impervious to ice. If you were launching into hard IMC with dew points perfect, maybe you could get some ice, but even then it is pretty unlikely. I've flown in freezing fog situations while on skis and never got carb ice (some airframe ice, but that's a different problem). The design of the Bing CV carb and it's location in the cowl does not lend to ice formation.

The only time I have ever heard of carb ice on a 912/Kitfox was on an installation that utilized the carb-heat, air box, exhaust-muff setup. The only reason for this was the location of the intake, taking cold air from the front of the cowl right behind the prop, when the heat was not engaged. This is the main reason I took this system off my plane.


Very nice looking plane, BTW!!

[kmach]

Hi,
It sounds like a fuel flow problem to me.
I would check everything in the system from the wing tanks to the carbs. something could be blocking the flow intermittently. any recent work done ? sometimes hose clamps can be tightened and restrict flow.

I really doubt carb ice, though in theory is possible.

Hope you find the problem.

[DesertFox4]

Welcome to TeamKitfox Brannan. I also doubt the carb ice just yet and also suggest a close scrutiny of your entire fuel delivery system. With the info. provided it sounds like fuel delivery restriction. If the aircraft is brand new check your fuel filter/filters for contamination from fiberglass from the fuel tanks. Make sure your are flowing enough fuel at the engine to maintain full power for extended periods of time. Your engine will consume between 7 and 8 gallons per hour at full throttle. You should be delivering a good percentage over that for safety. 50% over max fuel usage should not be at all hard to acheive sitting on the ground in any configuration. That is of course with no ram pressure at all from the fuel cap tubes. If not look for any restrictions and that goes for fuel lines that may look normal from the outside. If you are useing the old fuel valve supplied with many of the older model 4 kits I would inspect that real close. The old models used o-rings inside and they are susseptible to damage and blockage of fuel flow.

Good luck and please let all know if you find the problem. Be careful in the meantime.

[DesertFox6]

Welcome aboard Brannan; you can easily tell this group isn't very interested in your problem, huh? What a group! Wouldn't it be nice if our medical service here in the states was this responsive and diligent?

I'm going to cast my ballot with the "fuel restriction camp" in regard to your intermittent power "sag," and I agree with all the data presented here with that cause in mind for checking things out. Before you start selecting wrenches though, I'd like to suggest a simple trick that won't hurt a thing, but will definitely make your entire engine happier in the process if it doesn't cure or at least mitigate your problem while you're checking everything else.

Go buy a quart of Pennzoil 2-Stroke (yes, 2-stroke!) motor oil for air-cooled engines available in the aviation department of your favorite Big-Box store. Thoroughly mix this into about a gallon or two of 100LL (since that's what you're using) and then split it evenly between each wing tank. Doing this just before filling up will obviously allow quicker dispersion of this cracker-jack, fool-proof, engine/carburetor/pilot's-mind-cleaning compound: This is what solved all of my similar power-sagging predicaments on the recommendation of our local ROTAX 912 guru, Steve Beatty of Airscrew Performance here in Glendale, Arizona. I'm willing to bet your early-in-flight fuel-flow drop, resulting in your heartbeat-altering experience, may be caused by carburetor contaminants that break loose early in the high-power-on phase (think needle-jet area here) and then disappear for the rest of the day's flying: Altogether possible and just as maddening.

You might want to do this 2-stroke stuff every 3-4 months or so even if you find a fuel restriction problem elsewhere, as this will nicely polish everything in the engine except the wiring and, as I said, your engine will thank you! (I do and mine DOES! :))

Sometimes the simplest factor is the fastest fix; it's pretty easy on the wallet too. Give it a try while you check the items the rest of the gang has suggested and please keep us informed of what you find!

"E.T."

[leptronjohn]

I had what seems to be the same exact same problem 2 years ago, I fought it for a year.
I replaced the carbs nothing, I had the ignition checked, nothing, I checked fuel flow at carbs, nothing. then I pulled a cylinder... I was appalled.. I had been using 100LL for the last 150 hours and had sulfur build up in the heads, pushed up by the pistons. the rings were clogged.. it looked nasty. I wanted to cry, only had 400 hours on engine.. it went off for a top overhaul..
they actually just cleaned the rings and groves, heads, completed valve job, they were sticky... lead, dam
runs good now.. john Oakley
801-821-6175

[Av8r3400]

It is absolutely imparative that when you run 100LL in a 912 you keep your temps up to burn the lead off. Even with a lead scavenger additive. Several comments on this at airventure.

[Dorsal]

So you are saying my high CHTs are a good thing given I primarily burn 100ll.

[Av8r3400]

According to the seminars here at OSH, yes heat and high power settings good, cool low power, bad when running LL.

[Dorsal]

Leptron, out of curiosity were you using Decalin and 25 hour oil changes? I often wonder if these procedures help much or just delay the inevitable. I have seen pics of engines after a steady diet of LL vs Mogas (reminds me of the healthy lung smoking lung pictures). Would like to see pics of an engine after 1000 hours of LL following all the best practices.

[leptronjohn]

dorsal,
I always change oil at 25 hours, the decathlon was discontinued the last 100 hours. I did not feel it was going to be detrimental.. I was wrong.
I think there are others that have suffered the same. the first tail tail was pulling the exhaust, the valve stems were covered with crap. I broke my heart.

[Av8r3400]

What oil were you running, John?

[leptronjohn]

I was and am still running the Honda Simi synthetic oil.. the oil shows no problems at all and was agreed by Lockwood who did a top on the engine.
I have not been able to keep on the site like I should, you guys can contact me at john@leptron.com also.