Switch_639

i am running an os91fx 2stroke with 12x8 prop and ultra thrust rossi muffler... on the ground the engine is fine, revs 12900... once it gets into air and I do a declined pass at WOT the engine starts puffing little bits of smoke out the exhaust... any ideas what can cause this? it doesn't sound well after this then once I get to lower revs it cuts and I do a deadstick landing... engine then is fine on ground again, compression is still high too... and the head is not hot at all...

Lnewqban

[sm=confused.gif][sm=confused.gif][sm=confused.gif]

Phoenixangel

My guess is a fuel delivery problem, its leaning or flooding due to the attitude of the airplane in the air. Try running the airplane on the ground while another helps you lift the plane to vertical it should not really change how it is running if you are getting good fuel flow. If it does change or causes problems I would replace my fuel lines and check tank height in relation to the spray bar in the carb.

Just a gut feeling but it really sounds like a fuel issue.

Switch_639

^^^ my question also... I am doing a wide open throttle pass at full revs basically... the engine then starts puffing smoke... not leaving trails anymore...

Switch_639

its fine when I point its nose straight up into the air... it runs fine up until it reaches max revs basically... not really lean on HSN, loads of smoke trails behind until it starts puffing... and the engine head is not hot at all when I land...

Phoenixangel

Once the engine unloads it will need more fuel as it is getting more air, it really sounds like it is leaning in the air if you have a filter in the tank it could be slightly restricted also. Many tank clunk things have filters in them.

Switch_639

^^^ ok thanks... I had the same thought... I think its unloading too much with that small prop... I will try a 13x8 next weekend and check it out again... I don't have an air filter... you don't think its over revving perhaps also?

Phoenixangel

I was talking about an actual fuel filter on the fuel line inside the tank,it could be over rev but turning in the 12s on the ground sounds perfect.I would probably try going richer on the HSN and pinch the fuel line to make sure it revs a little and runs for about 2 seconds before it loses fuel.

Switch_639

its basically 13K... inside the tank I don't think I have a filter... would have to check... don't think so... everything was done for me... got it in the kit... its a GP revolver 46size...

flyinrog

My Feedback: (1)

It sounds like your running too rich to me, puffs are fuel building up in the muffler...the head is always gonna be cool to the touch if your deadsticking it in.....Rog

Switch_639

cool... something inside the muffler might be blocking it or something... will clean the muffler up also and see how it goes...

Lnewqban

It sounds to me like a plug that fails glowing properly at high rpm's and max chamber pressure.
As I understand, you dive at high speed and then go horizontal for the pass; when exactly the puffing starts?
Does it look like a four stroke operation when that happens, like missing one explosion each two strokes of the piston?

Switch_639

I also thought it might be the plug... but its fine... its a number 8... I also have a colder plug... didn't try it... the puffing starts once it gets its max/highest revs...

Lnewqban

Then, the piston is moving faster than the combustion shock wave, and the breathing is not enough to provide proper mix for each stroke, I believe.

In other words, your engine is reaching a condition at which the HP's begin to get reduced by "asphyxiation".

I am not sure how to solve the problem, but I believe in the solution of a bigger prop that was suggested above.

The mix may become too rich at that point (even when it is proper for lower rpm's), where the volume of air thru the carb throat reached the limit, and the spray bar "feels" a stronger vacuum coming from the crankcase.

For reach mix, the combustion becomes slower, and it drags behind the speed of the piston.
The retarded combustion shock wave reaches the exhaust port when it is already open, doing too little of useful power over the piston, and making that puff sound.

That is the reason for full scale engines to have means of advancing and retarding the spark that initiates the combustion.
Our miniature engines don't have that flexibility and operate properly within a narrow range.

Of course, this is only theory, but it could serve you to understand that your engine is OK, except that it may be "over speeding".

This article explains it better:

http://www.flyrc.com/articles/tune_your_engine_1.html

Frank Ts Stuff

My Feedback: (130)

FYI... I have had at least six of these engines in various planes and they have always been happiest with a XOAR 14 x 8 prop so you may not have enough prop on the engine.

Switch_639

yeah I know... I use to have a 14x8 prop on... but revs were only 8500 max... and OS suggested anything under 10500 I am "overpropping", not getting full power that I want basically... what kinda revs you getting?

to LNEWQBAN

your explanation sounds very accurate, think this can be the problem... I am hoping so actually... don't you think a hot plug can help with this? I have ordered a 13x8 prop its on its way...

Srewinkel

You could try reducing the throttle while in the dive.  I know it will slow the plane down, and therefore reduce the effectiveness of your high speed pass, but this could confirm the theory of over speeding the engine. witch cause a retarded detonation, witch would explain the puffs.  Just a thought.

Switch_639

true, yes I could give it a shot...

TimBle

you  running a OS91 Fx in a 46 size Revolver.....?!!!

So its running erratic once up in the air at max RPM, I'll buy into the too rich theory.

Piston moving faster than the combustion shock wave.... WOW, faster than the speed or sound. thats incredible and science fiction. there would need to be ero fiction and compression for that too happen and even then its impossbile.

Darn this hobby has its own rules of physics...

Switch_639

jip 91 in a 46 revolver... why not? its not as fast as I thought it would be... use to have 14x8 prop on there but not enough revs for speed... so I dropped to 12x8 which it doesn't like it seems... gonna try 13x8 and see if same thing happens... I cant really see how fast it will be at moment with the puffing at top end...

CKLLOYD

ALOT OF GOOD ANSWERS HERE BUT I DONT SEE ANY ON THE FUEL ITS SELF. WHO OLD IS YOUR FUEL ? WHAT % ARE YOU RUNNING. I ALSO THINK THAT YOU NEEDED TO CHANGE PROP BACK TO 14' AND BY WHAT YOU HAVE TOLD US I THINK THAT YOU ARE RUNNING TO RICH.. HEAD NOT HOT, SMOKE TRAIL, SURE IT MITE BE RUNNING GOOD ON THE GROUND BUT 200' UP IN THE AIR YOU WILL RUN RICHER. LISTEN TO THE MOTOR IS IT RUNNING 2 OR 4 STROKE ? AND AS FOR THE GUY THAT TALK ABOUT THE EXSPANTION WAVE AND OVER REV. GREAT JOB.

CKLLOYD

TIMBLE ITS NOT S/F THE COMBUSTION WAVE DOES NOT TRAVEL AT THE SPEED OF SOUND. EXSAMPLE IF YOU DROP BOMB INTO THE OCEAN AND IT GOES OFF 10 FEET UNDER THE WATER. THE BOAT A 1\4 MILE AWAY WILL HERE THE SOUND WAVE LONG BEFOR THE COMBUSTION WAVES REACH THE BOAT

Switch_639

On the ground I lean the HSN until the engine starts losing power and revs drop... then I set it 3clicks back... can't be too rich really I think... so it must be something else... will see what it does with the 13x8... fuel is new... 10% morgans (pink)...

Mr Cox

I wouldn't worry about the piston moving faster than the combustion...

Isn't the ultra trust muffler a tuned muffler?
Sounds more like you are getting out of its working range, or possibly starting to get into its working range in the air. They might be tuned for higher revs than you get on the ground. Just try a few different props to find where the muffler kicks in, and work from there.

Switch_639

ill try the stock muffler again also, just to see if there's a difference...