angelotti

I bought a new OS75AX a couple of weeks ago to put in a WOT4 XL. I ran-it-in initially on the ground according to the instructions - after a rich setting short run, I alternated with short bursts of full throttle at rich and less rich settings. This was using short runs of about one minute after each of which it was very hot.

After about one tankful of the above ground running, I gave it a short successful flight of a few minutes. However, since then I have not been able to get it to keep running. It requires the main needle to be more and more open. Yesterday, I could only get it to stay running with the nose up with the main needle four and a half turns open. Again after a minute or so it started misfiring and stopped.

I guessed that perhaps there was dirt in the carburetor. I took the fuel nipple and main needle boss off and took the barrel out without finding any dirt. However the part where the needle and fuel meet seems inaccessible. The manual says "also, clean the carburetor occassionally" but does not say how.

Another possible cause is the distance from tank to engine in the WOT4 XL. The front of the tank is two and a quarter inches behind the back of the engine and four and a quarter inches from the carburetor. The tank is only slightly below the carburetor.

Any suggestions gratefully received.

blw

My Feedback: (3)

Holding the nose up creates a lean condition. Did you set your low speed needle to be lean enough?

Ernie Misner

Are you running a cowling? Could be cooling issues there if so. The tank CENTER is only slightly below the carb? I don't know why the tank is that far behind the carb but that seems pretty far. Closer the better. Your tuning might be a compromise this way between nose up and nose down.

thailazer

Things that have caused me problems like that include leaks in the tank where the muffler pressure is not consistent, dirt in the carb, as well as split clunk lines inside the tank. You might indeed have some dirt in the carb so take the needle valve completely off the carb, put your finger over the carb barrel, and spin it up good with no glow plug driver to get some fuel flowing out to purge any dirt that might be in there.

blw

My Feedback: (3)

Being a new engine, maybe it isn't dirt or trash in the carb, etc.

angelotti

There is no cowl fitted. The WOT4 XL usually has a bigger and heavier engine fitted so this one is further forward to reduce the lead needed. That results in the distance from tank to engine which may be part of the problem. lt still needs three and a half ounces attached to the engine mount to get an acceptable cg position. The engine had plenty of power for the model on the occasion I did get to fly it.


I'm going to give it another go this evening and will try thailazer's suggestion for purging dirt from the carb.

angelotti

I got a good flight this evening with the main needle set at four and half turns open. I tried with it further in and there seemed quite a lot of smoke but it died after the climb-out. The required needle setting seems well open compared to engines I've had previously. Perhaps it's due to the largish distance from tank to carb. Anyway, I'm happy to now be able to get successful flights.

the pope

Your prop isn't too big ? Had a friend with a 55 and the recommended prop was too big . Very quite but ran hot as buggery . Fitted a smaller prop and she ran like a train . Hate too big a prop on 2strokes . As mentioned the muffler line to the tank is clear of obstructions ? Cheers the pope

angelotti

The recommended size for the 75 is 13x8-10, 14x6-8 and I've been using a 12.5x8 Bolly prop so seems already a bit on the small size. Will try one a bit smaller if I can find something suitable.

wcmorrison

My Feedback: (2)

Sounds like a fuel problem. Just because the engine is new does not mean there isn't crap in the carb. Four turns is way rich, so I am surmising it is not getting fuel.
Take a look at your tank. Inside the tank. Could be a clunk stuck up in a corner, could be a sliver of plastic in the fuel line, or at the carb or in the carb. If it flew once okay then it is not the engine, so it has to be the fuel path. Got to check it end to end.

flybyjohn

My Feedback: (2)

Judging by your first comments of it running good and then requiring more and more opening of the high speed needle as you continued to run tanks through it, I have two guesses. 1st there might have been some stuff in the fuell tank that started blocking the high speed needle requireing you to keep opening it up. I have had this happen with an old tank that had old dried caster oil in it and when I filled it up with new fuel, I had a lot of floaty obsticals in there cloging my high speed needle. 2nd the low speed needle might be slowly turning in while running and cause a need for more pressure to push the fuel past the low speed needle if it is not getting out of the way. I have had several engines that the needles will vibrate open or shut on me.

Sandyt

Perhaps you are using a Pitts style muffler? Many of these do not provide enough pressure to the tank for proper fuel flow. I don't think I have ever seen a pressurized system need over 4 turns on the needle, so for some reason either the tank is low on pressure or there is something obstructing the flow.

falcon_56

I would make sure your fuel tank is wrapped with foam rubber and not touching the fuselage. It is amazing how engine vibration can make fuel bubble like it is boiling, causing erratic performance. A few drops of armorall on the fuel will reduce foaming and maybe help pinpoint the problem.

If your engine overheats, it is running too lean