jasona

Dear All,

I've been in aeromodelling for years, but I've got a problem with an OS91 Four stroke that has me completely stumped! It's an OS91 FS Surpass, non-pumped model. On half throttle it feels like it's on full throttle, on full throttle, the power and revs back off. If you then go back down to half throttle, the revs pick up again and it pulls hard! So, half throttle is like full throttle and full throttle is like half throttle.

* It does this no matter how rich or lean I make the main needle valve
* Fuel supply is fine - the tank is pressurised, the pipes not too long, no kinks, etc.
* Yes, the servo adjustment is correct - on half throttle on the transmitter the carb is half shut, on full throttle on the transmitter, the carb is fully open.
* I can't play with the low running needle adjustment too much or it buggers up the tickover (this shouldn't be relevant to full throttle running anyway, but I wanted to try everything)
* I'm using an OS Four-stroke glow plug.

I've been fiddling with this for ages now and I'm stumped. Can anyone offer any suggestions? I've never encountered this problem on a four-stroke before!

Incidentally, the engine is in a brand new Great Planes Fokker DR1 Triplane, which I would have test-flown weeks ago if it wasn't for this problem!

Many thanks in advance,

Jason.

aerowoof

has this engine run with out problem before mounting in this airplane?have the valves been properly adjusted?what prop are you using and what rpm readings are you getting at half and full throttle?

jasona

Hi there! Thanks for your reply.

The engine has never run without problems in this airplane. As for whether it has run without problem BEFORE mounting in this plane - I don't know, I bought it and put it straight in this model. It has supposedly run OK before according to the person I bought it from.

Valve adjustment - I don't know, it came off eBay second-hand (but supposedly nearly new - not much running since running in) but I know that route can always be a bit of a gamble.

The prop is within the recommended range and I'm afraid I don't have a rev counter for taking the RPM.

aerowoof

I had a feeling it was a used engine from your post.there can be several thinks to check for,leakage around the carb and intake manifold and leakage at the manifold and head,could be a missing or dried out o ring,check valve adjustment both intake and exhaust sorry I sold my os fs91 and have a ys 91 also valve timing may be off a tooth.if I remember correctly at tdc the indent on the cam gear should point at the valve guides not straight up as most think.post what you find out.

jaka

Hi!
Are you sure you have set the high speed needle correctly? And low speed??The way you describe it, it sounds that the engine doesn't get enough fuel at full throttle.

Be sure to run 5-15% nitro fuel! 14x6-15x4 APC or RAM prop.
OS F glow plug is a must!

MinnFlyer

My Feedback: (4)

I would say 10 - 15% Nitro is better.

Is this a Surpass II? (If it is, it will have a hose running from the intake manifold to just in front of the cylinder - and if it has a nipple in those two places with no hose, that could be the problem right there)

I would also check valve clearence - When there is no pressure on the valves, they should have the slightest amount of play in them - Almost undetectably slight.

But one of the things that raises a red falg with me is that you say it won't change if you open the needle valve. You might have an obstruction in the valve housing. Try removing it and giving it a thorough cleaning.

jasona

Jan - I've fiddled endlessly with the high speed and low speed needles trying to get it to run right! It's on 10% nitro. Yes, I think it's not getting enough fuel at high throttle, but it does that no matter how rich I make it with the needle.

Aerowoof - Grateful for your input, I'll check the valves when I can.

MinnFlyer - It's a Mk1 surpass without those nipples you described. The only nipple on it that doesn't have a tube connected to it is the one on the very bottom of the crankcase. Good suggestion about the valve housing though, thank you. I'd thoroughly cleaned the carb through in case of blockages, but not that area.

Thanks to all of you - it's so nice of you all to try and help.

Jason.

Gray Beard

My Feedback: (-1)

Jason, the tappets should be set between .04mm to .10mm. My 91 and 70 both have the dots on the cam that you aline at TDC for correct cam timing. If the engine was in A crash there may be A bent needle valve or the O-ring may be worn, this could be on both the high and low ends.
My .70 was doing something like yours and it was fixed by buying A new needle valve assembly.

jasona

Thank you very much for your input.

Kimhoff

My Feedback: (22)

Your low speed needle is set wrong. Put it back the factory setting and start from there.

FlyingPilgrim

My Feedback: (26)

It sure sounds like it is becoming too lean at full throttle. Possible causes are split in fuel line, obstruction in tank lines (crimp), or maybe a mechanical problem in carb. Since you say it runs fine at half throttle, I suspect the cam is timed correctly, and the valve lash is close enough that it should throttle up OK. If there was some kind of an air leak in intake attachment, it would have shown up at the midrange throttle setting, so I don't think that is the problem, so again it leads me to believe it's a fuel delivery problem. When you observe the throttle barrel rotate as you open and close the throttle, do you see the barrel moving in and out properly (cam action to open up flow of fuel as it is opened)? If you have another fuel tank and lines, switch it just to rule out that possibility, and make sure it is mounted level with (or maybe 1/4" above) centerline of carb spray bar. Maybe clean out the carb assy to make sure there is no obstruction as well. Not exactly sure what you mean about the low speed mixture buggering up the tickover, might have to elaborate to us "Yanks". Keep us posted, curious what it will turn out to be.