simo707

I have an OS120III pumped which I have rebuilt from a prang last year, upon rebuild it ran fine, but hasn't been used for six months. I have since moved location to a higher elevation, and trying to restart it has caused many problems. Firstly it will only run with the glow starter up to 1/3 throttle, upon a whole bunch of high speed and low speed needle adjustment, I can get it to idle and seems to be on tune at about 1/1/4 turns on full power, however when I return to idle, it runs for about 5 seconds then dies. I have tried factory suggested settings for the idle needle valve, but still no luck. I have cleaned all fuel lines, the regulator and needles. The only thing I can think is the throttle barrel is a bit stiff to move. Before any tuning suggestions I have a basic re-assembly question. With the 'T' mark at the top of the crankshaft, (valves closed), where should the dot on the cam gear be positioned? Could it be timing, or should I simply run more fuel through the engine to un-gum anything I have missed?

Bax

My Feedback: (11)

The dot on the cam gear is aligned with the pushrods when the piston is at top dead center.

You may have problems with the engine if the regulator has been disassembled at all. They are finicky and can cause problems if they've been disassembled and reassembled. Same for the pumps. Usually, a full replacement is needed, which means you'll need a new carb body.

simo707

Thanks, the regulator and carb body are new, the pump is not but fuel runs through ok and it seems to draw fuel at least at full power. It hasn't been disassembled but I will try and assess it at idle. Could the pressure fitting from the crankcase to the pump be blocked? I believe I have the cam gear in the correct spot, at TDC, it sits just left of 12 o'clock in the casing, which is aligned behind the cam follower. Can four strokes be 180 deg out like car engines? I have no other ideas to solve this problem.

Cheers Simon

Bax

My Feedback: (11)

The cam gear can be either pointing up and aligned with the pushrods, or pointing down and aligned with the pushrods.

If the pump's not pushing fuel, there's the problem. If you have very soft fuel line, is it collapsing when the pump is sucking fuel from the tank?

simo707

Thanks for the tips Bax. After much frustration and changing glow plugs and the mixture all different ways, we finally changed one parameter that we had not thought of until now.....fuel. A new batch of fuel proved to be the cure. Idle and transition to full power was flawless. My previous fuel had obviously absorbed to much moisture. It was 72% methanol, 18% synthetic oil and 10% nitro, but about 8 months old in a 5L container, that had about 1/2L left, and didn't have an air tight seal. We actually found this out on another engine, a OS50 Hyper, doing exactly the same thing, would idle and run at full power, but not transition. So one more tip for your bag of tricks!

Cheers
Simon

NapaFloyd

My Feedback: (1)

Bax, a question.. I have a OS 120 II with pump and have installed a new piston, piston pin, ring and con rod. I am having problems with setting the timing. I have aOS 4 stroke book on engines which depics the timing. It reads that you align the woodruf key at 12 o'clock and then set the cam dot at 6 o'clock.. I am not sure in the other comments on this threadbut should the cam lob be at 12 when the piston is at TDC?

Do you have a pic of the cam in possition ?

NapaFloyd

My Feedback: (1)

Bax, sorry but the engine is a 120 III with pump.. not a 120 II.

Bax

My Feedback: (11)

Please see post #23 in this thread:  http://www.rcuniverse.com/forum/m_1921937/tm.htm

The dot on the cam is NOT 90-degrees to the crank when the piston is at Top Dead Center.