jvalovcik

This was asked by YRUSSO back in 1-25-2004;
----------------------------------------------------------------------------------------------------------------
Can someone help to solve the following problem?? I have bought a used OS FS-91 4-Stroke,
but it idle setting is not good. I can not get an idle under 3400 r.p.m. I have turned the
idle mixture screw clockwise almost to the end and the idle setting still seems to be rich.
On the other hand, I am able to get a very good high settings and the engine is not loosing
power.
------------------------------------------------------------------------------------------------------------------
Bax replied with this 1-26-2004;
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Check the valve/rocker arm clearances. Then "ball park" the carb per the instructions. If you don't have them, email us at the address below to get a set emailed back.

If the carb's set correctly, and the valve lash is correct, then the compression seal is starting to go. When an engine starts wearing out, idle and acceleration are the first things to go south.
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

IS THE COMPRESSION SEAL PART OF THE CARB.?

I HAVE THE SAME PROBLEM WITH MY OLD FS-91 4-STROKE I BOUGHT USED, I JUST CAN NOT GET IT TO IDLE BETWEEN 2000 & 2500 R.P.M. AND I JUST CAN NOT GET IT TO DO A SMOOTH TRANSITION FROM IDLE TO HIGH R.P.M.

I BURNED A EXHAUST VALVE, WHICH I THINK WAS CAUSED BY A BROKEN VALVE SPRING.

I WENT AND REPLACED BOTH INTAKE & EXHAUST VALVE ASSEMBLIES, CYLINDER LINER, PISTON RING AND GASKET SET.
STILL CAN NOT GET THE ENGINE TO IDLE. IT SEEMS TO RUN BETTER FROM IDLE TO FULL POWER AND IDLE BETTER IF I LEAVE THE GLOW BATTERY ON. WHEN IT IS REMOVED I LOOSE THE SMOOTH TRANSITION FROM LOW TO HIGH POWER AND THE IDLE GETS RICH AND QUITS.

MY FUEL TANK IS ABOUT 1/4 IN. ABOVE THE CENTERLINE OF MY CARB CAN NOT GET IT TO LINE UP BECAUSE OF THE ENGINE CENTERLINE AND THE PLANE DESIGN.

I AM ABOUT TO GIVE UP AND JUST BUY A NEW ENGINE BUT AFTER READING YOUR REPLY, DO YOU THINK I NEED TO BUY A NEW CARB.?

I USED WILDCAT FUEL 10% NITRO 18% OIL, 15% NITRO 18% OIL AND YS 20% NITRO 20% OIL
PROP SIZE IS MASTER 14X6 AND APC 15X4

Thank You
Jvalovcik

Bax

My Feedback: (11)

The "compression seal" is the seal made when the piston ring makes good contact with the cylinder liner and the piston, and also involves how well the cylinder head seals to the top of the liner, and how well the valves seal when compression loads are imposed.

If an engine won't idle well, you must first make sure it's well broken in. If the engine can be run at full throttle when leaned to just rich of peak RPM, and hold that setting "all day", then you can get the idle adjusted. Many times, the idle mixture's not adjusted properly, and then the modeler adjusts the high-speed needle. This gets the two out of balance.

You must first start out with a too-rich idle mixture. Lean the top end to just rich of peak RPM, and then throttle down and work on the idle. If the engine's not had sufficient breakin, then it may not idle well. If it has bad compression or leaking valves, it won't idle well.

If the engine can hold 2,500 RPM or lower, with a good transition, then it's OK.

jvalovcik

Thanks Bax,

I put the engine on a test stand and fine tuned it. I get it to idle at about 1,900 to 2,100 R.P.M. and the transition from idle to full power is OK.
my high R.P.M. is 10,000, will run it at 9,800.

I'm using a ACP Prop 15x4 and wildcat fuel 10% Nitro 18% oil.

Thanks for the reply

JVALOVCIK