Im confused. My brand new Surpass getting broke in on the test stand. Needle settings rich, 15% champion fuel, OS F plug, Even though it will run decent and transition well it surges at the higher rpms. And the idle while reasonable is not smooth. I have tried different plugs, prop, fuel and nothing. It will surge regardless of needle setting both high and low. I have taken off the carb and sealed the mating surfaces , disassembled the carb and checked for blockages. Even checked the cam to be sure its in right and the lash on the rockers.

It sounds like fuel delivery,
carb air, or engine temperature.

Maybe the vent tube inside the
fuel tank is pressing up against
the top. Maybe the vent line is
kinked behind the firewall. Maybe
there is something in the muffler
pressure nipple.

Maybe the klunk I hitting the back
of the tank. Maybe the fuel line is
kinked behind the firewall. Maybe
the needle extension is getting
pushed by the cowl.

Maybe there is not enough air entering
the engine compartment. Air passing
over the cowl draws air out. If not enough
enters, it causes a partial vacuum inside
the cowl making the engine run slower.

The same thing could cause the engine
to get hot and slow down. Heat related
surging has a slower cycle than air pressure
related surging.

Jenny

Edit: Sorry I misread your post. I thought
you said it was ok on the bench but surged
in the air. Since you are still working on
bench running, disregard (but don't forget)
the stuff about cowling.

[QUOTE=jeffie8696;12073549]Im confused. My brand new Surpass getting broke in on the test stand. Needle settings rich, 15% champion fuel, OS F plug, Even though it will run decent and transition well it surges at the higher rpms. And the idle while reasonable is not smooth. I have tried different plugs, prop, fuel and nothing. It will surge regardless of needle setting both high and low. I have taken off the carb and sealed the mating surfaces , disassembled the carb and checked for blockages. Even checked the cam to be sure its in right and the lash on the rockers.[/QUO

I really don't know OS 4S, but maybe the crankcase vent system is clogged or not working. Just a thought, I don't even know what that would cause. Good luck. But, be careful, don't void the warranty.
Sincerely, Richard
AMA 861960
Club Saito #635; Saito 56, 100, 120abc, 130T, 180
YS F 120; OS 15, 25, 40, 46, 60, 61, 80

If the engine is too rich, then surging is normal. Also, you mentioned that you are still running them in. After you are able to get the engine to run through a tank of fuel at full throttle with the high-speed needle leaned to just rich of peak RPM, and you have found the correct idle mixture and RPM, then a reason surging would have to be tracked-down. Usually, it's a fuel delivery problem...bad muffler pressure, cracked fuel line, dirt and other trash in the carb, air getting into the fuel via vibration induced fuel moving around, a bad fuel tank position, badly-routed fuel lines, and so on. If you have trouble tracking down the problem in the model, then put the engine on a test stand and see if you still have operating difficulties. A test stand makes finding problems much easier than trying to do it on the model.

It is still on the test stand , I use my test stand all the time and so I am confident the system is ok. It will tune to very rich on both the high and low speed needles. I have tried leaning the top end to stop the surging .

I agree with all of the above suggestions, but here is an odd one I ran across once. I used to use thin walled very flexible fuel line in the tank from the clunk to the fuel pick up tube. Worked fine on some 40 sized engines and even a lower powered 60. But with a high powered 60 the same set up caused the engine to surge. Tried everything, none of the lines leaked the engine ran fine on the bench so I swapped out the tank with the one on the bench that it ran on. The surging stopped. I figured the flexible thin walled silicone line would partially collapse, then open when the engine slowed down, then collapse as the engine picked up, over and over again. So you might try that.

I will install a larger engine on my test stand and verify my setup, thanks for the advise.