Just rebuilt top end starts up fine , but the motor cuts out when you give it gas. runs Ok at topend but taking off on even the slightest up hill it just quits. I cleaned the carb but it has not helped??? ... Baffeled.@#%$&!! I've been wrenching on my bikes for twenty years and I am just stumped!

You only give it air when pushing the gas pedal. Clean the throttle body and idle air control motor.

There is not an actual rev limiter. The top end is controlled by the carb. You actually want it to sound a bit rich at full throttle so that it has enough fuel when cutting under a load. What causes me some alarm is the revving up part. If it revs too fast that is a lean condition and can quickly ruin the piston and cylinder much like running straight gas. A torn intake boot can cause this. The high speed spec for the TS400 is 9,600 rpm + OR - 500 RPM. Your saw may not have a needle to adjust the high side as some have only the low speed adjuster. Also when you disassemble and clean the carb always use a new kit with the gaskets and diaphragm. Any Stihl dealer can furnish the kits and they aren't terribly expensive. Often between $10-20.

It sounds like the pair valve hoses have come loose or un attached to where they are supposed to go.You have constant velocity carbs on that bike and those hoses need to be correct in order to gain top speed.They attach to the line that goes to the velocity diaphragms for the carbs.

check your belt and clutches

Check to see if FUEL SHUT OFF SWITCH Went BAD..............Located in Passenger side kick panel. will have red button on top. Unplug it. take paper clip and complete circut. If car starts , then replace fuel cut off switch.It is an INERTIA SWITCH,similar to a MERCURY SWITCH.

I'd bet you 10 to 1 it's not the coil. It sounds like it is starving for fuel and needs a carburetor adjustment. When you rebuilt the top end, did you put a bigger jug and piston? If so, your problem most definitely is the carburetor needs to be re-jetted.

What caused you to rebuild it? Did it seize, or quit running? Was it losing power before the rebuild?

That's where my money is but if you still believe it is the coil, you can check it with a spark checker. This is a handy little tool you can find at AutoZone, Harbor Freight, etc. It's a one piece tool that connects between the plug and the coil. The plug stays in but you remove the wire and put the checker on, one side connects to the plug, the other side you connect the plug wire. There is a light in the middle that will flicker when you crank the engine if it has a good coil.

Put the checker on and start the bike and see if you are losing spark...if the light stops flickering at top end, then you do have a coil issue...but like I said before, having seen this problem hundreds of times, my money will be on re-jetting the carburetor.

Check the plug. I have had a very similar problem just recentley yesterday infact. It could be that you are running the oil mixture too rich as well...Good luck

prob need topend rebuilt hate to say it rings are shot sounds like

I don't think this is a float issue. One way to check is to turn the gas off, take the bowl off the carb, hold the float up, and turn the gas back in. If fuel flows, the float needle is toast. It sounds suspiciosly like a worn topend, but it could also be a clogged jet ot fuel filter, or a faulty CDI.

is it a shovle or an evo any way most top end noise is valve related pushrod adjustment could be off hope correct piston to cylnd clearence is right, piston slap or nocking sounds like top end noise also hope this helps give me a rate please