4-stroke Motorcycle hesitates with throttle?

Question

I just rebuilt the top end of my single-piston 4-stroke Yamaha XT550 1983. Before the rebuild when it started blowing smoke, it also started hesitating real bad, pretty suddenly. I assumed it was due to a bad oil ring. Now $500 later, I put it back together and it starts and idles fine, but will not rev smoothly. I can't even get it to hold a constant 3000 RPM's. When I give it gas, it revs a little, then chokes and sputters and refuses to go faster. I don't think it is timing, since I just put on the cam and made sure it was lined up with the timing mark. I do think the carb is flooding, but I don't know if that would cause this--plus I try to control it by turning off the petcock valve. I get RPM hesitation all the way until the carb runs out of gas. Any ideas of where else to look?

Answer 1

what was the $500 spent on, exactly? Is the carb totally worn out?  Is the tank full of cak? Is your"petcok" filter completely blocked? do you have a dead bird inside the air filter? Did someone forget an old rag inside the bottom end? is there some grit blocking the main jet? was the one who you spent the $500 a cowboy who did tons of damage on your bike EG cracking the rings, stripping  or overtightening all bolts.. how to say why your motor won't work after some unknown entity wrenched on it..... bit difficult to say...

I'll tell you one thing, the XT550 is the one bike that made me give up kickstarting singles, and I knew all the tricks you know put it slightly past TDC, etc etc it wil still backfire on you and break your leg, just torch the thing and throw it down a cliff.

Answer 2

Does it have the stock airbox and exhaust system?

Answer 3

Did you check if the valves were sealing properly when you had the head off?

Whenever doing top end work on a 4 stroke - check for valve leakage.

Hold the head so the intake, or exhaust port is facing up.

Pour liquid in and fill up the port.

If the valves and valve seats are in good condition, no liquid will leak out.

Tight or leaking valves can cause hesitation.

Did you adjust the valve clearances after installing the head? - if not, check them.

Perform a cylinder leak down test to check for leaking valves.

Answer 4

i would have started with the carb personally. but if thats not it you might want to work your way up-

its either carbs (floats ect) or the timing in this case. but if it is idling fine i doubt its the fiming, you can usually hear a difference there as well