Motor hesitates at high RPMs

[rjwenzel]

Ok, so I built this bike up from a kit and an old yard sale 10-speed. It's an "80cc" 2-stroke kit (as I've seen up here, it's pretty much standard). Right out of the box it ran pretty well but I wanted to make some motor-mount adjustments to take away some of the vibration. So I moved the motor around and did a few things with rubber bumpers and whatnot -- nothing extreme. Well, the motor mount experiments all went bust and I decided to just clamp it back on in its stock configuration and I figured I could just deal with the vibration another way.
So I got it all bolted back up and now I have another problem. At low RPMs it runs very well (that's about 15-20mph or so). However, when I open up the throttle, it starts to speed up and then hesitates and sputters. This last problem is KILLING me! I can't seem to figure it out. I've replaced and re-gapped the spark plug: I've replaced the fuel filter: I've replaced the air filter: I've replaced the intake manifold gasket: I've completely taken appart the carburator and put it back together: and I've remixed the gas to a better less-oil ratio (because I was still running on the burn-in ratio).
Could the angle of the motor make a difference? Right before I put it back to stock, I had the motor tilted pretty far forward in order to take the slack out of the chain. The vibration in the seat was extreme and downright painful, but man that thing would MOVE! I'm tempted to put it back to that again just to see if it has an effect on the hesitation.
In any case, if anybody has any thoughts on this I would really appreciate it.

 

[skyl4rk]

My guess is that you need to raise the needle in the carb.

 

[ozzyu812]

My guess is that its running too lean. Play with the air fuel ratio. Don't have a HT so I can't tell you a step by step.

 

[Mountainman]

My guess is that its running too lean. Play with the air fuel ratio. Don't have a HT so I can't tell you a step by step.

think there is a clip on the needle inside this carb
that would be the adjustment -- leaner to richer

I know -- we are guessing here
one of you all with the HTs can set us all straight

so as to ride that thing MM

 

[h0tr0d]

Mount the motor so the carb bowl sits as level as possible. Then remove links of chain to take the slack out. If the chain tensioner doesn't quite do it then use half links. If all of that doesn't fix the hesitation then start messing with the clip on the needle. Start with a level carb first.

 

[h0tr0d]

Also remove the tank filter from the fuel valve and use an inline fuel filter that has a clear bowl. The larger filters can fill up with fuel so the carb isn't straining for fuel from a tiny clogged filter in the tank.

 

[Honda50]

I fully agree with ensuring the CARB's float bowl is level. Other wise 'Tip-In' and Main Metering Circuit enrichment problems may arise with a symptom you describe.
Fuel delivery to the carb should be checked and found more than sufficient, allowing the carb to take only what it needs. I removed my tank strainer and added an external, in series clear fuel filter. The flow to the carb improved from a dribble to a solid flow.

Use propane enrichment to diagnose if it is in fact a lean condition while the engine is giving it's symptoms under load and is thirsty for more. Ensure your existing carb float level is set properly to factory specs. This will be used as a starting point. If a lean condition is of fault due to an insufficient float level for the engine's consumption requirements, you can simply make a simple but precise adjustment to raise the fuel level in the float bowl by say, 1 or 2/32nds inch. If that fails to improve performance, reset the float to stock specs and possibly consider a Main Well Jet modification or increased flow rate replacement jet?

Keep in mind that the carb should be suspected only after the ignition system is determined to function optimally and the engine has sufficient compression.

Sounds like an interesting problem. I am curious of the outcome!

 

[Donald M. Haney]

I have a similar problem. Twist to the max and it sounds like it's running lean, but WOT going up a hill under a good load, and it does fine. No sputtering, no hesitation.

 

[rjwenzel]

I've had the carb appart

I've had the needle in every possible position and it's still a problem (in fact that was one of the first things that I tried before completely taking the carb appart). I've already replaced the fuel filter. It's interesting that the motor ran very well when it was tilted the farthest forward (and the carb float was in the least-level position). I still can't be sure that the tilted position was what made it run better or whether it was something else. I'll take a closer look at the current carb position and see. One of the other posts suggested that if I run the motor at WOT up a hill (i.e. under load) then I might not see the hesitation. I'll try that just to shed more light on the subject.

 

[arceeguy]

In all of my builds, the engine seems to run a little rich at low throttle positions and the engine "4 cycles" a part throttle. When you open it up, the engine fires on every stroke and picks up speed. I haven't "redlined" one yet, as I am just running the first quart through "fine tuning" the engine and breaking it in a little before the new owner picks it up. My personal "demo" bike has been up to 27 mph, and while it sounded a litle frantic, the exhaust note was steady and strong.

Hesitation at WOT and high rpms is usually a lean condition. Two strokes don't like lean conditions, even if your oil is 16:1, you can still blow it up.

If you can't get it to run slobbery rich with the clip at the lowest position, the fuel level in the bowl is most likely off.

My engine is set at the middle position, with the float raised about 1mm. This gives me a crisper low end and good top end. Without raising the float, the engine would bog slightly at WOT. When I dropped the clip one notch and it was too rich at low speeds but good at WOT. Raising the float slightly was like a 1/2 notch adjustment on the needle.