Motor hesitates at high RPMs

Discussion in 'General Questions' started by rjwenzel, Sep 29, 2008.

  1. rjwenzel

    rjwenzel New Member

    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.

  2. skyl4rk

    skyl4rk Guest

    My guess is that you need to raise the needle in the carb.
  3. ozzyu812

    ozzyu812 Member

    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.
  4. Mountainman

    Mountainman Active Member

    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
  5. h0tr0d

    h0tr0d Member

    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.
  6. h0tr0d

    h0tr0d Member

    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.
    Last edited: Sep 29, 2008
  7. Honda50

    Honda50 Member

    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!
  8. 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.
  9. rjwenzel

    rjwenzel New Member

    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.
  10. arceeguy

    arceeguy Active Member

    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.
  11. rjwenzel

    rjwenzel New Member

    Still have not done anything

    Ok then, I'll take the carb appart this weekend and change the float level. I'm sorry I don't have more time durring the week but work gets in the way . . .
    Here's a side view of the current carb position. The way I had it before was tilted quite a bit more to the front (left in this photo).

    Attached Files:

  12. rjwenzel

    rjwenzel New Member

    No Luck

    I took the carb appart and bent the clips a bit so that it pretty much puts the float at the top . . . and it did nothing! I guess the next thing that I'll do is put the motor back in the same position it was in before I started to have this problem.
  13. userix

    userix New Member

    Pardon the necrobump, but I am having the same problem as the OP. Was running perfectly fine, decided to remount the engine in attempt to lessen vibration, and now whenever I hit WOT, I get hesitation and sputtering. It was running perfectly fine before and nothing else changed or broke. I have now tried to remount the engine like it was before, but it's still hesitating and sputtering at WOT.
  14. crassius

    crassius Well-Known Member

    that could be a vibration problem that is foaming the fuel in the bowl - does it feel like it is running out of gas for a sec than come right back as soon as it slows a bit?
  15. userix

    userix New Member

    Yeah, If I slow down for a bit, it runs again fine. The vibration is pretty bad once it gets up to max speed. How would I alleiviate this foaming gas problem? I just find it weird how it does this after only remounting the engine slightly different. I made sure every bolt is tightened down and not loose
  16. crassius

    crassius Well-Known Member

    different parts of the frame will vibrate at different levels - I usually try to get the motor as low as possible in the frame & use a bit of PVC between motor & frame
  17. KCvale

    KCvale Motorized Bicycle Vendor

    Just a few things...

    You don't adjust carb angle by engine angle.
    The engine has only 1 way to mount properly, that is the back mount metal to metal at a 90 to the seat post, period.
    You can go up and down to help with the front mount but unless it's a V frame with the right size downtube, get a front mount.

    Do 'THE test' on your mount...
    Hold your top bar with one hand, the top of the head with your other, and push back and forth AS HARD AS YOU CAN, if the engine moves AT ALL that is a fail and where your vibrations are coming from, hence the good front mount.

    I suggest you get an NGK 5944 (BPR7HIX) Iridium spark plug while you are there as well, I put them in everything I build.

    And remove that useless boost bottle and plug the intake hole, that could very well be causing your problems at high RPM's.

    Just tips.
  18. rjwenzel

    rjwenzel New Member

    Ok guys, I've pretty much had this thing parked since I failed on the last attempt. But now I have to fix it because I'm moving and I need to sell it (because I'm not going to ship it). Thanks crassius and KC! I'll mount it lower, more solidly, and I'll pinch off the boost bottle. I was going to pull the boost bottle anyway but it's good to know from an expert that that might be causing the problem. When/if I have time to fix it, I'll post back here.
  19. Ridin clean

    Ridin clean Guest

    I have a lil of the same problem, almost feels like the spark plug gives up. Before i changed the carb, WOT,i would do 33mph,back off (just lil) and would hit 35ish. Another question, moving the needle down means raising the clip?
  20. Nino

    Nino Guest

    Hi Guys,

    I have the same problem too with my motorcycle a 155cc Honda bike. I change the carb to size 28. after that. My bike sputters in high RPM. And Some time if I am going to force it to the top speed the bike dies. with a sputtering sound. This problem is more in the carb.but some would say it might be. Pistons. don't know wich one is correct.