Please Help. Rear Engine Is Stumbling.

Discussion in 'Motorized Bicycle General Discussion' started by 5-7HEAVEN, Apr 18, 2009.

  1. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

    Joined:
    Aug 2, 2008
    Messages:
    2,364
    Likes Received:
    38
    I have a problem that started about a week ago. On my morning rides both TLE Mitsubishi 43cc engines on "The Dragon Lady" perform flawlessly. The temperature is in the mid-70's.

    On the afternoon ride home both engines start out okay. Temps are in the low-80's. Everything's fine until I reach and maintain 8900rpm or higher. This is where maximum hp is attained. Then slowly the rear engine starts sputtering like it's running outa gas. If I floor the throttle the engine falls flat until I let up. If I give it part-throttle it'll struggle and catch itself. Then I can go full bore for a few seconds before it falters again.

    Both engines have their 24oz tanks being fed by a single Happy Time 2-liter tank. Fuel/oil ratio is 50:1.

    The loss in performance is only when top speed is above 34mph and 8500rpm. Any speed lower and both engines respond beautifully.

    Both engines have ADA S1 expansion pipes. The front engine has 39" exhaust tubing connected to the bleed port and exits below the crankset. It has no muffler/silencer.

    The rear engine has the standard silencer and four inches of connecting exhaust tubing.

    I washed both air filter elements but that didn't help. Tomorrow I'll replace both sparkplugs. Then I'll replace the expansion pipe's silencer.

    Can anyone figure out what this problem is? It doesn't seem to happen in the morning:-||
     
    #1 5-7HEAVEN, Apr 18, 2009
    Last edited: Apr 18, 2009
  2. bobdirt

    bobdirt New Member

    Joined:
    Apr 12, 2009
    Messages:
    13
    Likes Received:
    0
    Did you replace the fuel filters? A fuel filter can begin to clog, still pull fuel at low to mid RPM's even reach high RPM for a breif time before the engine begins to starve.
     
  3. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

    Joined:
    Aug 2, 2008
    Messages:
    2,364
    Likes Received:
    38
    Thanks. I'll check that.:-||
     
  4. Michigan Mike

    Michigan Mike New Member

    Joined:
    Dec 9, 2008
    Messages:
    509
    Likes Received:
    0
    Sounds like the rear engine is running out of gas at high speed.

    I'd start by replacing the fuel filter and making sure the tank vents properly.

    Always good to start with fresh gas and new spark plug too.

    .trk.
     
  5. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

    Joined:
    Aug 2, 2008
    Messages:
    2,364
    Likes Received:
    38
    Bob. Mike, both engines share the same large automobile fuel filter. It is transparent; when I prime the fuel lines I can see the air bubbles flow through the filter and into the Happy Time reserve tank. If it was defective both engines would have high-speed stumbling.

    I just replaced both sparkplugs. Since I use the bike every day there is always fresh gas in the tanks.
     
  6. ZnsaneRyder

    ZnsaneRyder New Member

    Joined:
    Nov 21, 2008
    Messages:
    163
    Likes Received:
    0
    Have you tried removing the carburetor and cleaning and reassembling it? That usually works for me when I have an engine issue.
     
  7. Michigan Mike

    Michigan Mike New Member

    Joined:
    Dec 9, 2008
    Messages:
    509
    Likes Received:
    0
    Heaven,
    If the fuel filter has some age on it I'd still replace it just to be sure ... shouldn't cost much. Seems like it might be possible for a partially clogged filter not to let enough fuel through to run both motors at high speed.

    But, as someone else said, it's starting to sound like the carb may need cleaning. Here's a way to check ... when the back engine starts to falter, try choking it a little bit. If it picks up speed with this richer mixture it may indicate that the carb jets need cleaning and/or the air mixture needs adjusting. This would at least confirm that it's probably a gas problem.

    Hope that helps ...
    .trk.
     
  8. HoughMade

    HoughMade New Member

    Joined:
    Apr 15, 2008
    Messages:
    624
    Likes Received:
    1
    Another vote for fuel starvation. You have the classic signs.

    I don't know where the blockage is, but I feel pretty confident that you have a fuel restriction. I had this exact issue on my bike and it required a thorough dissassembly and cleaning of the carb. I have also seen this exact behavior on numerous small engines (I used to work at an equipment rental, maintaining and repairing small engines) and it was pretty much always a lack of fuel.
     
  9. Bikeguy Joe

    Bikeguy Joe Godfather of Motorized Bicycles

    Joined:
    Jan 8, 2008
    Messages:
    11,529
    Likes Received:
    3
    Maybe it's just not-quite-keeping-up, unloads and breaks into a four cycle?
     
  10. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

    Joined:
    Aug 2, 2008
    Messages:
    2,364
    Likes Received:
    38
    Methinks it's fuel starvation too. The engine isn't sucking fuel from the reserve tank.

    This morning my engines' tanks were almost full. This afternoon I'll do a full throttle run on the highway and maintain that for two miles, traffic lights notwithstanding.

    If both engines perform well, then I'm gonna install an electric fuel pump in the line. That should solve fuel delivery problems.

    Usually if I have time it takes me about 30 minutes using the priming bulb to fill the engines' tanks.
     
  11. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

    Joined:
    Aug 2, 2008
    Messages:
    2,364
    Likes Received:
    38
    I believe I've located the fuel starvation/stumbling problem.

    This morning I left home with three full tanks. The roads were wet, friction rollers constantly slipping. It took a little longer to reach work.

    When I left work, the tanks still had lots of fuel. Both engines never missed a beat, even after two miles of sustained high speeds.

    To solve my problem I need to manually pump the primer bulb in the fuel line for 30 minutes, physically siphon directly from reserve tank to each tank...

    or install electric fuel pump and small battery.
     
  12. 5-7HEAVEN

    5-7HEAVEN Well-Known Member

    Joined:
    Aug 2, 2008
    Messages:
    2,364
    Likes Received:
    38

    Mike, the carbs on these engine have no mixture/high speed/low speed adjustments, just idle. I raised the fast speed and that cured the problem of the rear engine dying out when idling.

    Engine stumbling problem has been resolved, I believe.

    I picked a a Walbro WYK-58 carb. Throttle fits, carb bolts on but ports don't match. Need to find the right manifold.

    Does anyone know where I can find the matching manifold?
     

Share This Page