Losing power at high RPM or How I spent my Friday evening

Discussion in '2-Stroke Engines' started by Timbone, Jun 20, 2014.

  1. Timbone

    Timbone Active Member

    So, since I solved my latest problem (broken motor mount) and got the engine into a safe and steady condition. I am cruising again!

    But there's a new problem: as the bike builds up to cruising speed, the engine acts as though it has a governor. I end up feeding in WOT but 22-23mph is as fast as she will go. Considering this thing was cruising at 26-29 mph and hitting good RPM while not requiring WOT. It FEELS like the slide in the carb needle is slipping backwards. The RPMs get so high then will advance no more.

    This bike is on it's 8th or 9th gallon of gas which represents quite a few miles. It starts quickly and easily, has a sweet idle, and quickly ramps right up to 20 mph. Man, this thing is fun! I love it!

    I read through a great number of forum posts and here's what I saw as possible causes: mixture too lean, air leak, clogged air filter, broken seal, failing CDI, bad spark plug, etc.

    So here's what I did: I dissected my exhaust, removed the baffle, removed my heater hose rigged expansion and I found no issues whatsoever. No clogs, and it looked quite good inside. Put all that back together.

    I was able to add just a touch of torque to the carb/ air inlet connection. I removed the air filter and it looked brand new. Just a sheen of oil on the plastic cover. I wiped it clean and replaced it. Do not think this is an air leak issue.

    I was able to look in at at the slide needle and I had good smooth action from beginning to WOT. Everything looked good, clean, and tight.

    I opened up the magneto case (something I had never done before) and it looked shiny and clean in there. No broken seal!

    I removed the sparkplug and it looks a bit "carbonny". This engine is not running lean. If anything, it is running a tad rich. Gap looked good. Replaced the plug and took a test ride.

    After a positive start, I soon found the same problem: bike would accelerate rather quickly to 20 and when reaching 22-23, the same problem arose. No more RPM. A kind of surging. Complete lack of RPM advance.

    So, I figure I need to look at fuel flow. Not fun because I have a nearly full gas tank. So, on my final pass, I switch the fuel valve to "off" to empty the line from valve to engine.

    Well, guess what happened! RPMs jumped almost immediately! I was able to do a nice fast run before running out of fuel; it can probably make nearly a half mile with the valve shut off. But once I opened the fuel valve again - BAM! - the engine again gets to 23 mph and no more. Turn the fuel valve to off and and I have my speed back.

    What can explain this?



  2. crassius

    crassius Well-Known Member

    being too rich will explain it - start by dropping the needle to a leaner position - later, try lowering float level, then try a smaller main jet
    Timbone likes this.
  3. Timbone

    Timbone Active Member

    Thank you for the response. I'll move the needle one position.

  4. Decidium

    Decidium New Member

    It being rich definitely causes the surging at high RPMs. I had the same problem, and leaning it out one notch really helped.
    Timbone likes this.
  5. jaguar

    jaguar Well-Known Member

    yeah but it was running good before and then not.
    So something changed.
    And I doubt that the jetting changed.
    Sometimes with 2 strokes you have to go over absolutely everything till you find what is off.
    Could be anything.
    Plane the cylinder top and head to make sure it isn't a head leak. And apply Copper Coat upon re-assembly.
  6. Timbone

    Timbone Active Member


    Thank you for the response.Did a quick 2 mile run and instantly found a solution! It's something connected to the fuel valve. When the petcockis set full open, then I hit the 22-23mph bogdown. After experimenting, I found a setting where I was once again able to achieve advance and high RPMs.

    I'll attach a pic. When I set the fuel valve there, it runs great! Petcock.jpg
  7. professor

    professor Active Member

    Tim, what you are doing is restricting the flow- leaning out the mix.
    You have a carb/ fuel ratio issue.
    Timbone likes this.
  8. crassius

    crassius Well-Known Member

    true, it could go way lean on you without warning
  9. Timbone

    Timbone Active Member

    Yes! I guess that is what I am doing! The thing is flying now!

    OK. I need to move the c-clip out to a leaner position. I set it up dead center.

    Thank all of you for your help!

    OK. One more question: when I examined the sleeve, I looked it over well. I had no trouble whatsoever putting it together. But I wondered, how would you take it apart to get access to the c-clip and the needle? Do you need to free the throttle cable on the twist handle to get slack first?

    Thanks very much once again. I keep learning!

  10. HeadSmess

    HeadSmess Well-Known Member

    look at it harder and it will come clear...try squashing the spring, and when the cable nipple pops out the bottom of the slide...grab it! reef it out sideways! remember that slot the cable goes in goes towards a tiny lil pin in the carb.

    forget adjusting the needle, what you need is a smaller main jet. the needle does the mids. the lows as well on these. but has no affect on WOT!

    the trusty old method for people that dont like buying small drills and chucks or new jets? very fine copper wire soldered into the jet. one strand of the wire from a headphone lead, at a time, test riding in between, until you put one too many in. then take that one back out, and maybe the 2nd last one too, just too keep it on the "safe" side of right, ie...slightly rich.

    setting the fuel tap to a certain spot is not the way to do it ;)
  11. Timbone

    Timbone Active Member

    Thanks very much for the reply! No doubt you give good advice! I am starting to see that WOT mixture is controlled by the main jet.

    I tell you, though: I did manage to take apart the carb needle slide and I adjusted the c-clip one more position to 4 towards lean. The next move to lean would be the end. I DID see improvement at WOT, with the moto revving up to 24-25 before refusing to advance Rpms further. It's not as surgie as it was, but its still too rich.

    I just don't want to diddle with the carb when there are so many things to monitor on this bike. On tonight's ride, it was the muffler trying to work loose. Yesterday, the chain seemed a bit tight. It's always something! :)

    Here's the way I am thinking about mixture: airplanes have mixture control, going from full rich to full lean. I figure since I have a certain touch with the fuel valve, I have options. Since the motor runs great and idles well at "full rich", I'll just run it that way when I am at low to moderate speed. When I hit an open stretch of road, I tweak the full valve, lean it out and get my 26-30 mph cruise on. It's not difficult at all.

    I never run this thing WOT with the fuel valve constructing flow.

    Yeah, I am being stubborn.

    Gotta give you guys props: you all figured this problem out very quickly for me and I thank you!