Engine Trouble Engine only works under heavy load

Discussion in '2-Stroke Engines' started by RadicalxEdward, Sep 18, 2014.

  1. So I recently switched this engine to a new bike cause the old one was crap. For some reason it's lost 3-10 miles per hour on average. But that's not the biggest issue. I've tried various fuel /air mixtures and such, but for some reason the bike cuts out when it's not under heavy load a full throttle. So going up hills, fine. Accelerating, fine. but once it gets going about as fast as it's going to go (which is only about 19mph at the moment instead of the old bikes 30 despite being much heavier) it cuts out until it slows down a little, then runs/accelerates a bit, cuts out, slows, runs/accelerates, rinse and repeat.

    At first I thought was because of air getting into the fuel line because it used to start bubbling every few minutes unless i loosened the gas cap for a few seconds, but I'm pretty sure i've sorted that out. Now it's just this cutting out and low speed thing.

    It also sounds like there's something clanking slighty at the bottom of each revolution. but i can't figure it out. I looked inside and it's nothing above the piston and nothing obvious below it but i haven't stripped down the whole thing. Any and all ideas welcome.

  2. crassius

    crassius Well-Known Member

    sounds like a loose motor mount
  3. As in the motor to the bike? That's super tight. Any other ideas? I'm gonna check for leaks tomorrow but I've used silicon gasket maker stuff around everything so that shouldn't be an issue.
  4. crassius

    crassius Well-Known Member

    shaking the carb with too much vibration will cause surging - not usually reduced speed tho

    look carefully at all you did to see what changed between bikes
  5. jaguar

    jaguar Well-Known Member

    2 strokes like a rich mixture under load and a lean mixture at no load

    so logically we can deduce that your jetting is too rich or possibly the air filter has poor flow through it. take it off and press it against/around your mouth so that it seals against your face and then blow thru it. You should feel next to no restriction. If there is restriction then get a good filter.
  6. jaguar

    jaguar Well-Known Member

    and maybe you are living at high altitude which requires less rich jetting to match the oxygen poor air
  7. Thank you guys for the responses. There are no physical changes between the bikes.

    My cities altitude is only 200 ft. I have also tried each setting on the jet needle (using the stock nt carb) and it always happens (though I do notice running leaner gives more power but makes the engine much hotter. At it's leanest setting I went to do a full throttle plug chop and when I killed it there was smoke coming through the carb.

    Also I tried running it with no filter at all and with just the inner spongy stuff and no outter plastic to the air filter. Feels like it runs slightly leaner but still has the same issue under light loads.

    I don't mind if it only does 20 cause that's not gonna get me killed. Jerking around and starting/stopping will.
  8. It's possible I'm not doing the plug chops well enough but they never come out white or obviously tan. They're always black. Which made me think it was running rich. But at it's leanest setting the engine gets really hot compared to other settings and the plug still looks the same (and like I said I still get the no load issues)

    I really appreciate your guys help. I've been avoiding asking for help while researching other people's issues but nothing has worked. I'm not as smart as jaguar when it comes to this stuff and this is my first engine build but I am technically minded.
  9. HeadSmess

    HeadSmess Well-Known Member

    changing the needle height doesnt change the mixture, except at partial throttle and idle.

    everything says its too rich. get a smaller jet.

    hot? gets too hot? sounds like your idea of "hot" is lukewarm. tepid, even.

    clanking noise...either the main bearings are about to go, its plain old gear noise (possibly a cracked pinion gear), or the magnetos rubbing... could be others but theyre the first to spring to mind...
  10. jaguar

    jaguar Well-Known Member

    if the cylinders are basically the same then there could be a cranking pressure difference between the two.
    use sandpaper and glass to make sure the head and cylinder sealing surfaces are truly flat.
    other than that, it can only be that the main jet is too big for it.
    maybe you had an air leak previously that compensated for the rich jetting, and you "accidentally" took care of the leak with the replacement.
  11. My definition of hot is when it blisters your skin if you were to touch it for a full second or two. Is hardly call that tepid.

    Anyway, it's all the same parts but you're right jag, I could have accidentally fixed a gap that was helping before. Sounds like I'd be better off just getting a better carb rather than dumping more money into a crappy one. Any recommendations for a cheap one that's worth getting? I see people dis everything but the expensive stuff. Which to me is stupid. If I wanted to dump 2-3 grand into a bike I'd buy a real motorcycle.
  12. jaguar

    jaguar Well-Known Member

    you can get a good carb for around $75 but then you have to figure out what to do about the manifold and the new sizing required.
    lucky for you I sell manifolds. click on my signature link.
  13. crassius

    crassius Well-Known Member

    stock motors run best with standard intake & $10 NT carbs
  14. HeadSmess

    HeadSmess Well-Known Member

    even slightly modified engines with highly tuned exhausts run better with the stock carbs...

    sensible people know what needs changing and whats perfectly fine as is.

    what needs changing? removing weld/casting flash from ports and manifolds.

    matching manifolds to ports.

    the exhaust... there being nothing suitable on the market...just modify what you have :)

    the jet.

    everything else is a waste of time or money. and the main proponents of such "gimmicks" are the people that sell them!

    the cheapest modification ive made yet? replacing the POS head gasket with a few loops of plain old teflon plumbing tape, twisted up into a "string". raise compression AND ensure the PERFECT seal :) costs all of nothing if you have the roll handy already ;)

    save your money. read up on engines. ignore that signature link, its a load of drivel.
  15. Thanks for the various advice everyone. I've still been having trouble. I can't afford to go replacing stuff. I used to get the bike up to 30mph. Now I can't crack 17 and can only sustain about 9-10. which is almost useless. i've taken everything apart as much as i can. nothing is broken. Piston head is a little wobbly when the top of the case is off, not sure if it's supposed to be or not. done everything i can with the carb. Nothing has helped. I can't for the life of me figure out what changed between when it was running well and now. Oddly it's more reliable now. no stalling or fuel issues, just this WOT issue. (aside from my gas cap flying off and being run over :-(
  16. jaguar

    jaguar Well-Known Member

    wobbly? maybe you should check the cranking compression. some auto parts stores can loan out a compression tester.
  17. noxuz

    noxuz New Member

    jaguar please i really want to try your cdi.
  18. jaguar

    jaguar Well-Known Member

    so buy it and try it.

    Radical, maybe the ignition is the problem.
    clean the plug and minimize the spark gap in case the spark is weak. a bad stator coil is very common. I think they are only $10. read on my site how to test it.
  19. Can't afford to buy parts to experiment. Can only buy what i absolutely have to to fix things. However, I was trying to take the piston off to see why it was so wobbly (wasn't sure if it was supposed to be or whether it even mattered) and I figured out what that metal clanking sound probably was. It was...you guessed it...metal clanking. lol. bad jokes aside my wrist pin/crank baring is missing half of it. So i have to replace that and find the other piece of it that's been relaxing inside my engine all this time. Only problem is I can't get the wrist pin out. I took the little clips out but the pin is staying put even using a hammer and screwdriver to try to knock it out.
  20. jaguar

    jaguar Well-Known Member

    sometimes you have to heat up the piston and then cool off the pin before trying to knock it out.
    butane torch the piston and ice cube the pin.