engine leans out mid throttle

Discussion in '2-Stroke Engines' started by BSA, Apr 14, 2008.

  1. BSA

    BSA Guest

    My bike runs sweet, but at mid throttle were the powerband really kicks in, I tend to think it's leaning out.
    As soon as I hit the choke full when warmed up it kills the engine strait away. It smokes like a chimney in lower end and also in higher end, but in mid throttle there is no smoke. Also at idle or low power when you put the clutch out the RPM stays pretty much the same, high end is the same but mid throttle the RPM skyrockets when I let the clutch out.


  2. Have you tried playing with the needle adjustment? Is this a Happy Time?
  3. BSA

    BSA Guest

    Yeah it's a happy time. Yeah it's on 3rd slot so quite rich, it's rich enough on most of the time it's just that it seems to lean out at mid throttle. Might try the 4th slot tommorow, see if that improves it. I think it might be to do with the exhaust, I know some people who did the baffle mod reported cut outs at he top end.

  4. sparky

    sparky Active Member

    I'm having similar problems. Kerf replied in this thread, saying that the exhaust is clogged... I must try cleaning it out tomorrow.
  5. sparky

    sparky Active Member

    I looked inside the exhaust, and it's not clogged. Even with 20:1, I haven't had enough time with this new bike/engine clog it up.

    To be exact, my engine is bogging out at mid throttle as well. Sometimes if I keep it at mid throttle for a good distance, I can make it to the upper RPMs where it runs great. Sometimes if it's colder it can make it past this mid throttle "hurdle".
  6. SirJakesus

    SirJakesus Guest

    If it bogs couldn't that mean you're running way too rich? Like when an engine bogs when the choke is on... Check your sparkplug. If its really built up with black greasy carbon your too rich, if its dry and light grey/white you're way too lean.
  7. sparky

    sparky Active Member

    It was definitely not greasy black. It was more tan/brown with a bit of grey/white. If anything, I'm guessing I'm too lean. I guess I'll have to open up the carb & figure out how to adjust my fuel/air mixture later today.

    I think I'll just try a 27:1 gas/oil mixture first, then start messing with the carb.

    Thanks for the help, guys!
  8. BoltsMissing

    BoltsMissing Active Member

    I had a similar issue, I think we had a long thread on it a few days ago, but this is how mine has panned out so far.

    I replaced the gasket with a new standard HT exhaust gasket and a standard HT muffler. It went like ****e when uphilling, no power.

    I lost the baffle,it fell of,so the next day I did the Coke can trick, and I had that power up the hill again. It eventually blew the Coke Can while traveling to get the new mufler I had in stock at some place.

    This time I trimmed the same standard exhaust gasket exactly the same size as the exuast port on engine.
    On the new baffle, I drilled with a 1/8th pop rivet drill bit holes 1" apart all the way along the baffle's pipe right thru. Then 1/4 turn, another set of holes all the way thru along the entire length of the inner baffle pipe.
    Is a touch louder, but "respectable".
    Goes up hill better with none of that fade out.
    Fuel Ratio, 20:1 Castrol 2 Stroke, 95 Octane fuel.
    If that exhaust port shamozzle messes up again, I'll be making my own custom header for the muffler so it matches the ex.port, and cut off the header that comes with the standard mufler and weld on my own, rather than wait around for it to happen in HT Land.
  9. sparky

    sparky Active Member

    I rode around for a good bit yesterday, but I still couldn't use my last quarter gallon of mixed gas. So, instead of waiting to come up with a new batch, I went ahead and tried to figure out how to adjust the air/fuel mixture.

    This is the first time I have adjusted anything on my carb. There are two screws (one which I didn't see before) -- the one I knew about is for idle, the other is for gas. I'm assuming this one for gas is my mixture screw, and I don't have hi & low screws. I'm also assuming I don't have that slide needle thing, either... as the only place it could be is in the diaphragm, but I'm not opening that up just yet.

    Anyway, I finally got rid of the mid-range bogging, but it came at a cost. Adjusting the fuel screw finally led me to choosing between good bottom-end & mid-range -OR- good top-end only. I'd adjust at 1/8th turns or less & there were pretty big differences in results. Just a quarter turn could add soo much vibration, or it could run sweet until I get to the top-end where it wouldn't bog, but it sure doesn't have the top-end oomph like it used to.

    I think I'm really gonna waste this gas today & then start fresh.
  10. sparky

    sparky Active Member

    Well... with the tuned carb & freshly mixed gas, it seems to be running about the best it can, but the top end is still not quite where it was before. Seems like I got 2 or 3 mph better outta this thing before.

    The only thing that seems to be different is the spark plug boot being fairly corroded. It looks pretty beat up for being less than a month old. Seems strange that it would only affect a portion of my RPMs based on my air/fuel mixture than just sputtering all the time. If the small engine man can't find something cheap for me, I'll just get an expensive 8mm hi-performance one from the auto parts store.
  11. BSA

    BSA Guest

    I haven't had a chance to muck about with it yet, but I will do yet. I will clean out the exhaust as I am using a lot of standard dino oil (16-1 mixture), and I'm running a rich mixture. That makes a combination for a lot of soot and gunk to accumulate in the exhaust.

  12. sparky

    sparky Active Member

    I figured there'd be lotsa carbon buildup in my small exhaust since I was running 20:1 of dino... but I doubt I've had it long enough to clog it up.

    If I did want to clean out my exhaust, how would I do it? Put it in some boiling water and then blow it out with an air compressor??
  13. sparky

    sparky Active Member

    Could an exhaust leak be causing this mid-throttle problem?

    I'm thinking it's either all the leaks I've got or the plug wire for me.
  14. sparky

    sparky Active Member

    Large, I replied in that thread asking what you asked for at the NAPA place. They said they don't do single wires.

    See here: http://www.motoredbikes.com/showthread.php?t=3981&page=8

    I went to the small engine repair guy & he said I've already got solid core, but I dunno what kinda metal it is. The terminal seems beat up, but the wire seems pretty thick. Also, I can't replace the CDI (easily), so my only option is to splice in a new plug wire?? Would that still improve it as much as you guys are seeing? Is it just the terminal? That's what it seems like to me.
  15. gtowncronie

    gtowncronie New Member

    Similar but different

    Hey- i just got my new motor from Spookytooth and it rips hard, untill i throught a bit mor gas/oil that i may have not calculated right (i'm pretty sure i got danm close to the reccomended 20:1... anyway this morning was a bit chilly and when i fired it up it took a while to get idling smooth, and after about 5 min i tryed to go through the power band and about 1/2 throttle it started spuddering and then sa couple backfires out the carb! so i backed off and took it real easy to work and at lunch i went to the motorcycle shop and got another oz of oil and that owrked a little better, i was scared of being to lean (second guessing my mixing ability) when i move the choke up it runs worse but doesnt die, i play with the idle screw and it doesnt seem to make a difference at all, and im 95% sure im 20-25:1 mixture, i dont have many miles on it and like i said before it was rippin yesterday, i just dont know what it is!!! please help
    -frame mount ST 80cc on a GT Kustom Kruiser at sea level and about 50°F
    Last edited by a moderator: Dec 18, 2015