Home Forums Tech Talk KT100 stumbles/misses in only left turns

This topic contains 14 replies, has 7 voices, and was last updated by  Dustin McGrew 5 months ago.

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  • #28278

    Dustin McGrew
    Participant

    I’m having a bit of a weird issue with my KT100 engine. This issue first started happening when I was practicing 2 weeks ago. In 2 of the left turns my engine would stumble or miss slightly. Wasn’t really sure what was causing it. I thought it may have been low fuel at first, but that wasn’t the problem. I had the tank about 3/4 of the way full which should be more than enough. Unfortunately while I was trying more fuel in the kart my seat strut snapped and that ended my chances to try anything else.

    I thought it might have been a bad spark plug as my engine was also down about 500 RPM during those practice runs. So I figured a new plug would fix the RPM problem and the left turn stumbling problem. We had a race yesterday so I put a brand new plug in, it fixed the RPM issue but not the left turn stumbling/missing problem.

    The weird thing is that this issue only starts happening after I’ve ran about 4-5 mins. So I’m assuming it is not something that is simply loose. I’m also assuming its not a problem with something in the fuel line or pulse line as I would think that would cause the issue to happen as soon as I get on the track. Its almost like it is something that is triggered by temperature. It got progressively worse during the final yesterday.

    The only change I made before my practice session 2 weeks ago was to increase the clutch stall speed by about 200-300 rpm so now its at 8500. Not sure if I somehow messed up something there or not. I wonder if 8500 is just a bad spot for it?

    Here is a video of the entire final. The issue starts happening around the 5:30 min mark and after 12 mins the stumbling/missing is very noticeable in left turns.

    Any ideas as to what might be causing this?

    #28279

    Tim Koyen
    Participant

    Didn’t watch the whole video, but any chance its something silly like the axle sliding over and dragging the brake in the lefts?

    #28281

    Dustin McGrew
    Participant

    Tim, its funny you mentioned that because I had the same exact issue in our 2nd race of the season lol. That was just a case of me being a noob and not checking my bearing set screws. Had a few of them fall out and the axle shifted and rotor started rubbing the brake pads. That was more of a gradual feeling though and it bogged the engine down. This new issue with left turns its like the engine completely shuts off for a split second.

    #28316

    Gary Lawson
    Participant

    Check the fuel pickup tube. Make sure its long enough and not old/hard. Also make sure your leg doesnt grt near the holes in the airbox. Your suit can choke the engine slightly.

    #28317

    Dustin McGrew
    Participant

    Thanks Gary. You might be right about the airbox. I’ve heard that issue mentioned before and it never even crossed my mind that it could be what my problem is too. Maybe I lay my leg down a bit more after 5 laps. I do know that the airbox holes are pointed towards my leg a bit. Hope that is the problem, simple fix :)

    I’ll check on that pickup tube too. It’s been on the kart since I bought it in October.

    #28342

    Gary Lawson
    Participant

    Yep no problem. Always angle away and zip tie to the seat so it cant turn. Good luck

    #28353

    James McMahon
    Participant

    If its not the airbox, observe the rev counter (or review the data) and see if it behaves oddly when this happens. It does sound like an ignition issue. They tend to crop up when the motor gets some heat into it and disappear when its cold.

    #28375

    Walt Gifford
    Participant

    That’s a carburetion problem. You don’t see it too often but it happens in a certain series of turns/straights with some set ups. The track empties out the crank case and you open the throttle too fast and no fuel. Usually a long straight tight turn short straight another turn and when you get on it nothing.

    Gif

    #28981

    M Crotty
    Participant

    I’ve been working through something just like this. So far I’ve tried gearing shorter and playing games with the pedal in the problem corner, both of which do help. I’ve checked the frame for twist and it’s slightly twisted the wrong way to cause my issue, it’s lifting the inside rear more in that direction, maybe it drops too soon. Obsessively clean the power valve.

    Anyway my scenario matches Walt’s description, a fast S, short straight, then hairpin. What would you do to the carb? My guess is:
    Clean passages.
    Raise the fuel level in the bowl.
    Replace needle valve.

    Curious,
    mike

    #29020

    Dustin McGrew
    Participant

    Mike, another EKN member messaged me today and said that the low speed needle might need to be set richer (maybe an 1/8 turn more). He said the engine can start to run leaner as it heats up so that might by why the problem that I have only occurs after I’ve driven for about 5 minutes. Unfortunately I haven’t been able to get to the track to try different things yet but I should be on track this Friday. I’ll let everyone know if anything works.

    #29031

    Walt Gifford
    Participant

    What’s your pop off pressure? Maybe drop it down to 8 psi or raise the arm a little. Have you cleaned the carb screen every track day. Throw in a new pumper.

    Gif

    #29044

    M Crotty
    Participant

    Thanks Walt, I’ll try to find a way to keep a little more fuel in the bowl one way or another.

    The low speed jet fix sounds odd, why would it be directional? Maybe you’re barely on a threshold.

    I’ve cleaned my carb passages as a first attempt (and the screen filter), if that doesn’t help I’ll try bending the arms. Diaphragm type fuel pump has 4 races since a rebuild, I’m expecting to rebuild each season so that should be OK.

    I’m rotax, so my pop-off pressure isn’t controllable, however measuring it would probably teach me something. I can bend the float arms or use heavier floats to increase the amount of fuel in the carb bowl.

    My ribs are recovering, I’m hoping to make the next club race July 5th. I’m changing a bunch of stuff during my forced hiatus so it’s going to be difficult to tell what’s working this time around.

    #29057

    tony zambos
    Participant

    Don’t bend the individual float arms. Instead, bend the tap over the needle valve.

    #29196

    Walt Gifford
    Participant

    Actually I was speaking more to the OP.

    #29605

    Dustin McGrew
    Participant

    Finally got back on the track yesterday. I rotated my airbox so the holes faced outward instead of toward my leg. Made 3 10 lap runs and it never stumbled. So this problem had to have been created by my leg covering the airbox holes.

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