New Chain Problem - Lurch?

Discussion in 'Frame Mounted Engines' started by RoverGT, Jan 12, 2012.

  1. RoverGT

    RoverGT New Member

    I've done two builds now. My first build is working great. The second build is working well, but it has a problem that has developed recently that I don't understand. Off the throttle, everything is great. It pedals smoothly and coasts without any of the problems that happen when the engine is engaged. Once the engine is engaged, it introduces what I can only describe as a "lurch". It's as if there is one spot in the chain that is putting extra load on the engine and it causes the bike to accelerate and the decelerate over and over while on the throttle. The bike is not unridable, but it's definitely very very noticeable. The chain is nice and tight, but not too tight with about a 1/4 to 1/2 inch of "wiggle" on the top. I checked that the chain tensioner is not guiding the chain off kilter. I have a "mega motor" 2 cycle kit mounted on a beach cruiser. Any help would be appreciated.

  2. Al.Fisherman

    Al.Fisherman Active Member

    First off what type of build are you working. It's hard to diagnose without these facts. Are you using the sprocket that is supplied with the standard Chinese HT kits?

    Ok I Google it and found that it's the standard HT kit. You might want to bevel the chain gears, as they might be catching on the chain. OR go to a better made chain. Make sure the wheel sprocket is not warped, I have seen it on 1 kit I purchased. Also I have found that on my cruiser that the engine can shift due to torque, (even after a perfect engine set up) is yours, and there is a easy inexpensive fix.
    Last edited: Jan 12, 2012
  3. RoverGT

    RoverGT New Member

    Thanks for the response. I am using a #41 chain from tractor supply. The stock chain stretched so bad I couldn't keep it tight. It has, historically, worked fine up until the last day or two.

    Other things I've checked so far:

    1. The rear sprocket is true and hasn't shifted. I have custom 68 spoke wheels. I didn't use the rag joint. Instead, I patiently drilled the hub and mounted the sprocket using four 1/4" grade 8 bolts and lock washers to space the sprocket away from the spokes and align it with the front cog.
    2. The front and rear sprockets are aligned.
    3. The tensioner isn't pulling the chain off alignment.
    4. The clutch is adjusted properly. I loosened and then tightened it to see if it would have an impact. None.
    5. Revved the engine with the clutch disengaged to see if the throttle response was smooth. It was.

    It seems to be directly related to applying throttle under load. As I pick up speed, it gets worse and worse.

    A few more details on my build:

    It's a micargi falcon gs with, what seems to be, a standard "happy time" kit on it from (2 stroke). It has a boost bottle on it (nice addition!). Other than that it's a stock build. During the build, I did file the engines drive cog teeth down. I just rounded them off gently with a dremmel.
    Last edited: Jan 12, 2012
  4. Cavi Mike

    Cavi Mike Member

    Sounds like the rear sprocket might be worn uneven but it's really something that needs to be seen in person, that is, you need to really take a look at it and try to piece together why this is happening. It's not like it's some strange underlying electrical problem that can't be seen with two wide-open eyes, the problem is right in front of you and you just need to look harder.
  5. RoverGT

    RoverGT New Member

    Thanks for all your help. You guys were on the right track. The problem turned out to be related to how I'd mounted the sprocket to the rear hub. I'd custom drilled both the hub and the rear sprocket with a drill press to accommodate my 68 spoke wheels. I'd used 4 x 1/4" grade 8 bolts to fasten the sprocket to the hub with spacers and what not. One of the bolt heads had torn off, not completely, but just enough to where it would flex and rub the hub skirt under load from the engine causing the friction and hence the "lurch" i tried to explain. The torn bolt head routed out the hole I drilled for the sprocket over time making the problem worse, so I have a new sprocket on the way. I'll probably do the same setup again. I think I just got a bad bolt. I can't imagine any other reason why it would tear outside of over tightening, which if thats the case, I think over-estimates my arm strength. Took some tinkering to figure that one out. I appreciate all the help, even with my poor explanation of the problem.
  6. Cavi Mike

    Cavi Mike Member

    It doesn't take much to shear a 1/4" bolt. Maybe you should use a torque wrench next time.
  7. RoverGT

    RoverGT New Member

    Yea, seems like a logical suggestion and what not. I did try. Gap between the spokes is too narrow for a torque wrench. Didn't want the "ugly" end of the bolts to the outside. Easy problem to fix once I found it.