Brakes Right way to slow down and brake with these HT engines?

Discussion in 'Bicycle Repair' started by RuiGomes, Jul 20, 2012.

  1. RuiGomes

    RuiGomes New Member

    My bike is now ready and I went for a small ride around the block. However, I didn't manage to properly slow down and brake.

    I lightly pressed the brakes and released the throttle. Then I pressed the clutch lever, but as soon as I did that the motor sounded like it was going to explode (like 12k RPM) and everything was vibrating much more than when riding normally on the motor, I was afraid to damage something.

    How do you guys brake on these things?

  2. motorpsycho

    motorpsycho Active Member

    release the throttle, pull in the clutch and apply the brakes.
    if your engine revved out of control when you pulled in the clutch, it sounds like your throttle is stuck or you have a massive air leak.
    make sure that you have the slide inserted in the carb correctly. It can go in 2 different ways but only one way is correct.
    the slide, which slides down into the top of carb. You have to unscrew the top cap on the carb and lift to get the slide out.
    the slide has 2 slots in the sides, a long slot and a short slot.
    Make sure that the long slot is on the right side (when looking down at the top of the carb). The long slot lines up with a pin that's cast into the inside of the carb. The short slot goes to the left and lines up with the idle screw.
    if you have the slide in backwards, your throttle will stick wide open.

    It's also possible that your throttle cable is binding up and causing the throttle to stay wide open, or the slide is binding up in the carb.
    when you release the throttle and pull in the clutch, your engine rpms should drop to an idle, it should not rev up.
  3. Fabian

    Fabian Well-Known Member

    Sounds like your throttle cable is sticking; the engine trying to run at full throttle or your carburettor slide is sticking at wide open throttle.

    Either way, if you've pulled in the clutch, your engine has revved out of control. The big end connecting rod bearing is now totally f*&ked.
    Just a matter of time before the needle rollers in the big end eject themselves from the connecting rod and fly all around the bottom end before getting stuck in between the piston and cylinder wall; locking up the engine.
    Best case scenario is that the needle rollers fly up through the transfer ports and hammer the top of the piston and the cylinder head.

    At this point, it's best to purchase a new crankshaft and connecting rod assembly and rebuild the engine.
  4. RuiGomes

    RuiGomes New Member

    The throttle cable is installed the right way (biggest hole to the right side of the carb) and I am able to use it properly when riding (if I push the throttle it accelerates as it should).

    @Fabian, it that would happen, was it supposed for the motor to work properly when I'm not using the clutch? I'm able to start it and be pushed on idle and I can even give it more throttle and it accelerates very well.
  5. motorpsycho

    motorpsycho Active Member

    I doubt that the motor is trashed.
    I think that your throttle is sticking. sometimes if you over tighten the carb cap, it can distort the carb body, making the slide get stuck.
    ride it & when it does it again, hit the kill switch to kill the motor but don't touch the throttle.
    let the engine shut off, remove the air cleaner & look into the carb to see if the slide is stuck. it is also possible that your cable is too tight which will cause it to bind & make the throttle stick open.
    do you have wire ties holding the cable to the frame? If so, the can make the cable stick inside the cable housing if they are too tight.
    also, the twist throttle may be sticking.
    put some oil down into the throttle cable and grease up the twist throttle.
    if you have an air leak, it will rev out of control all the time.

    post a pic. of your carb & throttle assembly. also show the cable routing.
  6. Purple Haze

    Purple Haze Active Member

    That is great advice. I remember my first build, the throttle was a Murphy's Law kinda thing. Cable binding, over-tightened the cap, etc. One thing you missed was to make sure the carb throat is butted up against the intake, mine felt like it did, but had a small burr keeping it from sliding all the way in, creating a massive air leak which caused high rpm idle.
  7. Fabian

    Fabian Well-Known Member

    The motor may not be trashed "right now" but the damage is done. Those big end connecting rod needle roller bearings have all got flat spots on them.

    From this point onwards, it's just a matter of time before the case hardening breaks up and exposes the soft base material - i wouldn't give it more than 500 kilometers before the big end bearing fails, either locking up the motor, or spitting the needle rollers out the exhaust and bead blasting the cylinder head and piston face.

    After such high revs, the motor is effectively trashed...
  8. wan37

    wan37 Member

    okay another thing to try is adjust the set nut and screws on each end of the throttle cable.You might have them set to tight.If needed you also can take the set screw nuts off for more adjustment.That should drop the carb slide down farther in the cylinder for normal idle.
  9. Skyliner70cc

    Skyliner70cc Active Member

    If engine speeds up or won't drop down to idle right away, it means you are lean and probably have an airleak. as for braking, I just pull the brake handles until I come to a stop. My HT engines all have a centrifugal clutch and I don't have to worry about that nonsense.
    Last edited: Jul 21, 2012
  10. motorpsycho

    motorpsycho Active Member

    possible, but if it had a massive air leak, it would rev high out of control all the time. he says that it idle fine & only revs high while riding it & pulling in the clutch when coming to a stop.
    he said it isles fine until he tries to ride it.
  11. motorpsycho

    motorpsycho Active Member

    so, the times that I've held my throttle wide open while sitting still has trashed my engine? Wee eaters run at wide open throttle, as well as leaf blowers and chain saws. granted they probably have higher quality bearings,.but I doubt that his engine is trashed.
    think of how many people have these engines and have air leaks, which rev uncontrollably until they can fix them.
    they aren't trashed.
    it is possible that he has an air leak, but based on his description of the problem, it doesn't sound like an air leak. but his description of the problem may not be exactly accurate.

    another problem could be that the float bowl is running empty, causing it to lean out while riding. maybe he has bad fuel flow into the carb?
    Last edited: Jul 20, 2012
  12. Fabian

    Fabian Well-Known Member

    If the carburettor slide has been stuck in the wide open position and the engine has been allowed to rev to the point where it wouldn't rev any more,,,, the connecting rod bearings are damaged. It's just pot luck as to how long the engine will last before the needle roller bearings fail.

    As i've previously said:

    The motor may not be trashed "right now" but the damage is done.
  13. Skyliner70cc

    Skyliner70cc Active Member

    There is a difference betweeen running an engine at WOT with a load on it and unloaded. Weedeaters load is the cutting string that limits rpm to safe levels.

    With that said, the HT engine's quality is poor. Please consider using an oil that contains some castor, that should help prolong engine life a bit.
  14. motorpsycho

    motorpsycho Active Member

    ok, so we have different opinions, lets move on.
    did the o.p. ever figure out his problem?
    in his original description, (his second post about this) he says it idles fine, runs fine and accelerates fine.
    the only time that it revs out of control is when he's riding and he pulls in the clutch to come to a stop.
    personally, to me this is indicating a stuck throttle, binding throttle cable, float bowl running empty (leaning out, but the engine doesn't die), warped carb body (stuck throttle), carb cap on too tight, bent jet needle causing it to bind in the hole, making the throttle stick, or a possible air leak.
    or maybe, he's forgetting to release the throttle with his right hand before pulling in the clutch even tho he says that he is...ANYTHING is possible.
  15. Fabian

    Fabian Well-Known Member

    Looks like the OP will need a new throttle cable and crankshaft assembly and engine gaskets.
    Might as well replace what's likely to have caused the problem and purchase parts for what is likely to "become" the problem...