Short surges in RPMs at low speed

Discussion in '2-Stroke Engines' started by Capitol Motorbikes, Feb 9, 2008.

  1. Sorry if this issue has already been dealt with somewhere else...

    MY 70cc 2-stroke will occasionally surge for about half a second during accelerations when the engine...usually around 12-15mph and again at 18-20mph. It tends to happen only during take offs during the first few minutes of riding. Any clues as to why this is happening?

  2. fetor56

    fetor56 Guest

    You said "occasionally'...what,so it doesn't happen all the time...and even then it only happens when the engine is cold?
  3. HI Fetor,

    Yeah, you pretty much summed it up. I've added a boost bottle recently...not sure if it has anything to do with the surging. I'm going to check for air leaks this afternoon.
  4. fetor56

    fetor56 Guest

    Check for air leaks...try adding and subracting the boost bottle and see if that makes any difference.
    I'm assuming you've o-ringed your carby?
    Make sure the air filter is clean and that you've sealed it properly.
    Other than that i'm afraid it's trial and doesn't sound like a major problem,just one of those HT annoyances.
  5. Thanks Fetor. IT's too windy to do any testing today. I'll play with it tomorrow.

  6. JemmaUK

    JemmaUK Guest

    Since you mention the boost bottle it might be alot to do with that... it sounds like the inverse of a fuel starvation problem - ie the engine is suddenly getting an uneven boost of fuel mixture hence the revving..

    You say that it happens at two speed periods while the engine is cold (under choke therefore higher mixture). I think what might be happening is there is a effect similar of that of a tuned pipe happening within your induction system at the two given speeds that is sucking the boost bottle dry and providing the motor with a super-choked amount of mixture (ie very rich) hence the engine is racing...

    What might be worth trying to see if this is the case is having the machine on its stand with a cold engine and trying to replicate the problem - if the exhaust suddenly becomes darker (black/blue-grey) at the given RPM/speed point it will indicate that this is what is happening..

    Are you using the standard manifold and have you checked the inside of that part for obstructions as well - since you have fitted the boost bottle there might be areas that would facilitate such a velocity wave to build at certain engine speeds and cause this problem...

    hope that helped

    Jemma xx
  7. Thanks Jemma,

    Since the surges are fairly consistent (I can predict them now), I wondered the same thing about it stemming from a resonance issue, thereby pulling from the boost bottle. Looking back, I did make some mods to the exhaust around the time the surges began. I'll try the experiment you suggested and see what happens. Thanks for your really "get" these engines.
  8. mikaleno

    mikaleno Member

    Cap MB,

    Did you fix your surging problem? My HT 70 is surging at speed too, I have a boost bottle also, but it works fine on my other bike. I guess I'll start digging into some of these fixes on this thread.