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vintage power band

Discussion in 'Technical and Troubleshooting Torque' started by Lennyburger, Oct 3, 2006.

  1. Before I rebuilt the botom end in the Yamaha MX80 [ 1981 model ] it seeemd to have a reasonably high revving power band. Since rebuilding it maxes out a lot lower and starts to splutter and misfire. Should I investigate the fuel side of things or ignition timing? I adjusted the points to open a couple of degrees before TDC as they had to be removed for the rebuild. Nothing was done to the carby while it was off. Do these old two strokes have much of a powerband to start off with? It does seem to have a lot of torque down low as you can pop monos easily, just no crisp clean get up n go attitude. :twisted:

  2. Timing would be the logical place to start since that was changed. You may lose some of your new bottom end power though.
  3. I'd be looking at the timing to start with. Do you have a dwell meter? I have found this a much more accurate way of setting the points gap than with feeler gauges. Once you have the gap right it's then pretty simple to move the timing back or foward to find where the engine is happiest.
    Then pop a new spark plug in & do a run to see if you're in the ball park as far as your jetting goes. Then go from there
  4. I'll give it a go

    Thanks for the advice, I'll start with the timing. I read somewhere that retarding the timing will deliver more top end power so that's probably where I went wrong. A dwell meter wouldn't be any good as there isn't a 12v supply to use, unless an external battery is handy. I haven't got one anyway, I used a multimeter to set the opening of the points. Next weekend I'll give it a go and experiment I think, with advance and retard to see the effects.
    Cheers! :grin:
  5. No peak performance

    I played around with timing yesterday. Retarding the advance made it lose bottom end torque alright, but gained nothing in the top. Still doesn't want to rev out hard. I'm guessing she might just be a little too old to behave like the newer ring-dingers. Maybe I'll upgrade to a mid 80's 125, then I'll be satisfied!
  6. Andrew said this, but at the risk of stating the bleedig obvious, have you replaced the spark plug?
  7. And strip, soak, and clean out the carburetor very well. After sitting for
    that long it is guaranteed that it is all blocked up with gunk made from
    dried petrol.
  8. I was going to suggest main jets, but as its just been sitting there idly and accumulating crud in the carbs, i agree. take the carbs apart, use carb cleaner, but make SURE you remove all the rubber bits before attacking it with carby cleaner, that crap makes the neoprene rubber go funny (it expands and weakens it, can really screw things up)