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Struggling above 7000 rpm

Discussion in 'The Garage' started by Beakster, Mar 15, 2020.

  1. Hi,

    On my 81 special it runs great to 7000rpm, then it struggles.
    It has pod filter on the Y hose and stock exhaust.

    I switched the main jets from 135 to 140 but same problem.

    Could this be a float level or needle issue.

  2. pcream

    pcream XS400 Enthusiast

    Working on other projects (49cc scooters), I've found pod filters to be more trouble than they are worth. They never seem to work right and seem to have variable flow, making tuning quite difficult. I would put the stock airbox on, but if you don't have that I would replace with a large foam universal filter and then try to retune.
  3. xschris

    xschris a lifestyle not a trend Top Contributor

    Agree with stock air boxes/filters. If you still have a good set of stock exhaust it's the way to go. Make sure you have synced the carbs with a manometer. Floats should be 26mm for brass floats and 22mm for foam ones. Make sure to measure both sides of the float and remove the bowl gasket. Make sure you have copper core non-resistor wires and caps if you run resistor plugs. Compression test the motor? Set valves?
  4. NewHavenMike

    NewHavenMike 1976 XS360C Top Contributor

    Switching a jet by 2 sizes surely would change full throttle high end performance. Does the bike sputter or does it run out of steam? Earlier or higher in the rpm range? If the plugs are sooty and black they will start misfiring. Sooty would indicate a rich condition. The bike has to be at operating temp before you start making decisions.

    Is this same problem youre having from 135 to the 140 worse? If youre in say 3rd gear, at 5000rpm, you should be going to full throttle and up to redline with minimal break-up. You may have to deal with some running issues with your setup but youll be able to get it close enough.
  5. Difference between 135 and 140 doesn't seem noticeable. It will be a bit richer now but it makes me think that the main jets aren't the problem. Everything is perfect up to 7000 rpm. I'm wondering if the needle setting or float settings could be the cause of such an issue, I've never changed them before.

    I do not have an original airbox, the bike came with the after market filters. Stock exchausts are still on there.

    Valves are set and carbs are synced. Don't have a compression tester but the fact everything feels perfect below 7000rpm is a good sign that compression is healthy.
  6. xschris

    xschris a lifestyle not a trend Top Contributor

    Small problems at lower rpm's/load that don't show can show up at higher rpm's/loads.

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