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Backfire and cold left side

Discussion in 'The Garage' started by Eike, Dec 29, 2021.

  1. Eike

    Eike XS400 Member

    81 xs400 special 2

    So got it running. Had to clean the carbs about 4 times. Run it with the pilot 4 turns out. But it backfired a lot (see below). Set the left carb at 3 turns out and the right carb at 4 turns out on the pilot and that did better.

    Stock air filters are back on. Carbs are stock.

    Issues now: idles great up to about 2k. Then at 2-5k it will backfire from the exhaust. Also sometimes a surge from that range as if something is coming online. Backfire will continue after surge happens.

    After running Left side (driver’s side) pipe is barely warm. I can easily hold on to it. However I think the backfire is coming from this side. The right side is too hot to touch.

    Pulled the left side plug and turned over engine and there was spark. Below is what the plug looks like.

    Checked the left coil with 2 different ohmmeters. Primary (red/white, orange) registers anywhere from 1.4k-3.68k then drops to 0 and won’t re-register until you take the leads off and put them back on. Secondary (red/white, cap off plug wire) registers just fine at 8.6k.

    Carbs are only bench synched.

    So what do you think is/are the issues?

    -Is the left side coil bad? If so, what is the cheapest replacement? I’m using this in a go kart so it doesn’t have to look pretty.

    -If not the coil is it something to do with the carb?

    -Any other ideas? All of the above?

    Thanks for your help.
    0240303D-48C4-4F0B-8A5A-B3D35EF6B5C8.jpeg 0240303D-48C4-4F0B-8A5A-B3D35EF6B5C8.jpeg
  2. xschris

    xschris XS400 Guru Top Contributor

    Looks like those plugs are soaked in fuel or oil. Also a xs400 sohc should have NGK BP7ES or BPR7ES plugs in it. Depending if you running resistor on non-resistor plugs in it. Make sure they are gapped correctly. I would make sure your carbs floats are set to the correct 26mm height ( brass floats). Then sync the carbs with a manometer. Check coils without anything attached to them including plug wires and harness wires. All the tests are in the manual. Make sure the motor has good compression (125-155)

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