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  1. #1

    New engine in 1200xlt backfire

    New motor won't crank then backfired. So checked compression got 95 psi across the board any suddgestions what's going wrong


  2. #2
    Waveruiner pauladave's Avatar
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    If it's a new motor, you should be seeing 120psi at least. If it won't crank, how are you getting compression readings? Backfiring, and not starting, could be caused by a thousand things, including fuel, electrics, carbs, reeds, plugs, timing, air leaks etc. Please give us more diagnostic info. Where did this 'new' motor come from (SBT, Yamaha, Ebay, garage sale), and did it run before you fitted it? Cheers, Dave.

  3. #3
    i know compression should be up just trying to figure out why its down if there is anything on the engine that could be off that is causing the low compression. i am not very familiar with the power valves i thought it could be that but i don't know i have replaced 4 or 5 of these models for customers and i just stumped trying to make sure its not something else before i pull it and check flywheel. I have not experienced this before on a rebuild job.

  4. #4
    Waveruiner pauladave's Avatar
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    I guess the motor is spinning but not starting? The compression check should be done with wide open throttle to get a true reading, and a good gauge is essential. The fact that you have even readings across the 3 cylinders is a good sign, even though it is lower than normal. The power valves need adjusting as per the manual, and you need to be sure they are cycling open and closed after you switch the ignition off. If not, the servo may not be good or not getting a signal from the CDI box. If you are in doubt, disconnect the cables and close them by hand to get an accurate compression check. You don't say where the motor came from, or if it ran before you fitted it. I've had engines that backfired after cranking for a while. That can be down to raw fuel building up in the crankcase then igniting, or bad timing, or weak spark etc. You'll have to be methodical in checking fuel, spark, reeds etc. Maybe pull the head before pulling the motor and inspect the pistons and power valves, and the head gasket. If it's a new engine from SBT or the like, contact the supplier before digging in, as they'll void the warranty if you start tampering!

  5. #5
    Moderator OsideBill's Avatar
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    backfire usually means an air leak, did you pressure test the engine? also compression will be low until the rings break in. I would be more concerned with finding the cause of the backfire

  6. #6
    went back over every connection today could not fine a source for the leak i am calling the company monday to talk to them before i put any more time in it if somehow they agree to a faulty engine i will hope they give some credit towards other motors i have to order i have already lost a couple hundred dollars on this rebuild trying to diagnose a problem that is the first of its kind and can't afford any more time i have to keep it timely cause i am about a week and a half behind and got a lot of boat to work on thats the reason i am trying to find a quick answer.

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