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  1. #1

    GP1200R Backfiring Issues

    Hi all,
    I did a full rebuild recently on my engine all the way to the crank:

    * New seals everywhere on the case, pressure tested <1psi drop per minute (over 6 min) w/ 3bond 1211.
    * Oside Bill-esq full carb rebuild with salt treatment, metal prep, and paint, added new Mikuni kits and had pop-off 42-45psi w/ added t-handles.
    * Jetted per OSB specs for D-plate, F/A's, and Free Flow.
    * Compression was a bit low, so I bought some new OEM pistons/rings from Lowell.
    * New plugs

    The engine overall appears to be in great shape. I searched the topic and there is so much ambiguity except to check the reeds. I checked the reeds and, although I didn't replace them, they appeared fine. No cracking or damage, only blackened tips. Some of them are "sticky", meaning with a slight push they open just fine, so i'm not sure if that is an issue.

    Put it all together, primed the carbs (that took more fuel than I thought) - and finally managed to start only to have continued backfiring. One was pretty large. I'm stumped and frustrated guys....any thoughts? I'm going to pull the carbs and replace the reeds anyways - but unless the crank is out of phase, I have no idea what can cause this...


  2. #2

    Join Date
    Sep 2013
    Bennettsville, SC
    I replaced some sticky reeds on my wife's venture 700. The reeds were leaking back, backfiring. I think the stickiness was due to it being oil injected. Maybe some additive in the oil making the reeds want to stick to the plastic sealing surface. FWIW these reeds were stainless. I replaced with fiberglass. That being said, I don't think the reeds will cause backfiring unless they are broken. A slight amount of blowback is normal, especially under hard acceleration. Backfiring is usually caused by crank out of phase, which is unlikely if it ran OK before you rebuilt it, or electronics that have failed, or been installed incorrectly. Are the coils installed correctly? That's where I would look first. And, due to the backfiring, you will need to probably replace the reeds. Also check your fuel pump diaphragms for creases. Backfires kill the fuel pumps.

  3. #3
    Man, I don't think I have the mental fortitude to tear this thing back down to the crank again. I've done it countless times. In any case, how do I go about checking the crank phase?

    I'll open the electronics to make sure I kept the right firing on the coils. I did play with them about a year ago as I was having some electrical issues. I'm sure I put them back but I could have maybe switched two and gave up without replacing them!

    I'll pick up some extra diaphragms along with the reeds. I just fear if I don't find the root cause, it'll backfire again and i'll be tearing it apart continuously into the winter.

  4. #4
    TX21's Avatar
    Join Date
    May 2007
    North Texas
    Backfire says check spark to me. All of them backfire a bit on a trailer. Mine does but its very minor, runs perfect in the water. Get the multi meter out and start checking plug wires, coils, connectors all of it. Can't remember if the flywheel has a keyway but if it does did you put it on? Metal on the crank pickup? I did that on a dirt bike once. Took me forever to find. Even the Yamaha dealer couldn't figure it out.

  5. #5
    Spark is all ok. I had electrical issues before and replaced the CDI, plug wires, magneto, and plugs. I can see spark in all 3 plugs. Keyway was installed when I put the flywheel on. Cleaned all pickups before putting the engine back together.

    I originally thought the same as you - I wasn't sparking and vapors built up. But if I drop some gas into the cylinder, it will fire up and briefly (very briefly) idle like it is supposed to. That's whats really driving me insane....Something else must be causing the backfire.

  6. #6
    TX21's Avatar
    Join Date
    May 2007
    North Texas
    See any smoke in the engine compartment? Could be a air leak?

  7. #7
    Everything I am reading indicates this is a out-of-phase crank. I can do a quick test to measure - but I'm fairly certain that with everything I have replaced (and I have done everything in A+ fashion), it must be cause by the crank being out of phase. There is no other reason I can think of that would cause a backfire.

  8. #8

    Join Date
    Aug 2006
    What was the cause of the rebuild? Unless it was something that locked the motor up HARD, it is very unlikely to get an out of phase crank. Ignition is the most probably cause. Just because you see a spark does not mean the spark is at the correct time. Junk on the magnet on the flywheel, problem with rectifier screwing the CDI up, or a leaky carb pouring extra fuel that ignites in the exhaust.
    You are on the hose, right? Because you need water in the waterbox to have proper backpressure, and less air getting to unburned fuel.

  9. #9 runs now. LT1, I was thinking the same thing at work today...the amount of force to twist a crank must be pretty damn high. I purchased this crank used from a friend - and even then it's probably very rare that a crank I have no history on is twisted. Turns out, I had #1 and #3 wires flipped to the coils. It's been that way for quite some time since I had prior spark issues and hadn't ran the ski since.

    e ran great on the water. Lows need to be adjusted slightly - but highs felt great. Likely hitting low 70's with the mods.

    I feel like when I have a problem with a jetski, it's like going to WebMD with a cough and diagnosing yourself with cancer. I go right to the worst case every time.

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