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  1. #1

    Mikuni triples making a popping noise at lower RPMs

    I have freshly rebuilt carbs with genuine Mikuni parts, pressure tested and popoff set to 52psi on my 1200 non-pv SUV. New short block is broken in properly, idle set a bit high right now at 1500 so I will fix that but there is a noteworthy crackle or popping many times over when I run under 5000rpms. I assume this is a tuning issue on the low needles? They started out set to factory spec but I had to turn them open 1\2 turn each to get it to quit bogging. What is this pop, pop, popidy pop pop all about? I have the drilled out fuel pump return mod from Osidebill as well as the 100 jets across all 3 carbs.


  2. #2
    mudslanger's Avatar
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    you need to check your plugs to see if it is lean.

  3. #3
    Quote Originally Posted by mudslanger View Post
    you need to check your plugs to see if it is lean.

    thanks for the tip. I pulled plugs hear the results... The front 1 or number 1 I guess it's called is a little bit dark so I guess that means it is running rich correct? The other 2 have a nice tan or light brown color. 1 thing I did fail to mention is that these plugs were brand new and the running that I did was on the breakin with 32 to 1 oil mixture. Could it be that running 32 to 1 is causing this sound or could it be that 1 cylinder running too rich create this sound? Please take a look at plugs number 2 and 3 to let me know if they look like they are the correct color.I have a and a scope camera that I will take pictures of the piston wash with next week...or can a person even tell anything from a ski with brand new pistons and only 10 hours of heavy mix feel run through it?Click image for larger version. 

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  4. #4
    Happily Self-Employed WFO's Avatar
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    popping thru intake? check reeds?

  5. #5
    Quote Originally Posted by WFO View Post
    popping thru intake? check reeds?
    Hmmm, although I have not checked them since I broke the ski in last week I can say that the reeds looked excellent and were very clean when I put them in all tolerances were in specification according to the manual. thank you for the tip though... If it comes down to it I will just have to pull them and look. now I am curious if I will be able to turn the little endoscope camera in that direction to see the reed when I go to look at the piston wash

  6. #6
    Yamaha artisan Cutlass's Avatar
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    Have you tried pumping the primer or closing the choke while its popping? That's something I might try if I thought it was a rich/lean issue. Of course if its a problem with the reeds, ignition system, or compression then that probably won't tell you much

  7. #7
    Nope, have not tried that but certainly will do. Thanks!

    I also came across one of Bill B's posts on another forum last year that spoke to this issue

    Wondering if I explained the popping sound to u guys correctly. The popping sound would appear to come from the exhaust area if this matters? I need to document the entire rpm band where this happens and where it stops at. Where do these SBNs change over to where the high speed adjustment takes over? Is that below 4k?

  8. #8
    Yamaha artisan Cutlass's Avatar
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    The transitions are described by throttle opening and not rpm in the Mikuni manual. so 1/4 throttle, 1/2 throttle, 3/4 throttle, and WOT for example.

    It does matter if its out the intake/carbs or the exhaust. Out the exhaust would seem to indicate there is some unburned fuel/oxygen in the exhaust that is popping/burning in the exhaust.

  9. #9
    Quote Originally Posted by Cutlass View Post

    It does matter if its out the intake/carbs or the exhaust. Out the exhaust would seem to indicate there is some unburned fuel/oxygen in the exhaust that is popping/burning in the exhaust.
    Then it beggs the question...if I did Bills fuel pump drill-out mod and changed out the stock 1.2ns's for larger 1.5's to make it run fatter and happier is this the extra unburned fuel blowing out the exhaust or is that the consequence of that front plug being so dark? My assumption of the front plug being fat, oily and dark is that I have either my lowspeed or high speed adjuster on that carb open too much and that would allow excess premix fuel to dump into that cylinder...allowing some to escape unburned till popping in the exhaust. ???

    Thanks for the explanation on throttle position as well.

  10. #10
    Yamaha artisan Cutlass's Avatar
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    Tough to say. Just thinking out loud here, but if the cylinder was rich and it did fire, then theoretically all or most of the oxygen should have been burned up. Which leaves inert CO, CO2, etc and unburned fuel in the exhaust. It shouldn't ignite and pop/backfire. But if the cylinder misfired, then unburned fuel and oxygen enters the exhaust pipe and could ignite.
    So that begs new questions like : is the ignition system ok? is the crank out of phase so that the spark isn't lining up with the compression stroke properly? Is it a lean misfire? Like I said...just thinking out loud.

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