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  1. #1

    Yamaha 1200 suv is slow !! Help !!

    Last fall I purchased a 1200 SUV with a blown motor. #2 piston had melted, connecting rod had snapped off and became a rip saw driven by the other 2 cylinders. It took out the crankcase, reed cage, exhaust manifold and anything else in it's path. Being as it was not economically rebuildable I purchased a "new" rebuilt 1200 NPV engine from SBT. While waiting for the motor to arrive I rebuilt the carbs. They did have a fair amount of white corrosion on the aluminum carb parts which I carefully cleaned with a wire wheel. (Not Salt water) I also ran small wires through all of the holes in the carbs and blew compressed through everything. The 3 carbs were also run through my ultrasonic cleaner as well. I set the pop off to 40psi on all 3 carbs. The Low speed and High speed jets are at 1 turn each. The oil pump has been removed with a block off plate installed. 93 octane fuel, Maxima Oil, 30:1. So here's the actual problem; The motor will start right up and idle great all day. It will rev up good at the dock and briefly upon initial acceleration. As soon as it starts to get on plane it bogs way down, dropping to nearly an idle again even though the throttle is still pegged. Running it at half or quarter produces about the same results. Eventually if you gradually bring the speed up by giving it light throttle inputs it can be coaxed up to get on plane, lumbering along at 20-25mph, and only about 4000-5,000 revs, deep low sound and nothing as high rev as other Yamahas on the lake. It is so slow it is embarrassing! I have tried playing with the return, running a line directly into a bottle, bypassing the water separator and check valve, etc. which seemed to help it go a bit better, but still doesn't seem to be the real problem. I have contacted SBT who only said to look for a mouse nest in the exhaust. A local mechanic told me to just buy another rack of carbs to fix it. I am very familiar with 2 stroke engines and carbs having worked on countless motorcycles, ATV's and race karts for the past 30 years but this thing has me stumped! It almost seems like it is electronically governed down like in a limp mode. There is no flashing warning on the dash. PLEASE HELP!!


  2. #2
    Honestly it sounds to me that carb is set way too rich. Whats the size of the main jet, pilot jet, the needle and seat? Any engine mods like aftermarket flame arrestor, etc etc? Also 40 popoff is i think for for moderately modified engines. Have you checked your sparkplugs? If not remove them and inspect them. You'll know if its rich or lean since you said you've worked on motorcycles,atvs,etc. Also try changing the popoff pressure to 60psi.

  3. #3
    I didn't make note of the jet sizes figuring they are whatever comes stock from Yamaha. This boat is still in Wisconsin which is the same state the previous owner had been running it for several years before it blew up so the jetting should not have changed. I should have mentioned that I had previously tried a 60 psi pop off a week ago and it was no better and possibly even worse. I can agree that being too rich would slow it somewhat and burble on top end but this thing runs at only half the RPM's that it should.

  4. #4
    Yamaha artisan Cutlass's Avatar
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    Maybe its lean, like its starving for fuel. try running in the on position or reserve. Maybe the pickup in the tank needs attention (ie flapper door needs to be removed).
    Maybe the pumps in the carbs aren't pumping well due to weak pulse (not likely), fuel pump check valves are creased/not sealing, poor quality carb rebuild kits, etc. Maybe the internal filters are clogged. Maybe the gas is old and crappy.
    Just throwing out ideas for you to consider.

  5. #5
    I do realize this is a completely different engine, but I agree it sounds rich also. I just pulled my hair out for days on a dual engine seadoo boat, it was doing the same exact thing. Started with one engine, and then the next weekend both engines. So naturally I looked for things common to both engines. (fuel delivery and electrical) Who would have ever guessed that 3 of the 4 high speed adjustments (4 carbs) had backed out and it was just running too rich. It was just like hitting a rev limiter, it would run up to 5,000 rpm and back down to 4700. Spark plugs looked like a normal running engine, no sign of being rich!

    Don't know if you have a choke or not.
    If you do, pull the choke out slowly, if it speeds up, your lean. If it slows down, then your rich.

  6. #6
    If you're almost sure that the carbs jetting is stock then that's fine. I know the carb could be a pain to remove/install. Since you've rebuilt the carbs, did you use Genuine Mikuni carb rebuild kit? The consensus here is that anything other than that CAN cause issues. I know some have used other brand carb rebuild kits with no problems but I cant comment since I've always used Mikuni's.

    When you get the chance. DO a plug chop and check the sparkplugs. That's really the way to tell if it's rich or lean...

  7. #7
    I have tried applying the choke (richer) as well as removing the airbox (leaner) with no noticeable improvement either way. The spark plugs are light chocolate brown. While I am familiar with the Mikuni kits I used Winderosa rebuild kits which are high quality USA made items. I have had great success with their kits on rebuilding Mikuni fuel pumps in the past so I don't really think it is the fault of the rebuild kits. I can try leaning out the high speed mixture screws but they are a pain to get at.
    Regarding carb removal I have that down to a science; Airbox cover, flame arrestor, velocity stacks, line, cables removed and carbs on the bench in just 10 minutes. If my carbs are indeed just bad, I can get a rebuilt set from the Jet Ski Store for $200. I'm kind of tired of throwing money at this thing but I may have no other choice to get it working properly. The odd thing is that it takes off fine with all kinds of REVS, starts to get on plane and then just falls on it's face almost to a complete stop. It is as if it was suddenly laddened with an impeller with WAY too much pitch and it shifts into a low rpm lumbering mode.

  8. #8
    I just had another thought. Check the CDI box. I know sometime when CDI's go bad they kinda have that symptom...they may be good at idle and slow revs but when it gets warm then thats when they act up. I still do think that it's carb related problem.

    Here's another one, so lets say it's lean like what cutlass is throwing out there. What you could do is go ride the ski with the seat off, and bring a starter fluid with you. And then try to replicate that problem. When it starts to do the problem you're describing then go and spray on the carbs.

    If it is in fact lean then it will rev up and you'll get some nice boost in power. If it's already rich, then it'll bog down.

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