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  1. #1

    2001 GP 800r Carb "recipe"

    Like Bill used to say, "if you follow the recipe, the cookies always turn out the same." Never met the guy but miss him just the same. Hope there are a lot of skis to work on where he is now. RIP

    So, I am hoping someone has done Oside Bill's mod to a set of 800 carbs. I am doing a rebuild on an SBT engine (now there's a shock...) and I would like to get this engine hooked up with the best possible chance at longevity. I am boring the sleeves, chamfering the ports correctly, replacing the crank, all seals, new mikuni carb kits, etc. The engine caught a ring on one cylinder and lost a wrist pin clip on the other so I am thinking that the carbs were sort of OK. I see some dark carbon on one piston but the other was so (wrist pin clip) blasted that there was no surface that wasn't pummeled. Pictures of the carnage is below.

    Anyway, I am attempting to do Bills fuel pressure balancing mod on these and will start with a #90 restrictor jet in a tee in the return line per Cutlass's recommendation and measure fuel pressure to see if his guess is a good one. I am assuming that the 1-2 psi at idle and 4-6 at full throttle is what is optimal.

    There was no evidence of scuffing even though the guy I am doing this for uses 40-1 premix and it seems that others suggest 32-1 for the 800 motor. He used Yamacrud for oil. You know, the oil that is so good, you have to buy ring free to keep your combustion chamber clean?

    The previous technician removed the choke butterflies and put a primer on it when he went to premix. The accelerator pump is still active. I am looking for some suggestions on jetting as going with the stock setup with a stock airbox but with the butterflies removed may be causing some level of leanness. I know that on triples, the factory jetting is mighty lean and the pump helps with low end acceleration but I do not know if this is the same idea on the 800. Has anyone out there tricked out their 800 and removed the pump and gone with alternative jetting, pop off pressure and screw adjustments? I can keep it the way it is and start with the stock settings and err on the rich side but if someone else has worked out a set of ingredients, I would like to get the first set of cookies out of the oven in golden brown, crispy on the outside, chewy on the inside condition...If you know what I mean.

    Suggestions or the best person or persons to PM if you know of an 800 carb specialist? I realize that trying to make a GP800 go as fast as a 1200 is kinda silly since the 12-1300 ones go that fast stock. This setup is more about reliability so just getting an even power band with the least possibility of a lean condition at any point in the power band is the goal. Any help would be appreciated. Thanks in advance.

  2. #2
    70 folks have looked at this one and sadly, no one has any suggestions. Here is what some ridiculous time doing a lot of research has found. Back in the day (anywhere from 5 to 12 years ago, people used to remove the accelerator pump went to a larger main jet and lowered the pop off pressure a bit to flow more fuel at mid to med high throttle range. Someone noted that going to a 1.5 N/S did not work well as it was too rich. No one seems to remember or have any records of any settings.

    I spoke to a guy at Jetworks (they do the blackjack carb mods) and he said that the 800's don't really have as bad of a problem as the 1200's have and usually if it is kept stock there really isn't a mid range lean issue. He cautioned about low octane and old fuel as being a bigger problem for these and all two strokes.

    I have begun the rebuild process on the engine and the carbs and found that the low speed screws were out at 2 turns and the highs at 5/8ths of a turn. Both were the same and both have #150 main jets and 1.2 N/S. I have gotten someone to send me some choke butterflies to take it back to stock. (still keeping the primer since the linkage and choke cable is gone). The reason for this is that "opening up" the intake with this carb makes it leaner than it would be otherwise and since it is already lean, any mods like an aftermarket F/A or removing choke plates can really mess up the mix. The guy I am rebuilding this for indicated that the powerband was even and there were no hesitation or indication of richness or leanness prior to the losing the front cylinder.

    There was a bit more stubborn carbon on the rear cylinder's power valve and the piston has some real black deposits on it (see the pics) so I am thinking that both cyls might have been close to lean and since the rear cyl on 800's tends to run leaner, then I will start with maybe 7/8ths on the rear carb high speed screw and see if I am still good.

    Hoping for someone that might have done a accelerator pump delete and OSB balancing mod on an 800 and can share the magic formula. Thanks for any help.

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