1999 GP760 Carbs Carbs question???
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  1. #1
    PWCToday Newbie
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    1999 GP760 Carbs Carbs question???

    Hello and thanks for reading.

    I am rebuilding the carbs on a 99 GP760 (new to me, would only run on the prime and no fuel was being pumped to the carbs). Got into the carbs and they're pretty clogged up so am fairly sure that's the problem. For the most part I have everything cleaned and blown out and will re-assemble using Mikuni rebuild kits. While not easy was able to figure out most passages and how it worked but have one issue. Each carb has two fuel line nipples coming off of it (and of course the crank case line to the pump plate). I am fairly sure the top ones (as they sit in the ski installed) are the return gas lines for excess fuel and the bottom or lower lines are the fuel (into the carb from the gas tank) lines. Once cleaned I can blow air through everything (all passages) inside the carbs no problem except for through what I think are the fuel return nipples. When I blow into either of them (one on each carb), nothing. The air compresses and no sound until I pull off and the air comes back out. To me, looking inside the carbs, it looks fairly obvious where the passages from the return line nipples go - to a a small hole about a half inch into the carb and is in a cavity or port that includes the needle valve and inline with a larger hole where the fuel comes into the carb via the inlet nipple (which is isolated from the the needle valve and hole I am talking about via a porting cavities and gaskets). I've tried to clean out from the nipple to this small hole but no luck so far.

    So, I guess my question is, am I correct and this is indeed the return fuel line nipple and should I be able to blow air into the nipple and have it come out of the hole I described? If so (I can't imagine that when I blow air into that fuel line nipple that it shouldn't come out some where), how do I clean it out? Or am I completely of base here?

    Obviously, to answer this someone will have to be very familiar with these Mikuni carbs but, any input would be greatly appreciated!!!

    Thanks!

  2. #2
    PWCToday.com Is My Home Away From Home Grumpy Old Guy's Avatar
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    Re: 1999 GP760 Carbs Carbs question???

    You are correct on every point. Use spray carb cleaner & spray in the return hole some should come out...its a very small hole.

    Also use 95 gram springs instead of the 115's
    Last edited by Grumpy Old Guy; 04-26-2019 at 11:08 PM.

  3. #3
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    Re: 1999 GP760 Carbs Carbs question???

    Mucho thanks for reading my very dry post and your reply.

    After I posted I went back and looked at the carbs and thought a little bit and I actually wonder if I was right on every point ... or not. There is one hole I could not account for under the Valve body assy (per the Mikuni diagram). I found it odd that both carbs had the exact same problem and even more odd that I could not get any (I mean zero) air to go through. So, I cracked a beer and thought about it. Under the Valve Body Assy there is 5 holes, 2 screw holes, 1 main jet, 1 pilot jet and ... humm, one more hole. Couldn't figure out what it was for. An access port I figure but, there was threads in it. Why would you thread and access port? I had been thinking that, maybe there is some sort of pressure relief valve that has to be open to allow fuel to go through that passage (return to tank) and back to the tank. I looked at the carbs again and, indeed there is a plugged bore hole that aligns with the mystery hole under (the one I couldn't figure out why it was there) the "valve body assy." I think that is a pressure actuated valve that opens, under the right circumstances, the passage to allow excess fuel to go back to the fuel tank. I was confused by the fact that there is nothing, like gravity pulling down on floats in a float valve (I come out of the motocross world), to shut the needle valve. Just a light spring or, pop off. Then I realized, when the pressure on each side of the needle valve is equal, the spring gives the extra force to close the the needle so too much fuel doesn't through the carb but, where does that fuel go? At that pressure the valve I think is inside the carb opens, allowing the fuel not needed to bypass the carb and go back to the tank. When the pressure drops, that valve closes, and the fuel goes to the jets as needed. At least that is what I am thinking.

    I might be wrong but would love to hear from someone that regularly works on these things and can freely blow air through the return line nipple and out the hole I described above that is pretty clearly inline with it.

    I used to think that fuel was just pumped at a higher rate than needed and nothing regulated how or when the fuel went where (to the engine via the jets or back to the tank). I assumed that the last resort was back to the tank and unless it got to there it went where needed but I couldn't figure out how that could be possible, or work. I think, what I think now , clears that up a bit.

    Might be that second beer talking now but, would be interested in other thoughts on the subject.

    Thanks

    ps - yes, I understand this is a bit dry. Sorry about that.

  4. #4
    PWCToday Newbie
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    Re: 1999 GP760 Carbs Carbs question???

    To anyone that gets to this point, my above post was indeed the second beer doing the talking (or lack of thinking). I thought about it more last night and looked at the carbs again early this morning and there is no room for any kind of a pressure valve in the middle of it. Old Goat is indeed correct, it's just a very small hole and were quite clogged on both carbs. I used a fine wire to clean out from the nipple side which wasn't too bad. My wife, being an emergency room physician, has an array of hypodermic needles and the smallest one (I think made) is a #30 and it happens to be just a fraction smaller than the hole when cleaned out. I would stick the needle into the hole, pull it out and some of the clogging material would be in the hole in the middle of the needle. Clean it out by squirting carb cleaner out of the needle and then go back in again. Took a while but, all of the sudden it went in another quarter of an inch and I had a passage I could squirt carb cleaner through. So, for anyone looking to clean that passage, there's an option (I believe the needles and a syringe would be available via any vet supply store or site).

    Old Goat, thanks very much for your input, it was very helpful.

    Now I'll see if I can get these things put back together and installed.

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