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Discussion Starter #1
I have a 93 650SX which we have rebuilt the motor on, and in the process lost the original carb. After looking around I purchased a used carb and could never get it running... It would start but every time I gave it throttle it would go BWAA BWAA. After getting frustrated enough to chuck it in the trash I purchased a mikuni SBN-44 carb. Tuned it with original adjustment screw settings and it worked great. Took it to the lake, did a little fine tuning, rode it for about an hour:D, and left. Now I can't find my adjustment screw settings on carb or online, and it is doing the same thing it did in the first place, bwaa bwaa bwaaa lol.... I've about given up hope, but that ski is my favorite thing I own. Does anyone have any clue what could be causing this?? Or what a pretty accurate screw setting would be on that carb? BTW ski was bored out to around a 670 is what they said? I've though maybe it could be my primer creating a weird vacuum in there?? Thanks, I'd appreciate any help from this long, boring post.:rolleyes:
 

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So now I've got it tuned where it seems to idle and run pretty good, but only with the hood off or the little pop out on the hood open. Obviously it seems like it get's starved for air when the hood is on.... Any suggestions or ideas why this could be? Thanks, I'd appreciate it!
 

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What's the pop off pressure of the carb needle? It really might take some tuning since the engine was bored over. Might need a few jet sizes larger, might need a lower pop off than stock. Borderline I would say on pop off is 25 psi. Fuel pump working ok? What's your compression?
 

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Discussion Starter #4
I know the fuel pump works great, and compression in excellent.... Pop-off or jets very well could be the issue. I'm just not to familiar with tearing carbs apart myself. Jet's would be one thing, the pop-off I know nothing about. Any idea where i should even begin with that??
 

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The low speed adjuster, bypass holes and low speed jet work with the inlet needle pop=off pressure to transfer fuel when you go from low to high speed. You have to put a pressure pump with a guage on it to the inlet nipple on the carb/fuel pump. Take the cover off on the other side of the carb. Make sure the needle is wet. Pump the gauge up until it pops and adjust it by changing the spring under the needle arm.
 

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Discussion Starter #6
Oh I see. Sounds like something I could do lol. I'll check that out thanks. I've heard a lot about the pop off pressure but it always passed right over my head. I took the ski out this weekend and had it sounding great at shore with the hood off. Put the hood back on, took it out on the lake and it died shortly after at about half throttle, so I popped the little hole cover in the hood off and it ran great. Flew across the water, so I pushed the cover back in and immediately it bogged down and did the same old thing. Took the little hole cover off and away we went again. Do you think this could be due to not enough air getting to the carb in that closed off hood, therefore making the thing way to rich??? Seems like soon as it get's some air it great....
 

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i am maybe going through the same prob I have a js 440 that i rebuild and with the hood off it seems like its got more power. I might have to get different jets it has a super bn 38 on it with 107.5 main and a 65 piolit jet i don't know if that is to big or what because it take forever to get on plane does anyone know what the jets should be
 

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There should be some sort of vent into the engine bay. Is that clear and clean? The engine bore should not affect the carb setting that much. Sounds like your not getting air to the filter. or putting your hand over the venturi/intake.
 

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Discussion Starter #10
I can only find two things that look like vents, and they just go right out the sides of the hood. Like an inlet and outlet for water. Guys at the lake have been saying there should be no holes in the hood whatsover because then water gets in the engine bay....Which makes sense, but how else are you going to get air to a hungry 2 stroke ya know. It's never gonna run right if it keeps choking out. They all say its something with ignition because it always stops a cylinder for a second after a hard landing. Anyone ever hear of that problem?
 

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There has to be a case vent..

My waverunner has two white boxes located one on each side of the engine that have trap doors on them if water tries to go though..

The water that enters here gets diverted to the bilge pump sump..

Was the ski stored outside uncovered?..maybe leaves (or something) have accumilated in this area of your ski?..
 

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Discussion Starter #12
Nope never stored outside... There are two vent looking things but they don't actually have holes into the ski. Just little outlets for the water. Kinda weird.
 

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U sure we're not talking about an aqua scooter here?!..lol..

There HAS to be a hull/case vent of some kind..there just has to be..

Have you looked it up in an assembly manual?..

You have me curious enough to look it up myself..lol..
 

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Discussion Starter #14
Well now I'm curious too... I don't have any sort of manual for the thing at all, but... I checked out my two polaris s and sure as could be they both have vent type air inlets for the motor.... I'm debating just riding around on it with the adjustment plate taken out of the hood to leave it open to the engine lol. Just try my best not to submerge it.?.?.?
 

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I came across this thread and realize its old, but if its still a problem, here's my input. The 650 should have two rubber hoses on the under side of the hood that go down into the hull when the hood is place on. These tubes provide ventilation and combustion air to the hull cavity. The tube vent to the atmosphere through the interstitials of the hood and the vents on the hood. and create air changes when the ski is moving to evacuate any gas vapors and to provide fresh air to the engine.

Are the tubes there? Has anyone seal the vents on the hood or sealed the interstitial of the hood. some times people close these off and modify the air intake to come through the hand pole because they do allot of tricks. That usually means they have replaced the original hand pole with an aftermarket one that has an intake tube.
 

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If you have an exhaust leak inside the hull, the engine won't run well after a few minutes with the hood on when it starts sucking its own exhaust fumes.

I have also seen an outboard motor once with a fuel leak that did not run right with the cowl on because the leaking fuel fumes in the intake air made the mixture too rich. Ran fine with the cowl off.
 
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