Clivingston
New Member
Does seem like I get to about 25% before my motors start to rev
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Got it running but now this. Carb question and others. Please help
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jhjesse
Well-Known Member
Sounds like you are zeroing in on the rpm problem. If you have to advance the throttle 1/4 to 1/3 of the way your cables need adjusting or replacing as you stated you have one that is broken. I have mine adjusted to where there is very little slack.
Sportster-2001-951C-Stock
Well-Known Member
Honestly there really isn't much to adjusting these carbs, 1-1/4 to 1-1/2 turn open from lightly seated (never crank the mix adjust screws forced tightly closed) and the high speed mix screws are usually completely closed but you can add some fuel if you want by opening them a little (maybe 1/4 turn open from seated).
If the throttle cable is adjusted too tight, the inner wire will break due to overtension, so be sure throttle plates (butterflies) reach full open just as throttle handle reaches full position. The procedure outlined earlier will achieve this goal.
If still not running well, make sure fuel lines are leak tight, an inward air leak fills carb fuel chambers with air bubbles which displaces fuel.
Otherwise, if this is a lean hesitation at part throttle or off idle throttle tip-in lean stumble then it's time to pull the carbs and clean them WELL, internally. There must be 100,000 posts on cleaning carbs, they fill with decades of trash from the fuel tank and decayed fuel lines, and they corrode a little bit internally as well, all of which plugs the tiny precision drilled passages.
If the throttle cable is adjusted too tight, the inner wire will break due to overtension, so be sure throttle plates (butterflies) reach full open just as throttle handle reaches full position. The procedure outlined earlier will achieve this goal.
If still not running well, make sure fuel lines are leak tight, an inward air leak fills carb fuel chambers with air bubbles which displaces fuel.
Otherwise, if this is a lean hesitation at part throttle or off idle throttle tip-in lean stumble then it's time to pull the carbs and clean them WELL, internally. There must be 100,000 posts on cleaning carbs, they fill with decades of trash from the fuel tank and decayed fuel lines, and they corrode a little bit internally as well, all of which plugs the tiny precision drilled passages.
Clivingston
New Member
Thank you. I have to go offshore for work so it be a while before I can dive back in. Gonna do some studying while I'm out. Thanks again.
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Sportster-2001-951C-Stock
Well-Known Member
Welcome. I forgot to post a link to the Mikuni carburetor manual. Mostly from this you can see the three paths for fuel flowing through the carb, idle circuit, part throttle low speed ("bypass holes") and high speed venturi circuit.
At diel, fuel is only being metered through the idle mixture low speed adjuster needle, just enough fuel to keep stoichiometry while idling. On throttle tip-in, the "pilot holes" transition ports begin adding fuel as air velocity through the carb bore increases. Most of the actual fuel consumed is provided by this transition circuit, unless you're a genuine lead-foot. These are the small passages most prone to clogging and very important for low to mid throttle operation until air velocity increases enough to begin pulling fuel from the high speed venturi.
It's VERY common for those "Bypass holes" to become cloggrd or partially clogged, these passages are precision drilled in the aluminum bore and prone to close up slightly as the aluminum corrodes, the small diameter makes them a great place for trash to collect as well. The "Bypass holes" are the most important passages in the entire carb b/c most of the fuel is metered through them.
The high speed circuit is constructed using raher large passages to meter large amounts of fuel at wide throttle angle and air velocity thus usually aren't clogged or restricted due to large diameter.
Then there's the atmospheric-pressure referenced vacuum operated metering diaphragm, which shuts off fuel from entering the fuel chamber when there isn't much, or zero, intake manifold vacuum. When operating as intended, mostly this has it's greatest effect on low speed operation, and most important keeps fuel from peeing down into the intakes and fuel-flooding the crankcase when the engine isn't running (when there's no vacuum).
http://www.mikuni.com/pdf/sbn_manual.pdf
At diel, fuel is only being metered through the idle mixture low speed adjuster needle, just enough fuel to keep stoichiometry while idling. On throttle tip-in, the "pilot holes" transition ports begin adding fuel as air velocity through the carb bore increases. Most of the actual fuel consumed is provided by this transition circuit, unless you're a genuine lead-foot. These are the small passages most prone to clogging and very important for low to mid throttle operation until air velocity increases enough to begin pulling fuel from the high speed venturi.
It's VERY common for those "Bypass holes" to become cloggrd or partially clogged, these passages are precision drilled in the aluminum bore and prone to close up slightly as the aluminum corrodes, the small diameter makes them a great place for trash to collect as well. The "Bypass holes" are the most important passages in the entire carb b/c most of the fuel is metered through them.
The high speed circuit is constructed using raher large passages to meter large amounts of fuel at wide throttle angle and air velocity thus usually aren't clogged or restricted due to large diameter.
Then there's the atmospheric-pressure referenced vacuum operated metering diaphragm, which shuts off fuel from entering the fuel chamber when there isn't much, or zero, intake manifold vacuum. When operating as intended, mostly this has it's greatest effect on low speed operation, and most important keeps fuel from peeing down into the intakes and fuel-flooding the crankcase when the engine isn't running (when there's no vacuum).
http://www.mikuni.com/pdf/sbn_manual.pdf
Clivingston
New Member
I copy thanks for the info. Gonna look at all this.. Got three weeks to study and get a plan together. Want to get it running reliability before the. 4th. What kind of life do these 2 strokes have like hour wise. And what life to the 4tecs have?
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Sportster-2001-951C-Stock
Well-Known Member
Fresh or saltwater lifetime? Good thing about the seadoo engine is they're about the lowest replacement price marine powerhead out there, less than most any outboard even. 2 motors are twice the cost though.
I've seen some people claim to have reached ~700 hours with the 787 I think. The 4-tecs I've seen posts claiming 3,000 on the clock and I think these numbers are doable as long as it's kept in tune with good maintenance.
Most other marine engines will beat those numbers by far, reman powerheads are $$
Keep an eye on cylinder compression, that's your warning indicator and you can easily view the piston skirt exhaust side (the hot side) for scoring and gummed or coked rings through the RAVE ports while cleaning the RAVES. Learn to read piston wash and be a guru, LOL!
I've seen some people claim to have reached ~700 hours with the 787 I think. The 4-tecs I've seen posts claiming 3,000 on the clock and I think these numbers are doable as long as it's kept in tune with good maintenance.
Most other marine engines will beat those numbers by far, reman powerheads are $$
Keep an eye on cylinder compression, that's your warning indicator and you can easily view the piston skirt exhaust side (the hot side) for scoring and gummed or coked rings through the RAVE ports while cleaning the RAVES. Learn to read piston wash and be a guru, LOL!
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