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  1. #1

    Overbore - Rejet??

    Hey fellas.....been chasing this problem all last summer so I figured I'll get a jump start on it this year before spring. Long and short - - I have a '98 SLTX that runs flawlessly ONLY when the choke is applied slightly. Without doing this, it will have trouble getting out of the hole and will not rev since it is running slightly lean. If you apply the choke to my "sweet" spot, then BAM! Gets out of the hole, pulls RPM, runs great, plugs look good.

    The fuel lines have been replaced, carbs rebuilt, etc. But it is still clearly an Air-Fuel issue which means I'm either not getting enough fuel or too much air. I will be checking the pick-up lines and any of the in tank components next, as well as doing a leak down test to see if maybe I have a slight leak in the case gasket.

    BUT......

    This motor was previously rebuilt, which means it could have been bored. In my past, I've not had an issue with this but since I don't know the exact parameters of whats been done - - - could this be part of my problem??? If it has a significant overbore/oversize pistons I need just that little bit of extra fuel across the board and that re-jetting all (idle/pilot, mains) up one or two sizes would solve my problem. It makes sense in my head since right now applying a touch of choke is cutting of a touch of the air, so slightly larger jets/needle adjustment would add the fuel I need to compensate also.

    Thoughts? Opinions??

  2. #2
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Leak down test.

    Don't ever trust the Previous Owners work.

    What rebuild kit did you use for the carbs?

    Did you check the jets in the carbs?

    Did you verify the MID and PILOT jets are oriented correctly, and not reversed?

    Did you check your needle valve arm heights?

  3. #3
    Thanks Xlint - - - and agreed on both trusting previous repair and that a leak down test is going to be my first order of business as I'm going to be pulling the carbs either way. I bought the rebuild kit from SBT I believe so it appeared to be of good quality. I also verified that the current jetting does match the stock/factory specs when they were apart and being cleaned but I can't say for certain that I checked that position of the Mid and Pilot jet orientations although because of how it acts I don't really suspect that they aren't. I also did not check to see which position the needle valve was set at but I will do all of the above when I go ahead and pull everything off.

    So a leak down test will be where I start.....since one of my possible suspects was that there may be a possible crank case seal leak that is just enough to lean out the mixture in the motor. I did try to spray some starter fluid around the motor in various places to see if I could get an increase in RPM but no dice.

    If there is no other apparent air leak, fuel restriction, etc. - - - Carbs at factory spec - - - I think I'd have to try going with upsizing all jets a size or two??? Like I say, the amount of choke I have to apply is very slight and wouldn't just slightly reduce air flow.

  4. #4
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    you've got an air leak somewhere. while it's a pain in the butt you gotta pressure test the crankcases.

    this assume the pop off value is on the money and you've knocked off the vac fuel valve ( restricts fuel flow)

    increasing fuel flow also increases the gas/oil ratio. That's a dangerous game of "catch the sharp axe" you want to play.

  5. #5
    Yeah Pete.........I agree and really think air is the issue. I've never heard of a minor overbore on a rebuild having to require jetting changes since it's such a minimal amount of additional cylinder space but figured I'd ask just in case someone has had that issue. These watercraft motors seem to be more finicky than a snowmobile in relation to proper operation and A/F ratio. So my bet is that when they did the rebuild they may have re-used the case gasket or just did a poor job in putting everything back together which makes sense since that would cause a slightly lean ratio across the whole throttle spectrum which is what I'm experiencing.

    As for the pop-off - - - I didn't test them so likely will but for the vac fuel valve I don't believe the domestic engines have them only the blue fuji fuel pumps unless I'm mistaken on that as well.

  6. #6
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    A quick way to check for a air leak is to give a shot of gum out with a long straw around the case mating surface

    Could be a crank seal leak as well

    A pressure test confirms then the fun begins

  7. #7
    Click avatar for tech links/info K447's Avatar
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    Quote Originally Posted by nmpeter View Post
    A quick way to check for a air leak is to give a shot of gum out with a long straw around the case mating surface

    Could be a crank seal leak as well

    A pressure test confirms then the fun begins
    For clarity - Apply the solvent spray while the engine is running. If spraying around the seal area changes the engine RPM then it is sucking the combustible spray into the engine somewhere near where the spray was applied.

    Evidence that there an 'air leak', drawing air (or combustible solvent) into the engine during each intake stroke.


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