Thread: Keihin Carb tips
08-12-2011, 01:09 PM #1
Keihin Carb tips
if your ski still has the original gray tempo fue lines replace all of them
carb sizes seem to follow these guidelines.
3 cylinder carbs are 38mm w/accl----40mm w/0 accl----40mm w/accl
the 38mm carbs have a throat size that exactly matches the stock flame arrestor
the 40mm carbs have a throat size that is smaller than the stock flame arrestor
both of these size carbs use a 3 jet metering block
only the 97 sl1050 came with 40mm carbs and no accl pump
and used a 2 jet metering block
2 cylinder carbs are 38mm w/o accl----40mm w/accl as single or dual
almost all twin cylinders came with the 38mm carbs including the octane
the exceptions are the slh's which have the 40mm w/accl pumps
and virage/freedoms that are a single 40mm w/accl pump
the single carb 40mm w/accl pump uses a 4 jet metering block
the dual carbs all use 2 jet metering blocks with the exception of the slh's that use a 3 jet metering block both use only 2 jets
added info from casey
You forgot the 96-97 SL/SLT 700,that had 2 38mm carbs with accel pump and high and low speed adjusting screws. Also had 3 jets , with the main jet screwed into the front of the jet block.(All other stock Polaris carbs have the main jet screwed into the back of the jet block)
These are the carbs shown in nearly all the service manuals and cause sooo much confusion
as a side note the genesis 1200 carbs are listed as having a smaller venturi
than the other 1200 3 cylinders and a different jetting setup
Last edited by TxPro1200; 08-13-2011 at 02:36 PM.
08-12-2011, 01:17 PM #2
ski hesitates or dies from a dead stop or having to feather or pump the throttle for acceleration.this applies only if the carbs HAVE an accl pump.what i have found is that the zip ties are coming loose around all the fittings for the hoses between the accl pump discharge fitting and the brass squirter nozzles.when the ski is running the vibration shakes the check ball around inside the metering block and can allow the fuel to drain out of the hoses and back into the metering block.the pumping or feathering of the throttle reprimes the hoses and pushes out the air pockets.it also greates pressure which forces the hoses to press back against the fitting barbs and reseal so gas doesn't squirt out from around the fittings but instead pushes thru the brass nozzles that also have check balls inside them.if one of the brass nozzles has a stuck or damaged check ball the same thing will occur which is fuel draining back into the metering block or the inability of the accl pump to prime itself.it doesn't take much for air to find its way between the hoses and fittings to bleed the fuel back down into the metering block which is under a vacuum when the motor is above idle speed.redoing the zip ties and getting them tight has fixed quite a few of these issues for me
08-12-2011, 03:49 PM #3
ski bogs,stumbles,loads up,sounds like its misfiring but doesn't backfire.replace all gray tempo fuel lines with new hoses.this tip applies to 3 cylinder carbs specifically to the middle carb and on 2 cylinder carbs to the mag (front) carb.a green sludge will build up around the brass fittiings and also on the internal screens of the carbs.a piece of this sludge can and does break loose and clog the return orifice for fuel pressure on the carbs.the return orifice is cast into the carb body and can only be accessed by removing the return fuel line brass fitting.its located closest to the top of the carb body.this means you'll need to split apart the carbs from each other to properly reinstall the brass fitting into the carb body when done.a welding tip cleaner can be used to clear out the obstruction or you could try to use compressed air to blow into the brass fitting to clear out the obstruction.note that the metering block MUST be removed from the carb body so that the material can be blown clear.this method doesn't work very well as the orifice size is very small and usually the material will not be blown clear of the carb body.a way to make this repair way faster is to drill out the orifice to a larger size like an 1/8".the brass return fitting can be tapped and a mikuni main jet threaded into the end.if the jet clogs up simple remove the hose from the fitting.unscrew the jet,clean it and reinstall. on 3 cylinder keihin carb setups that run 2 pumps (mag & pto) the jet size i use is a #60.any body know what a 2 cylinder would use??
as a side note.on my keihin carb setups that use 3 pumps on three carbs i use a jet drilled to .060 for a return size.but these setups also have dual feed line for a supply
08-12-2011, 04:47 PM #4
rebuilding tips...the lowest or closest to the throttle blade of the small jets is the low or pilot jet.the higher is the mid jet.i use blue loctite on the metering block center screw.its important to set the needle arm height.it allows the needle to lift high enough away from the seat for proper fuel flow.lay a straight edge across the metering block face.there should be a gap of .060" between the needle arm button and the staright edge.there are 2 ways to sync multiple carbs.loosen the idle speed screw until the mag carb just seats the throttle blade in the bore and adjust the others to match or adjust the idle speed screw tighter until you uncover the first port in the throttle bore,i like to uncover half the port to the visible eye.i then adjust the others to match.you can the check how this compares to the first style by loosening the idle speed screw until the blades seat in the bore.forget which fuel line went to what???the line that goes to the crankcase connects to the ALUMINUM fitting pointing towards the rear of the ski.that leaves 2 lines.make sure the petcock is turned on.blow into one of them if fuel is heard gurgling inside the fuel tank thats the feed line and connects to the lowest (closest to the throttle blade) BRASS fitting on the carb.the fuel line that doesn't gurgle is the return line.it connects to the BRASS fitting highest on the second carb or the middle carb on a triple set
08-12-2011, 07:43 PM #5
- Join Date
- Jun 2010
- Milwaukee WI
Good stuff!!!! keep it coming!!!!!
08-13-2011, 11:58 AM #6
These are the carbs shown in nearly all the service manuals and cause sooo much confusion.
08-14-2011, 11:26 AM #7
How to test fuel pressure on two and three carburetor domestic red engines
fuel pressure...i run my setups around 1.5 psi at idle and around 5-6 at wot.
I've never checked any of the stock carb setups for pressure as i never really saw the need to, but here's how it can be done.
twin carb models running one pump;
the mag carb has the orifice for the return pressure cast into it.
there is a line that connects from the top of the pto carb to the middle of the mag carb.
you would add a tee and short piece of line to the connection.
the tee would then connect to a fuel pressure gauge that could be read while the ski is running at wot across the water. not exactly an easy thing to do.
triple carb with 2 pumps;
the middle carb is the restriction so the line from the pto carb top to the cen middle or the line from the mag carb top to the cen middle could be setup with the tee.
no simple way to check a single carb.
pulse lines should be kept as short as possible without coming close to kinking.
I've been told the stock fuel water seperator has a 19gph flow rate.anything bigger than 44mm carbs should have a dual pickup setup,and i would run it on anything over 42mm to be safe
Last edited by K447; 08-22-2012 at 07:17 AM. Reason: Formatting, title
08-22-2012, 12:32 AM #8
- Join Date
- Aug 2006
- Pittsfield, MA
08-22-2012, 07:07 AM #9
- Join Date
- Jul 2007
- near Toronto, Canada
So buy a few more tools
08-22-2012, 07:13 AM #10
- Join Date
- Jul 2007
- near Toronto, Canada
Instead of trying to read fuel pressure while flying across the water at WOT, would it be acceptable to test fuel pressure with the hull strapped down to the trailer on the launch ramp?
Full throttle on the trailer with the jet pump intake fully submerged should be close to full RPM, but you can watch the pressure gauge easily.
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