'92 SP 587? Rotax question

[steve1147]

Hi Guys, and thanks for this forum of free help!!!
I bought a non-running '92 SP with the two cylinder Rotax (600), single carb, single pipe last fall.
I rebuilt the motor,bore job, oversize pistons, etc., and rebuilt the carb, which seems to be NOT stock. It's a Mikuni SBN, but there's an obviously home-made aluminum spacer under it (about 1-1/4" tall) and a straight K&N air filter on top.
======THE PROBLEM=====
From day one after the rebuild (I don't know HOW it ran before since I didn't own it), it appeared to be starving for fuel. Would just die when throttle applied.
I re-disassembled the carb and found it had a 105 main jet, when it should have had a 127.5, according to the manual, so I changed that, and changed-out the fuel lines and the check-valves at the bottom of the fuel tank dip tubes. I also eliminated the oil pump and started running pre-mix.
I checked the air-inlet vent to the tank which is good and wide open. All hoses, check valves, etc. are good and operational.
+++++THE CURRENT RESULTS+++++
Same Problem basically.......
It starts phenominally, almost immediately every time and idles well.
Then you have to pump/work the throttle to get take-off, but usually after a little nursing it'll start going. It "surges" through acceleration, and never seems to reach top end, IF it does, it'll surge at high speed, and drop around corners. I've spent HOURS adjusting the low and high speed screws to no avail.
After a while (10-30 minutes) it will quit doing anything but IDLE. And I'm lucky to get it back to the trailer. As I stated, the vents are clear/functional, the fuel dip-tubes are good, the carb is rebuilt (x2).
COULD this be still too small main jet????
OR is this an aftermarket carb which is just too big/small for the ski????
Thanks for any advice!
Steve W.

 

[steve1147]

Just checked....

OK, I took off the carb (obviously aftermarket as I explained above), and the intake diameter is approx. 46mm, and the exit diameter approx. 45mm, guess this is a 44mm carb????
Thanks, Steve W.

 

[kustomkarl]

Steve, take the spacer off the intake and see what it does. I have a feeling that is your problem.

Karl

 

[steve1147]

Thanks Karl,
I'll try that, so you do not think having an oversize carb bore or the maybe less restrictive air breather would cause these problems?

The spark plugs are burning nice, deep chocolate brown. Why would 'they' have made this riser piece for the carb?

ANYWAY, I'll take it out and try again hopefully this weekend and let you know.

Thanks Again, Steve W.

 

[steve1147]

OK, Removed the spacer, but still SAME problem. Can adjust jet needles to run OK for a few minutes, but does NOT reach top end and surges while it tries. THEN acts like it runs out of gas and will only idle back to trailer. Could this be "pop-off pressure" whatever that is? It's definatley NOT the factory carb.......
Will try bigger main jet (135+ instead of factory recommended 127.5) and see what happens. Don't know what else to do.
thanks, Steve W.

 

[timmyboy76]

That carb sounds big, for that motor. Is their any mods done to it?, it happens, but...are pistons in right(arrow toward exhaust), and timing right?

 

[steve1147]

The pistons, etc. are all right, I'm an experienced mechanic and I just rebuilt it, bored .5mm over. It's most definately a fuel delivery problem. Maybe I'll look on ebay for a new carb, this one was modified just to fit the manifold and has an aftermarket air breather. I wonder what size (bore) carb this thing came with?

 

[derek]

that is a 580 motor. that carb on it is like putting a 780 holley double pumper on a cavalier. go get a stock one. nobody wants them so they are cheap on ebay. get a whole rotary valve cover-intake-carb set-up together. the rot valve cover is shaped differently for single and twin carbs and you don't know if and how much the one you now have has been butchered by somebody who thought he was porting it. on the question about the pistons; you have the hole below the rings pointing toward the intake-not the exhaust side?

 

[robin savell lloyd]

I am no pro but I have a 650 with 2 mikuni 44's and was told that it will not work because that is to much carbs for my 650 cc engine but I will say it seems to haull a**. I am not sure if this will work for you but to me you do not seem to be over carbed as you only have one. but I can allways be wrong. :cheers:

 

[newforest]

rough running

Hi, I am new to Seadoo ownership,I sufer very similar problems that todate remain un-cured, mine came as a barn find ! non running 1991 single carb with a few tunning bits, like jetnetics flywheel and a performance exhaust but what seems a standard carb apart from it's fitted with a K&N filter, my problem is that it ticks over fine, reaches about mid range rpm just great then struggles after that with a mis-fire lumpy running and if you hold the throtle open it will die. If you blip the throtle it picks up fine. I have put a new pump diaphram in the carb so I guess its pumping fuel ok as this seem to be an issue before, hence non runner. Any advice welcomed !

 

[masta2000]

i too am having the same problem with my 97 xp. professionally installed twin 44mm mikuni with factory pro pipe. replaced grey fuel lines. cleaned fuel filter. machine starts fine but when anything over 1/4 throttle it dies and goes back to idle. machine will not even hit powerband. can it be a problem with fuel pump?. are you supposed to use fuel pump with 44mm carb?. its as if i am dealing with fuel starvation.