300/440/550 js300 carb issue???

I

iveexcaped3

Location
riverside ca
ok new to stand ups but im mechanically inclined and will try to understand what your saying. ok so i bout a 88 js300 off a friend for $250 (sat for over a year) and wouldnt start up so i took off the starter unstuck the contacts and not turns over every time. then i wasnt getting spark pulled out the plug wire and split the ebox and came to find out the plug wire wasnt all the way down so there wasnt any contact between the coil and wire so now i have spark. compression is good. onto the issue im stuck on. i dropped some fuel into the carb since the primer system doesnt work and it fired right up no problems dry start. shut it off then 10 minutes later fired it up again (had to drop fuel down) and it revved up as if in WOT without even touching any thing i pushed the kill switch nothing, pulled the spark wire off it and it was still running, didnt want to blow the motor so i unscrewed the fuel pick up and snapped the throttle and it died out..... this happened 2 times. im gonna take a best guess its a carb issue since its most likely just dumping fuel and wont shut off but where do i start?
any help will be greatly appreciated
 
yamarider

yamarider

Location
new zealand
Every 300 with a standard carb that i have owned (i love 300s!) has had this same problem. After searching for air leaks and timing issues and as many other possible causes I could think of i learned that that's just what 300 motors do whenever they aren't under load. I can almost 100% guarantee you this wouldn't happen if your ski was sitting in the water at the lake.

Its a real pain when you come to flush the ski out after riding salt water' but you can manage to keep it from "running away" by installing a primer kit and using it to slowly pump extra fuel into the carb the whole time whenever the motor is free running. (It sounds stupid but it works).

The other thing you can do which i recommend is swapping the stock carb out for either a bn38 or sbn38mm carb. They bolt straight on and totally eliminate the problem you are having you will be free to "free rev" the motor all you like.

If you go for a sbn carb I can give you the jetting specs im running one on one of my 300s and it goes mint.
 
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iveexcaped3

Location
riverside ca
thats the thing we had water running through the system so it acted as if it was in water, i called a shop and they said maybe the float valve? is maybe clogged up or some thing like that. with the primer you talking about flooding it out if it revs like that? the whole engine was "redlining" when it should have been just idling.
 
Jetpilot106

Jetpilot106

Kickin it old skool
Location
Kansas City,Missouri
check the throttle cable make sure it doesnt seem like its getting stuck.If that doesnt help then you need to either you need to pull the carb apart and check for broken,dirty or stuck parts.Clean and rebuild.Or you could just go to a bn/sbn 38.Their isnt much else that would make it have a effect like that other then the carb.
 
I

iveexcaped3

Location
riverside ca
like snap the throttle so the plate is open and kill it? i snapped the throttle last time after i pulled off the spark plug wire and it died after i snapped it. ill see if i can get a video of what it does
 
yamarider

yamarider

Location
new zealand
iveexcaped3 said:
thats the thing we had water running through the system so it acted as if it was in water, i called a shop and they said maybe the float valve? is maybe clogged up or some thing like that. with the primer you talking about flooding it out if it revs like that? the whole engine was "redlining" when it should have been just idling.
Click to expand...


We are talking about the exact same problem but you are not grasping what im saying the ski needs to physically be submerged in a body of water in order for the engine to be under load. Running the motor on the hose will give no torque resistance to the motor whatsoever. For some reason 300 motors with stock carb run away and its pretty normal.

The best way to kill the motor or keep the idle under control whenever the ski is running out of the water I have found is to tip a regulated amount of additional gas down the throat of the carb constantly when the motor is running (or when it runs away) Or install a primer kit to make it easier to do this same thing without having to remove the air box.
 
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iveexcaped3

Location
riverside ca
can anyone else vouge for the 300's running away? i appreciate the help. i have the primer on there but need a new one its not sending fuel through to the carb, its sucking in air and losing its pressure. where abouts in NZ are you?
 
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