300/440/550 Help with JS440

X

Xluben

Location
Minnesota
I just got a 78 JS440, and in order to keep it running I have to stay on the throttle, and If i give it more than 1/4 to 1/2 throttle it just bogs down. Im guessing it is the carb, I'm going to clean the carb tomorrow and maybe get a rebuild kit. What carb is on a stock 78 JS440? Do you think the carb is my problem?
Thanks
 
Austin

Austin

Location
Loveland, Colorado
The carb is your problem. First of all, the previous owner probably had it so you have to keep throttling it to keep it alive so when you fall off it dies. That is how I have my x2 tuned Because most of the time when I fall of it never circled around so just have it so it dies but you can tune that out. Then your second problem is it is mostlikly running really rich so it is bogging out. There are two tuning screws on your carb. I would tune it first then if that doesn't work then rebuild and clean it.
 
cookn

cookn

Kamikaze
Location
where you live
the stock carb is a bn38. they are really touch and go on the tuning. i found we had to tinker with the screws every time we rode the thing. the old bn's don't have main and pilot jets so its all up to the screws. I would just look into a new carb, some say you can't run a sbn44 on a 440 but we are with only a pipe, no internal motor mods except a slight timing advance and the thing rips.
 
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X

Xluben

Location
Minnesota
Where are the adjustment screws located(in what order)? I mean like the high speed, low speed, and idle. And how far out should they be? Thanks
 
cookn

cookn

Kamikaze
Location
where you live
The idle will be where there throttle cable hooks up, it pushes the throttle valve open to set your idle. if your all stock then you don't have a kill switch so i would set it low enough that the motor dies when you fall off. The high and low speed screws are on both sides of where the pulse line goes into the carb, one will be marked with an H and the other L. i believe if you are looking at the motor from the carb side the low is on the right and high on the left. start at 1 turn out on the low and 1.5 on the high. i think thats at least where we started, but you then have to keep riding it and making only small adjustments of maybe an 1/8 turn. Ride it long enough for the change to take effect. You really need a day down at the lake to tinker with it and even after you get it, if that thing is all stock it will still run like a vacuum cleaner.
 
cookn

cookn

Kamikaze
Location
where you live
i would ditch that carb and try and find at least an sbn38. its not worth your money to rebuild the bn. I don't think you need a intake spacer but i could be wrong on that and you can use your 38 intake manifold. check out that ultimate picture thread and there is pictures and info of my buddys 440 that is also a 78. its was bone stock outta some guys barn last year and hadn't been run for who knows how long and we put it in the water and it ran soo bad, i think it took, all summer to burn the crap outta of it and once we got the carb tuned it was still slow. We did a few mods that were pretty cheap. the sbn 44 and intake with spacer was the highest cost at about 200 bucks. a cheap no name pipe and a skat prop and now it faster then my 550 on the top end.
 
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