Js440 jetting

[trevangillette1]

Can someone wise give me a starting point for jetting? I have a JS 440 it’s all stock. One day I’m going to convert it to a 550 top end and so I went ahead and got a SBN 44 mm carburetor so that I won’t have to change down the line, but I currently need to jet this carburetor for a 440 rather than a 550. I know that it’s a little big for 440 but can someone help me choose some jets that should work. I will be riding mostly at about 4500 feet elevation . I have track 16 pitch impeller and a stock for 40 pump 120 psi on each cylinder stock water box. I think some sort of aftermarket pipe but I don’t know the brand.

 

[fastgtfairlane]

That's a stock 440 pipe. A sbn 44 will work with proper jetting. This should be a setting that should at least get you started and will probably a little rich but worked well on a piston port 550 engine. 2.0n/s 65g spring, 100 pilot 110 main. 1 turn out high and low speed screws and go from there.

 

[trevangillette1]

That's a stock 440 pipe. A sbn 44 will work with proper jetting. This should be a setting that should at least get you started and will probably a little rich but worked well on a piston port 550 engine. 2.0n/s 65g spring, 100 pilot 110 main. 1 turn out high and low speed screws and go from there.

Thanks. This give me a perfect start point. Am I aiming for pop off pressure of around 20?

 

[fastgtfairlane]

16-20psi should be OK. If it's too high you can somewhat compensate with more jetting.