jetting 38mm Mikuni for HL500

[Marc.com]

anyone have a hot jetting spec.

I know VMX had one but I think that was for 36mm. Engine is 540, big valve head and high lift cam.

much obliged

[TT5 Matt]

send a pm to colj500 he gets into things like that,he should have it written down as well

[Graeme M]

Try the following, it's my settings for my HL500 running a 38mm Mikuni. The only thing I'd suggest is starting with a 240 or 250 main jet, as other 500 owners I spoke to were running about that. I was never real sure why mine ran the smaller size, but that's what it liked. Altitude was about 600 metres. A P8 needle jet may be a better bet too, as mine seemed to prefer a slightly leaner setting and the P8 was recommended.

Slide 2.5
Main 185
Needle jet P6
Needle 6F8-3
Pilot 37.5
Air screw   1.5
Float level 19mm

[Marc.com]

thanks Graeme, interesting I also thought 240 was about as lean as you wanted to go.

[Colin Jay]

Marc,

Below is the jetting for the VM38 Mikuni on my SR500 salt lake racer. Basic engine specs are; Gas flowed SR500 head with std SR valves, WEB 88A camshaft, Wiseco 11:1 piston, giving 12.5:1 CR with the modified head, open megaphone exhaust system.

230 main jet
6F9(2) needle (clip position)
P6 needle jet
2.0 slide
20 pilot jet

This jetting was setup on a dyno and gave a maximum of 43HP running 102 octane unleaded racing fuel. There is a major flat spot / hole at about 3000 rpm, but we were unable to correct this with jetting and beleive it is the result of the open megaphone exhaust ststem.

I am running exactly the same jets in the the VM38 Mikuni that is fitted to the XT500 that I am currently campaining in road trials. The XT engine has a gas flowed SR head, 10:1 540cc Wiseco piston, Tighe 503 camshaft and a fairly free flowing exhaust. This engine is running great at low to medium speeds, but is  a bit rich at 3/4 to full throttle, so I problably need to drop the needle and go down a size or two on the main jet. I intend to get it on a dyno and sort it out properly once the road trial season is over I get a chance to take it down to Adelaide.

CJ

[oldyzman]

Hey Guy's,
there is a small air jet just inside the intake bell of the carby on a VM38 (YZ250 1979), this feeds the needle jet with air,
Does anyone know how important this is... and what size it should be on this bike as I understand that for a YZ2501980 it should be a 2.0
Cheers Brett