490 jetting

[mico]

hay guys just looking to buy new carb for 82 490 wondering how to have it set upany advise would be great cheers

[smith]

main jet  #370
pilot jet   #50
needle middle   clip
pilot air screw   1 and half turns

[lms6201]

only if u want to cook it !!!    370 main is only  for a stock std it 490   (unless that is what u have ?)or possibly very high altitude  or u are riding it like miss daisy , yz490 82/83  can be jetted a few different ways , one that works without too much fiddling is (for mikuni vm38) 440 main (std)  6f16 needle(std) 40 pilot (leaner than std 50) R0 n/jet (richer than stdQ8)   3.5 or 4(leaner than std#3) slide cut ,1.5 turns out with the air screw , start with clip in mid  run on 33 to 1  highest octane , booster added helps    have also seen a q2 n/jet with a 460 main work very well with the std #3 slide and 50 pilot  , i  use the 1st setup with a #4 slide and i think the needle dropped  a position  , if it,s not digging trenches or doing wheelies everywhere or not pinging it,s sick   

[Ji Gantor]

What Ims said.

I have found that Yamaha had their Mikuni carbs retooled and you should stay with the original carb if at all possible.

First check to see if the screw top is an extended one. If so then the slide is longer.
The new Mikuni's don't have this extended top nor can you purchase a new long slide I found out yesterday for my YZ125 H 1981.

The rest of the jets are standard Mikuni.

But if your carb is no good at all I guess you will have to go down the new trail.
Best of luck

Ji

[lms6201]

ji the 250 and 490 j,k use a std mic carb ,for your 125 try kawasaki 125 about 1983 , i think i have one in the shed  , what slide cutaway ?

[Ji Gantor]

The cut away is 2.5

Ji

[mico]

thanks very much guys i am ordering a new vm38 tomorrow i,ll give ims6201,s specs a go

[lms6201]

just beware of the fact that there are two vm 38 ,s  i believe the guide pin can be on the opposite side , i,d try a new n/jet and needle and a slide first in the orig carby ,slides can be filed   these are the parts that seem to wear , confirm with mikuni the side of the guide pin for the slide   , is there something drastacly wrong the the carb u have ?

[Marc.com]

good advice I would stick with the original carb if you can, it is one less variable.

like the 440 jetting spec, you can always lean it off later. Buy #4 slide.

[hotdog1975]

just beware of the fact that there are two vm 38 ,s  i believe the guide pin can be on the opposite side

the guide pin is on the opposite side to the compared to the original carb so slide change over is not an option, also the idle speed screw is on the opposite side. I have one in the shed I bought off TD Hatrick for my 490J (now sold) & ended up re-kitting the original carb. My IT490 will be keeping the original carb on it also..

[lms6201]

thanks for that hotdog , i knew there was something going on with left and right pins , but that means he can,t buy a new slide from mikuni to fit a yamaha ,so u have to go the whole hog , i,ve been lucky , everytime i get a box of parts there,s been a carb  or part there of , 

[Marc.com]

if it,s not digging trenches or doing wheelies everywhere or not pinging it,s sick   

I used mine in the sand quite a bit and it used to ping like a MOFO, I found the best way to sort the 490 out is to trade it on a 250.

[lms6201]

marc did your 490 s have a bigger tank than std ? just that i saw one with tecnosel seat and a lumpy top tank and was wondering if it came from yamaha

[Marc.com]

no Mine had a small tank.... maybe it is OW tank, or the bike was set up for European GP circuit, Tecnosel seat sounds like it.