Yamaha Jetting Q’s

[dennis da menaceMember]

the old carb on the YZ im rebuilding is pretty much rooted and im gonna whack a new one in it.

the original is a Mikuni TM38SS (this is basically a TM carb made for yamaha OEM)
its factory specs from the manual were…

45 pilot.
370 main.
6DJ8-63 needle…….3rd clip pos.

the settings as it came out of the bike were…

55 pilot.
340 main.
6DJ8-63 needle…..3rd clip pos. (same as stock)

from what i can work out, the equivalent carb is a Minuki TMX38-27.
it ticks all the boxes with regard to outer dimensions etc. but has different jetting out of the box…
its jetting is…

20 pilot.
370 main.
6EJ12-55 needle, 3rd clip pos.

now, the motor has been rebuilt. slug is 20 thou up.
its running VFORCE reeds and has an FMF gnarly pipe and powercore muffy.

i need to jet it to suit Craigs riding. he doesnt ride hard and generally uses the bottom to mid side of the rev range

all this carb business is doing my head in :S as i cant understand the needle numbers :pinch:

i plan on upping the pilot to the 45 and leaving the 370 main for now.

but can anyone explain the needle number jargon to me???
and is there a reason the pilot is so small on the new carb????

and tell me the difference between a TM series carb and a TMX??

cheers in advance carb guru’s

[drewMember]

hey Menace, i just copy n pasted to the Yamaha IT forum in a thread someone was asking similar Q?’s for an IT. hopefully someone has some answers.

the only issue one member found was that the TMX was slightly shorter and had to make a spacer tube. but i dunno what differences there are between IT and YZ dimensions.

the only other remark was that they would be better off looking at the TM over the TMX, but no reason as to why…

here is a chart someone posted.

i have it in pdf form that is much clearer to read. if you want it pm ya email n i’ll send it.

[Steve WyethMember]

The TMX may have been used on a bike with a bigger motor which has a stronger vacuum signal and therefore needs smaller jets.

Its why a KTM 250 runs bigger jetting than a 300 but both use the same carb. Strange but true.

XY has covered the rest with the chart

[dennis da menaceMember]

so basically, if i throw the same jets in as the manual says, it should be near enough to right……

near enough for run in and testing anyway……

yes???

i think the TMX has an extra “power jet” for sustained WOT… as in open desert hardcore fanging :laugh:

….that chart may as well be written in chinese…..

it makes little sense to me :laugh: :laugh: :laugh:

[Steve WyethMember]

Pretty much yes. You know you’re stuff and will be able to pcik if its scarily lean anyway.

You won’t go wrong with the stock jetting but you can improve from there once run in.

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