jetting the 250 desmo

Hi- or me again

Does anyone know if there is a minimum size for the main jet on a 250 desmo?

I currently have a 105 but the plug is still oiling horrendously. It had a 120 which is apparently about right for a 450! Since tried 110 and 108- both oiled horrendously. Is there a lower limit? I’m wondering about simply buying everything from 95 up! Or should I start at 75???

Many thanks

020 7722 3215
07960 173903

Hi Ian,

I’m a tad confused (Ok, I know that’s not hard …:slight_smile: ). You say you’re oiling the plug, but you’re asking about the main jet size? :confused:. I’m not clear why you think the two are related …

So I need to ask: Is the plug oily, i.e. is the fouling black and shiny? Or is it carboned up, i.e. is the fouling black and dull?

  • If it’s oily, oil is getting into the combustion chamber either thru worn valve guides or worn rings/cylinder. A simple compression test will help here.

  • If it’s sooted up, (most likely) the carb is running too rich either 'cos the choke isn’t sealing or an airways partially blocked or you’ve got too large a pilot jet or the float height is wrong. It’s unlikely to have anything to do with the size of the main which only comes into action from mid-throttle opening up.

But I thought you said Brian S. had sorted all this for you? :confused: Btw, what plug are you using? Should be NGK B7HS, possibly B6HS.

Btw, the standard settings for a 250 Desmo running a VHB 29 are, according to Mick Walker:
Needle: V13, clip in middle position
Main: 110
Pilot: 40
Atomiser: 265M
Slide: 40

According to Mick, the smallest main used on a w/c 250 was 95 on the 250 Scrambler. But this was with a smaller carburettor, the VHB 26.
Note: If you go too lean on the main, the engine will run too hot, be liable to pre-detonate and may suffer some bad damage. So be very careful before you go any smaller …