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
IM
020 7722 3215
07960 173903
Hi Ian,
I’m a tad confused (Ok, I know that’s not hard …
). You say you’re oiling the plug, but you’re asking about the main jet size? 
. 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? 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 …
Ciao
Craig