150 super,which carburettor

Hi ,i have a 66 mod 150 super VBC1T, framenr 38287. I need to buy a new carb(old one has stripped threads). The one on my bike is a SI20/17D but in your parts drawing is says it should be SI20/15D. I see that SIP recommends a bigger carb due to cooling?? What is the difference between these 2 carbs? Power?? Which carb do you recommend? Do these new carbs come complete with jets or do have to order new jets? If so,what jet set do i need to order?

I'm building a Super right now also.  This link is pretty handy:



OR just google "sip tech tip si carburetor"

I'm playing with a 20/20D and a 24/24E but I'm tuning for a 177 kit.

I ordered the 24/24E from SIP and it came with the jets installed as noted in the link above.


Hi, you've asked about the difference beetween a SI 20/15 and a 20/17 carb.

Well, it's the diameter of the main air opening...for the SI 20/15 it's 15 mm, for the SI 20/17 it's 17 mm.

Be careful, you cannot just put any bigger carb on the engine, as the inlet opening in the engine case has to correspond with the carb size... a too small opening in the case sometimes makes a bigger carb useless...unless you open the engine and modify the inlet opening.

Regarding the jet size it's highly recommendable to perform a jetting procedure - nobody can tell from a distance what size of jets you exactly need... Please remind, the task of the carb is to mix fuel and air in a very precise ratio (1 part of fuel  : 14,7 parts of air)...so for sure  if you change to a bigger carb you'll also need bigger jets in order to obtain the correct ratio between fuel and air. 
Otherwise, if you put the jets of the small carb into the bigger carburetor, the engine could be damaged (danger of piston seizure) as the mix is too lean.