FCR carburetors
#11
09-13-2008 | 04:47 AM
Senior Member
Joined: Oct 2006
Posts: 3,106
From: Misawa Japan
1st Gear Member
Usually on 300cc and above. I personally wouldn't go with a carby that large on this bike. The guys with the 39 report more power in the range they want, but also say the fuel efficiency noticeabley decreases.
#12
09-13-2008 | 05:00 AM
Senior Member
Joined: Oct 2006
Posts: 3,106
From: Misawa Japan
1st Gear Member
Yeah all these ideas agree with me except the 39 mm carb on a stock 250 engine its just to big and i would have to agree with the reasons. I wouldnt wanna go any smaller than 34 then your gonna be pulling too much gas from the main jet and just give you crap mpg perfect fuel air ratio is 14.7 parts air to 1 part gas plus if the carburetor is too big for the intake port then its wasted size to have a bigger carb on small intake port, IMO
You're only talking about difference of 0.5mm on either sid of the bore (33mm vs 34). It's easy to get to the right mixture with the right jetting. Ideally you want to run at slightly richer than 14.7:1 A/F mixture. There are FCR33s over there in Japan configured specifically for KLX250 or 300. How much is pulled from the main jet is related to the size of the MJ. When it starts getting pulled from the MJ is related to the size of the slide cut-out.
I'm telling you the FCR33/300cc combos I've seen over here go like a raped ape and get excellent gas mileage.
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