I got a new (old) Keihin PE carb with a 31mm bore. I'd like to use it in preference to the pos GN250 one....however I suspect 2 stroke needles are different.... Main jet- 125 pilot-50 needle- 1KB (8?) ...does not appear on any charts. slide 3.0 the main has a dished ring around it. its for the FXR 150
A 2 stroke carb usually has a hood around the needle jet...because there is not much suck on a 2 stroke,creates an extra little venturi effect. They seem to work just fine on a 4 stroke,but then my ''just fine'' is possibly unriderable to others.
yes it has the hood. I'm thinking the needle will be the main difference...?
Sometimes also the emulsion tubes, this is for the Bucket? FXR150? GN250 carbs work better if you drill the lift hole in the slide slightly bigger
GN250 carbs work better I must be old skool as I assocaite diaphram carbs with BMW and I think they are junk compared to a Dellorto.....all that extra crap there to reduce wear on the drive train. Yeah its for the bucket...the GN carb is ok but it seems to lack get-up-and-go.....mind you std gearing on a tiny track can't help....barely use 3rd gear.
A 4S carb is different in it's air passages and uses an emulsifier jet in the main jet circuit. have a look here http://www.sudco.com/Carburetor01.html
Keihin XR250 carb works great on Tylers FXR, its a pumper too. made a big differnce to the power I will get the jet sizes, needle and post them over the weekend. Had too modify the throttle cable, but that was all. The carb spigot? (where it fits into the manifold) size was 1mm bigger that the FXR carb but fitted easy, just had to put a slightly bigger hose clip on it Have you got a picture of the carb?
You can solder up the emulsifyer tube and replace the main jet with a 4-stroke one
You can solder up the emulsifyer tube and replace the main jet with a 4-stroke one I don't think there is such a thing as a '4 stroke' jet....a jet is a bit of brass with a calibrated hole in it...... I think the secret is the needle diameter and taper..... If I recall the Mikuni they fit to the BMWs as a Bing upgrade is usually for 2 strokes....its just a matter of working out the combination of main, idle, slide and needle.... ummmm
.its just a matter of working out the combination of main, idle, slide and needle.... ummmm That's what dynos are for
