Carbs

[Chino886]

What is the best carb for these motors?

I have heard that 38mm A/S are the cats meow. I also have heard that the 40mm Keihin is also good.

I have a 36mm Mikuni also.....

Too many carbs, too little time!

Any info or enlightenment is greatly appreciated!

[86 Quad R]

what porting does your engine have and what type of riding do you mostly do?

[wilkin250r]

That's like asking "What is the best pipe?" Well, it really depends. A top-end drag pipe will give you the most horsepower, but it won't be too good for woods racing, and vice versa.

Carburetors should be taken into account with the entire engine package, and the desired characteristics of the motor. The reason you have so many choices, is because they each are good for their own purpose. If there was just one "best" carb that fit everything and every purpose, then that's the only carb you'd ever find.

Smaller carburetors (36mm-ish) have better flow characteristics at low and mid RPM ranges, and better throttle response. So smaller carbs are good for stock motors and tight terrain where you're not able to be at WOT for a long time.

"Small" is also relative, a 38mm carb is big for a stock bore 250cc, but it's rather small for a 400cc+ big-bore motor.

Larger carburetors flow more air, so they are great for max horsepower machines and extended WOT riding styles. This can often encompass drag racing, and wide open areas like dune riding. Larger carburetors are usually better suited for top-end porting and high rev riding styles.

Again, let me say that "large" and "small" are relative. A 38mm carb is rather large on a stock-bore motor, I'd only go that combination on a drag-racer or wide open dune machine with high-rev porting. But that same 38mm carb is really small on a 500cc Saber.

The 40mm Keihin is really a 39mm keihin that is bored out, I believe that's the max you'll get on a Keihin carb. Anything larger will have to be a Lectron.

[hartwill]

I switched from a 34mm to a 39mm air stryker on mine and what a difference it made in throttle response and mid to top end. And I'm only at 67mm bore with mild porting. I will never look back.

Wilkin is on the money, however…

If I was to have to pick one carburetor for use from mildly modified 250R duner to a 370R duner/hill racer, I would go the Hartwill direction with a 39MM Keihin Taper Bored & “Super Flowed” to 40.5MM…yes, a bit over the top for the mildly tuned 250R, yet almost perfect for the 370R…

Carlos.

[jcs003]

in my experience a smaller carb is too limited. it is good off the bottom, but power seems to drop off quick. i have a 38 mm lectron and it seems to spread out the powerband without losing much throttle response. the wider band makes it easier to ride. im also riding an 85 atc(unported) cylinder if its any consalation...

[wilkin250r]

Let me backtrack just a little, when I said I wouldn't put a 38mm on a stock bore unless it was a drag or high-rev porting. What I SHOULD have said was high-rev riding style.

The smaller carbs like 35 or 36mm are really dwindling in popularity. They were great carbs for tight terrain and an XC-type riding style where the terrain simply doesn't let you hold WOT for more than a couple seconds. But the 250r simply isn't ridden in that type of terrain much anymore, the 4-strokes are just better at it.

I wouldn't hesitate one second to put a 38mm carb on a stock bore with stock porting, but only if you're riding in fairly wide open areas where you get to stay in the higher revs for longer periods of time, and you can actually see a benefit from the WOT performance.

[jcs003]

gearing and pipe selection also play a part. like one of the wise members here once said, "the combination of porting, pipe, carb and gearing must work together like a symphony"

[86 Quad R]

Originally posted by jcs003
gearing and pipe selection also play a part. like one of the wise members here once said, "the combination of porting, pipe, carb and gearing must work together like a symphony"

and that my freind is the truth the whole truth and nothing but the truth. :D

[jcs003]

Originally posted by 86 Quad R
and that my freind is the truth the whole truth and nothing but the truth. :D

i wish it was my quote.haha. cant remember who said it either.

[86 Quad R]

i've been known to say it but im inclined to say that ole neil is the guilty one. :cool: