I have an 89 trx250r that has the stock carb on it and i have been riding alot of other peoples bikes and have noticed the 250s with the larger carbs al seem to have more low end to them. I have asked what has been done to these and the biggest common factor is the 39pwk carb. Any comments would be great.
thanks, sand-racer
note from atvbbs
one topic..one forum. Your duplicate post in "Performance mods" has been deleted

 

I have always noticed less low end with a bigger carb, maybe those quads also had boyson reeds because that would make a huge differnce

 

sorry but in most cases that is wrong in fact its almost the exact opposite. A lager carb will usally give you more top end at the sacrafice of the low end. if you care about the details, adding a lager carb will increase the ammount of fuel/air going into the engine, the problem is if the exaust pipe you have is desigend in combination with the smaller carb you will have more fuel/air going in only the same ammount of exaust going out, a problem you ask? yes. an engine is only a strong as its weakest link and if the exaust cant keep up with the intake the engine wont run as good as it should. but there is a solution get a better pipe (fmf or sutch) and then put on your lager carb and should have more power than if you just addad a pipe.



1990 polaris trail boss 350L 4x4

 

The big-carbed quads you rode may have had more low end, but chances are, other variables (than the carb size) were involved. Typically, a bigger carb doesn't atomize fuel as well as a stocker at low rpm and lower-velocity airflow. That's why automobile 4-barrel carbs work "progressively;" at low rpm, only two barrels open--hey, even the hottie, hottie rigs, like "Tri-Power," with THREE two-barrel carbs, ran on only the center one at low speed.

Tree Farmer

 

SAND-RACER IT MUST BE THE TYPE OF PORTING THAT QUAD HAS [OOPS I MEAN YOUR QUAD NICE PICTURE].i DEFFINETLY WON'T BE TRYING IT ON MY 88 THE LOW END SUCKS NOW AND TOP END AND MIDRANGE AND ANYWHERE ELSE IT IS SUPPOSE TO HAVE POWER SEE YA LATER ON TODAY

 

bigger carb equals more topend

 

You beat me to it Tree Farmer, I was about to point out the exact same thing.

I've always thought it would be cool for someone to make a 2-barrel carb for ATVs, one primary and one secondary venturi. A lot of four-cylinder imports have this type of carb. I know there's nothing better than the feeling of the secondaries opening up on my big-block V8, and I'd love to have that same feeling on my EX.

 

Motherhead: The mid 80's honda XR's had a 2 carb setup, I had a 84'XR500R with this setup. the first would open to 1/2 throttle then the second would start to open. It worked ok, but it had a few hickups when letting off slowly from full throttle, they just did not carburate cleanly. My friend had a 84'XR250R with them too, and I could smoke him with my 91' single carb and both bikes have the same exact engine. (heck, the old 83'XR500R is nearly the same as the 400EX's engine) Honda ditched the 2 carb setup around 1986 on the XR600R if my memory is correct.

 

It ain't really relevant; but, since we're walkin' down memory lane . . . I had a Honda Super Hawk (CB77) with twin carbs, but the separate carbs were synchronized and there for an entirely different reason--the Super Hawk (and the smaller-displacement Hawk) had 180-degree cranks; one carb couldn't feed two intake strokes efficiently at 180-degree spacing . . . also, two separate coils and point sets were required for the 180-degree firing order . . . oh, man; nostalgia's not what it used to be!

Tree Farmer

 

ACTUALLY A BIGGER CARB CAN ADD LOW END.YES A BIGGER CARB USSUALLY TRADES BOTTOM FOR TOP BUT THE MAIN FACTOR HERE IS CARB DESIGN.AN FCR 39 ON A 400EX ADDS 4HP TO THE LOW END-3.5MM BIGGER ASWELL AS MORE TOP.CARBS INTENDED FOR RACE USE WILL ALWAYS BE BETTER EVERYWHERE THAN A STOCKER.