carburetor size pros and cons

[daveciak]

Could someone please explain to me what size carb I should have on my new 340cc sphynx. It is ported for midrange/mx. Icurrently have a 36 PWK on it but am wondering if a 38 or
39mm would be a better choice. Then on top of that, is there really a difference between a pwk and pwk airstryker. If you get to large of a carb will it at some point start to hurt other areas of the powerband. Any advice is appreciated.

[utah250r]

With the bigger carbs you start to get a stiffer throttle and the power shifts to more mid and top. I would be interested in your carb if you decide to sell it.

[utah250r]

That is a pretty small carb for that big of a motor. Im sure other guys have more experience with the sphynx motors but I would assume you would want to go bigger

[8686]

I'd def have a 40.5 PWK on that beast.

[addictedtomud]

I run a 38mm PWK airstriker on my 350 motors and love 'em! The airstiker just has little fins on the airbox boot side that guide air in when you barely crack the slide open - gives you better low end throttle response than a regular PWK

[wes350x]

When you told me a while back the size of the carb I thought it was a little small...if you do trail type riding or riding where low end power is essential, then a 38 a/s or 39 would be good. If you do more mid-top end power riding then go with the 40.5 carb. How's the power of the new motor?

[dabeechman]

Yup, mid to topend type riding...40.5

Woods/Low end...39

Pros of a bigger carb is more top end horsepower.

Cons of a bigger carb, less low end and harder to tune.

[wilkin250r]

Ditto, ditto, and ditto.

A 36 is really small on that motor. Smaller carbs have better airflow characteristics at lower RPMS, it atomizes your fuel better which gives a little better power at low and mid-range RPMS, and gives you better throttle response. The drawback is they don't flow as much air as larger carbs, which means they choke off high-RPM and peak horsepower. If bigger was always better, we'd all have 80mm carbs on our bikes.

Larger carbs flow more, which means they are better suited to high RPMs, like drag racing, high-speed TT, or even just a high-rev riding style (which is exactly how I ride at the dunes). Better peak horsepower. The drawback is that they suffer from poor airflow characteristics at lower rpms because the airspeed is lower. This gives inconsistent jetting characteristics at low RPMS and poor throttle response.

But even with all that, a 36 is really small on that motor. Even for low-end and midrange, I'd never suggest anything smaller than a 38mm for that motor, and even bigger if you tend to rev it out more often.

[Grande Huevos]

ESR sells a 40mm pwk a/s. this could be the best of both worlds right now, you still get the low end punch with all the top end as well. I wish they offered this when i built my motor :(

[morse250r]

id do a 40.5

[daveciak]

I ordered a 39 PWK today with a new intake as well. Right now the bike pulls like a freight train but as soon as it gets on top it just starts breaking up. I have been jetting on this thing for 2 days of riding now and I just cant get rid of it with jetting. I have jetted and jetted some more so I'll see what the new carb does. I run a NGK B9EGV plug as recomended by FTZ and the bulder. I'll post my results for those interested. As of right now though it moves my 240 pound butt around pretty rapid till it gets all the way on top. Thanks everyone for the great replies.