36 or 38 carb

[bnau267]

Seems like a lot of the 250r XC pros ran 36mm or smaller carbs back in the day.  Everyone seems to prefer the pwk38 now days.  What about a pj36?  Is it old technology...help me understand why a pwk38 is better for XC?  Engine PC2000 275 and eventual LED 300.

[Tbone07]

You might be able to get away with the 38mm on the 300. I'd stick with the 36mm for the 265. I'm running a 38mm on my 350 for example.

The larger the carb you stick on there, the more air/fuel that goes in. This is great for top-end power but you'll lose bottom end power and throttle response. Your motor won't be able to efficiently process that larger air/fuel charge at lower RPM's.

[ledperformance]

The 36 is not a great choice for carburetor. The 36 will loose almost as much low end as a 38, but does not make up for it on the top like a 38 does. The 38 is a clear winner. I gives acceptable low end and roll on power. It makes more mid range and top end power. It works well when you add other mods like porting or big bore kits.

[wilkin250r]

It has been a long time since I have studied carburetor theory, but I seem to recall that flat-slide carburetors have better throttle response due to their decreased length, and also something about the airflow characteristics as it relates to turbulence over the main jet.  (I would appreciate anybody posting articles or links to support or deny this.)

In theory, a 36mm carb would have better throttle response than a 38mm due to size.  However, because the 38mm PWK is a flat slide, it will have better throttle response than a round-slide.  Also, the 38mm PWK is available in an Air Striker version, which has fins to direct airflow over the venturi to improve throttle response even more.  It's entirely possible that the 38mm PWK will gain all the throttle response that was lost with the increased carb size, and also have the added benefit of improved top end performance.

Unfortunately, "throttle response" is something very difficult to measure and quantify, so I can't back up my theories with any hard data.

[mx250r91]

I used the Keihin 36mm PWK A/S carb only found on newer (~2006 and up) KTM 200/250/300 bikes last hare scramble season. It had great low end with the PT Type 6 pipe but did sign off a hair sooner on the top end compared to the PWK 38mm A/S. It is popular to oval bore these carbs out to 38-39mm to keep the low end snap of the 36 but have the top end of a larger carb. It is shorter in length than a standard Keihin so I had to use a 1/2" spacer with the RAD valve to reach the airbox boot. The cap also uses two screws instead of being threaded so needle adjustments are a little trickier. The nice thing about it though is that the pilot jet is right next to the main jet so you can access it through the drain plug and don't have to take the whole bowl off. I think the 36mm smart carb when it comes out will be the cat's meow for a low end XC engine.

[bnau267]

If only the KTM 300 had a counter balancer  You have to be a fan of KTM.  Love em.  I had I hopes for the smartcarb but who knows how long before they get production squared away and start shipping orders.

[udontknowme]

as for vibration i dont know how well the ktm300 would do in a qaud chassis but it does fine in a bike. ive got a 2013 and  see no need for counterbalancer. maybe the qaud chassis would change the vibration aspect ?

[bnau267]

as for vibration i dont know how well the ktm300 would do in a qaud chassis but it does fine in a bike. ive got a 2013 and  see no need for counterbalancer. maybe the qaud chassis would change the vibration aspect ?

Wish I had the resources to test the theory...

So Duncan racing sells a Keihin 37.3mm PJ Carburetor.  Anyone have experience with this one?  Seems to bridge the gap.

[05450r]

My opinion is that Arlan and Wilikin hit it dead on. Especially the part about being an Air Sryker, which if Im not mistaken was developed as a mx carb, and its ribbed intake design for throttle response. With the quad vent helping solve fuel delivery problem while hitting the jumps, I'm assuming this would be good for bouncing around in the woods along with steep angles of climbing. Maybe Im wrong though..lol
There more that goes into carb selection other than size, the Air Stryker by design is far more advanced than a Pj

[ledperformance]

Don't test, either stay with the stock carburetor or go to the 38. Don't waste your money on 35, 36, or 37.5.
Barry Hawk preferred the stock 34mm carb on his LRD 300 for cross county. Bill Balance preferred a 38 AS. I believe Chris Borich used 35 PWK instead of the stock 34, because it it had a throttle stop not for performance. All my MX riders after 1995 used the 38 air striker.

[bnau267]

Don't test, either stay with the stock carburetor or go to the 38. Don't waste your money on 35, 36, or 37.5.
Barry Hawk preferred the stock 34mm carb on his LRD 300 for cross county. Bill Balance preferred a 38 AS. I believe Chris Borich used 35 PWK instead of the stock 34, because it it had a throttle stop not for performance. All my MX riders after 1995 used the 38 air striker.

Done deal.  That's one way to put my wandering to rest - Thanks Arlan.

[F-Red]

38mm Air Striker, Awesome all around carb! :victorious:

[bnau267]

38mm Air Striker, Awesome all around carb! :victorious:

That's the first time you answered a question without providing a visual to help us better see your perspective.

[F-Red]

That's the first time you answered a question without providing a visual to help us better see your perspective.

Sorry. I don't like high jacking, ones thread's.
NICE CARB!

[bnau267]

Sorry. I don't like high jacking, ones thread's.
NICE CARB!

High Jack away!  Good end to the thread plus I got closure - 38mm is what will work best for my application.