Good pipe for airstryker???

[two85atc250r's]

Will an FMF fatty pipe and a power core silencer work well with a 38mm PWK? I am thinking about upgrading from my stock carb as part of my rebuild. Any carb advice would be appreciated. Thanks.

My motor is only .030 over(66.75 bore) w/ wiseco piston and I am running V-force reeds.

[my88r]

what kind of riding are you doing??

[two85atc250r's]

all sand and dunes man. Would a 36mm work better for my setup?

[86 Quad R]

got any porting done?

[Buttermilk]

You really should consider running the ESR TRX5 pipe if you are looking at changing pipes.

I don't know a whole lot about the FMF Fatty pipe, but I do know a little about the ESR pipes.

The ESR pipes are really hard to beat for most applications. Especially if you are riding dunes a lot.

The ESR5 pipe is considered to be one of the best all around pipes available.

I would certainly change from the stock carb to the 38 as well. The Keihin PWK 38mm A/S carb is a really good carb.

[two85atc250r's]

No, I don't have any porting done and I already have the full FMF exhaust system. I wasn't looking to buy exhaust, just want to make the best of what I already have.

[86 Quad R]

what you should do is bump the compression up a bit, add a 35pwk a/s or 36pwk carb and have the exhaust cleaned up and polished. :cool:

[wilkin250r]

If you were riding trails or mx, I would say stick with a 36mm. But since you're riding sand, I think a 38mm would be just fine.

Smaller carbs mix fuel better at low RPMs. That better mixture improves combustion, and therefore gives better power at low RPMs. This is important for mx, it gives you better control and traction coming out of corners.

YOU, however, are in the sand. There's no traction there, no matter what you do. Wheelspin is just a part of life. Therefore, you don't care about low-end power. You care about HIGH END power, max revs. This is where a larger carb will benefit you. The larger carbs will flow better in high RPMs, making more peak HP.


Down the road, I would highly suggest some good porting. You would be surprised at the power increase.

[Buttermilk]

There are a lot of factors that come into play regarding engine setup.

The attached dyno chart shows my 250R setup with low port timings, lots (250 psi cold cranking compression) of compression and a 38mm PWK A/S carb - Dyno Run 008.

The other run (071) is with a 41.5 mm carb and a completely different setup on the motor. Dyno Run 071 has a lot less compression, a bigger carb, different pipe, a lot more port work and different port timings.

As you can see by the chart, the larger carb has better power all across the power band.

If it were me, I wouldn't run anything less than a 38 PWK A/S, and I would also get that cylinder ported.

That FMF pipe should work fine.

Regards,

Rog

[pdxbigfoot]

Buttermilk. Great info!!

[two85atc250r's]

Wow, thanks alot guys. I am always amazed at the instant information that you can find here. I found a really great deal on a 36mm PWK so I think I may run that for now and keep my eyes open for a 38mm. I didn't realize that the price would be that much higher for the 38, they must be really good carbs. Thanks again for all the help.

[wilkin250r]

Originally posted by Buttermilk
As you can see by the chart, the larger carb has better power all across the power band.

I'm not looking to dispute anything on the graph, but a dyno won't show you anything about throttle response, a very important aspect to consider when selecting a carburetor.

[All250R]

56 hp is a respectable peak number for sure. There's a chance though that if you put the bigger carb on the more mild setup you tested it won't perform as well as the 38. Carb size should be picked in part by where you want power, but also as noted how the engine is designed including pipe characteristics. Bigger carbs are not always better, just like big ports, pipes, reed cages aren't either. It really depends on how hard the carb is being sipped on by the engine and what the rider wants to experience.

[Buttermilk]

I agree with all that's been said regarding carb size. Bigger isn't always better.

I simply posted my chart to show that there are many factors that come into play regarding power characteristics. And to simply say that a smaller carb will have more power on the bottom to mid vs. a larger carb simply isn't always true.

I've had my 250R on the dyno for 72 different dyno pulls over the last year and a half or so, trying different combinations. It really amazes me how one small change can effect the overall outcome.

I'm still learning and trying different thing...

I also agree that had I tried the larger carb on the milder setup, it wouldn't have performed as good as the smaller carb. I didn't have the larger carb at that time or I would have tested it. But, I would have expected the smaller 38 mm A/S to perform better than the larger 41.5 PWK.

Still, I personally wouldn't go any less than a 38mm PWK A/S even on a mildly ported OEM cylinder. I know of two (one which was mine) 250R's with mild port jobs that performed better with a 38 than they did with the stocker, all across the rpm range.

I've attached a chart showing my current motor setup with a 38mm PWK A/S compared to a 41.5 PWK taperbored carb. Everything else was exactly the same on engine setup. Run 059 is the 38 A/S, and run 071 is the 41.5 PWK. I will point out also that my axle bearings were discovered to be bad while on the dyno with run 071, and that I had ~1mm mis-match between the carb and intake boot, inhibiting airflow and creating turbulance.

Had those two things been fixed, I would expect to see the 41.5 out perform the 38 by a slightly greater margin. The dips in the line on run 071 is most likely due to the bad axle bearings.

As far as throttle response, I've ran both of these carbs on the same quad, same day, both recreational riding and at the race track. The 41.5 mm PWK carb has virtually the same throttle response on my motor as the 38 A/S carb does.

Also, run 071 has about 2 Average HP more than run 059, which to me, is more meaningful than peak power (it's a better indication of which motor will be faster on a track than peak...)

The dyno chart shows that for all praticle purposes, the bottom to mid is virtually the same, with a better peak and better over rev going to the bigger carb.

Sorry 'bout jacking the thread. To get back on topic, my advice (for what it's worth...) it to go with a 38 PWK A/S carb (my own testing has shown it to be a really good carb) and run your FMF Fatty pipe and get some port work by someone you trust.

;)

[woodsracer144]

whats the lighter blue and red mean?

[wilkin250r]

Originally posted by woodsracer144
whats the lighter blue and red mean?

There are lots of different types of data taken on a dyno run.

There are only two runs, the red and the blue. The top set of curves is "horsepower", and you read the numbers from the left side of the graph.

The lower set of curves is "torque", and the values are listed on the right side.

The very bottom is the air/fuel mixture, again color coded by red and blue.

So although you have two runs, each run has three different measurements: HP, torque, and air/fuel mixture.

[woodsracer144]

thanks thats alot of info on this page... i never really looked at a dyno befor....