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carb boring

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I am thinking about boring the carb on my 250XC.  partly just for a project/experiment and also because some more top end would be nice.  I am an experience machinist, but have never experienced boring a carb.

 

The way i see it there are a few ways its done.  Using my 36mm PWK as an example.

 

oval bore-  boring a 36mm hole 2mm above centerline, making a 36mm by 38mm oval.

 

Straight out boring it to 38mm, but 2mm above centerline so that the slide floor is the same.

 

Keyhole boring- boring 3 or more differnt arcs to create more volume above half throttle than an oval bore.

 

Since there are those air fins, and the shroud cast into where the needle goes through the bottom of the carb I wont be able to go straight through.  I imagine I will have to use a CNC or rotary table (Not going to use a dremel!) to cut a half arc on the bell side of the slide, in order to match the engine side.

 

The other option I can see is to taper bore the carb so that it remains 36mm on the middle, but opens up to 38mm at the opening.  This doesn't seem like it would provide very much gain though.

 

Oval boring or keyhole boring sound like the best to me.  I really like how the bike runs at low throttle openings, so I dont want to mess with the carb size at under half throttle.  At least not yet.

 

Am I way off in any of my thinking??  Thanks for any input!

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I have zip experience on carb boring, but if you're going from 36 to 38 and want to maintain the same floor tangency, then you move the centerpoint 1mm not 2....

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You don't want to touch the slide floor, that's why he'd move it up 2mm on a 2mm larger bore.

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If you lower the floor 1 mm you may not be able to lower the slide enough to idle the engine down. I've just always read not to do it that way.

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I understand, but a two millimeter larger circle has a 1mm larger radius.  If you bore a 2mm larger bore with the center line offset 1mm farther up from the floor than it currently is, the new bore will be tangent with the original floor, will it not?   What did I miss?

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I understand, but a two millimeter larger circle has a 1mm larger radius.  If you bore a 2mm larger bore with the center line offset 1mm farther up from the floor than it currently is, the new bore will be tangent with the original floor, will it not?   What did I miss?

 

You are correct.

 

A 1mm offset from center will end up skimming the original floor on a 2mm larger diameter bore

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Thought so, and Adam is correct about it being the right approach to take, for the reasons he pointed out, and all the machine work involved with lowering the main nozzle in the carb body, re-cutting the slide "seat", etc.  Why get involved with that?

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I have zip experience on carb boring, but if you're going from 36 to 38 and want to maintain the same floor tangency, then you move the centerpoint 1mm not 2....

 

Correct. I realized I typed it wrong a couple hours later, but had no way of fixing it until now.  Looks like my mistake was caught though!

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There are shops that will do this for reasonable cost. Cycleplayground in PA is a specialist in KTM 2 strokes. If you want to do this yourself, keyhole is the way to go. If you look at the throat of a 36mm PWK you can see that the bore is not centered in the casting and that there is more material left at the top than the bottom. Keihin uses the same casting for 36s and 38s and offset bores the 36s. A 2005 250SX 38mm PWK made a difference in the top end power on my 300 with very little trade off in low end response. The jetting was different than what most others run but it was a nice step up in overrev power.

 

If you need any more convincing here are some dyno runs comparing a stock 36 , a stock 38 and an oval bored 36. The oval is the thin line making more low end hp (further left) with the same peak power, stock 38 is the thicker red line, stock 36 is the inside curve  (the curve looks odd like the PV spring was too tight). These are all on a 300 so it may not be as large of a difference on your 250.  I like to tinker so I have no reservations about trying new combinations. You might get some machining ideas from cycleplayground's carb mods page http://www.cycleplayground.com/wordpress/?p=65

 

148763449.MPCnnVUP.jpg

 

 

 

 

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here is a follow through on this.  I plotted a profile that would give me 38mm of hight and about 37mm of width and half throttle.  At 3/8 throttle and below the bore of the carb is completely untouched.  Using a long endmill on a CNC mill, I was able to bore all the way through and miss the fins and jet shroud.

 

The bike runs awesome.  I just winged in on the jetting and it was close enough to work.  Huge gain in overrev and top end.  More than I expected.  It did take alittle more clutch work to get the bike onto the pipe under load, but my jetting may not have been perfect and the track I was at had some heavier dirt than I usually ride.  Even if I did give up some throttle response, the trade off was worth it.

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