Carb Boring

[dlunn]

I found a good deal on a 35mm airstriker and was wondering if this carb could be bored out any. I was wanting to go bigger but the price is hard to pass up. Could I get another mm or 2 out of this carb?

You could have it tapered bored out a few MM’s and the bell intake side re-shaped…

Carlos.

[dlunn]

Who would do this type of work?

Most good builders would taper bore your carb without issue...PM sent.

Carlos

[Pumashine]

Originally posted by acecarlos
Most good builders would taper bore your carb without issue...PM sent.

Carlos

The price is a $100-$125 right?

Most shops charge around $90 to taper bore & reshape the bell curve side of the carburetor; ”super flow”…

Carlos.

[Pumashine]

Originally posted by acecarlos
Most shops charge around $90 to taper bore & reshape the bell curve side of the carburetor; ”super flow”…

Carlos.
Just want to let people know even if they get a "good deal" on a carb it may cost more than the carb itself to taper bore.

Yes…find a good used tapered bored carb; should be lots of them around.

Carlos.

[dlunn]

I only paid $50 for the carb, but I would have as much into it to bore it than if I went out and bought a 38mm airstriker. I think I'll just hang on to it and look for a good 38mm.

Buying a stock 38MM Keihin verses taper boring & re-cutting the bell shape curve or “super flowing” the carb, even if left at 38MM will give you different end results.

If done correctly; the “tapered bore & super flow” 38MM carburetor will give you better throttle response; smother transition through the power band and an increase (however small) in mid-top end performance.

Best to keep your eye out for a used 38MM-40.5MM Keihin that already has all the work done; I have seen these go used for as low as $90…

Carlos.

[Motofool250r]

pete aka piranha did my 39mm carb to 42mm and i love it i think his price is 125$

[Pumashine]

Originally posted by Motofool250r
pete aka piranha did my 39mm carb to 42mm and i love it i think his price is 125$

I did the same for each of my three Puma's. Got all my 39's converted to 42's by Pete. Best of both worlds on this planet.

[Motofool250r]

yea i only have one puma but im gonna bore the toerh carb jsut so their the same on the smaller bike =)

If you go to 42MM from a 39MM or 38MM Keihin, not much of a taper can be maintained. The bore will be too big and the taper to shallow…

Best to stop at around 40.5MM….at 40.50MM you can still maintain some taper and keep somewhat of the “venturi effect” desired in a carburetor. It is better to have a converging inlet (like the bell curve) to slightly before the needle center line followed by a narrow or small straight section (constricted section) past the needle centerline followed by a diverging (tapered bore) outlet of some significance.

Fluid or gas velocity needs to increase as it passes through a constriction (Bore Diameter at needle location; 40.5MM), and pressure must decrease, in order to maintain mass conservation.

Engine dyno runs have shown a properly shaped “venturi” carburetor to have superior performance throughout the power band over the biggest bore you can have machined within equal carburetor bodies.

At a 42MM in a 38MM/39MM Keihin carburetor body, does not have any significant "venturi effect"; it is basically a straight hole.

Certainly you will see a performance increase going from a 39MM to a 42MM carburetor bore, however; you will see a better performance gain if you can keep the “venturi effect” at a smaller bore such as 40.5MM in a 38/39 Keihin carburetor body…

Carlos.

[Motofool250r]

even at 42mm we are still restricting the motor quite a bit being were running 443cc which uses a 6mm stroker crank.

i agree with you venturi is best but when there is no bigger carb available for your application without using a lectron then this is the best that was reccomended.

me and Puma shine both Dune are bikes and the Lectrons suck for actual riding so the biggest pwk carb is 39mm and bore it as big as we can get. IE 42mm

Just curious, not arguing here…did you guys try taking a 39MM Keihin to 40.5MM with a taper bore & re-shaped bell curve (“venturi”) verses a straight bore 42MM with no “venturi”…?

Sounds like at 443CC’s, nothing but extra intake area seems to help…

Have you tried having your carburetor machined to an oval bore; then you can get some of the much needed extra area verses a round bore.

A 45MM Kehien, tapered bored, “venturi” carburetor would be ideal for a 443CC…

Carlos.

[Motofool250r]

i have not but pete may have hes pretty much tried every combo possible since hes been building these engines since they hit showrooms

*shrug*

if there was a 45mm pwk carb id have one.

[86honda250]

why do you Guys not like the lectron crabs for duning. I ran a 42 PWK on my 350 and liked it. But when i got my puma I put a 48 lectron on and have not had any problem with it. My dads PSI 425 started out with a 39 pwk and then we put a 48 lectron on and found a big diffrence in performance all acrose the bored.

When you do the math; a 46MM Venturi Shaped, Tapered Bored Keihin should be just what a 425CC-450CC engine needs on gasoline.

Do you feel a 48MM is better and why? I ask because we are in the middle of a 430R development, an intake system including the carburetor we will be needed & developed as well; time to gather a little VOC…

Carlos.

[Pumashine]

Originally posted by acecarlos
a 46MM Venturi Shaped, Tapered Bored Keihin should be just what a 425CC-450CC engine needs on gasoline.
Carlos.

When you get one of those Carlos let us know and we will get ours done that way.

As you know; getting “one of those” 46MM Keihin’s is easier said than done, requires a new carburetor body & slide + a couple of other parts...

Carlos.

[Motofool250r]

Originally posted by 86honda250
why do you Guys not like the lectron crabs for duning. I ran a 42 PWK on my 350 and liked it. But when i got my puma I put a 48 lectron on and have not had any problem with it. My dads PSI 425 started out with a 39 pwk and then we put a 48 lectron on and found a big diffrence in performance all acrose the bored.

us oregon duners spend 95% of our riding time at less then full throttle the Lectron doesn't meter fuel but at idle and WOT and has nothing to offer inbetween, so after years and year of riding usging the midrange power of you machine to go to a style of riding where is has no mid range just on or off. i just dont like the flick style riding a lectron brings, to do trail riding.

PWK's are wonderful carb for duning smooth great throttle responce havent found anything better for these bikes and i can tune it in 20min, versus a lectron i cannot tune for the life of me.