Machining Bellmouths

[diesel pig]

Since my thread on Alloy wheel repairers was so successful at getting a result, I thought I would start this one with the hope of similar results. So in the ESE thread Wobbly said that Keihin PWK Airstrikers have fences on the bottom part of the bellmouth to get better response at low throttle openings but these fences badly affected the airflow at high throttle openings (the one I am really interested in on my bucket) but since I want to keep using the PWK with it's Throttle position sensor,Solenoid Power Jet and short over all length, I did as Wobbly suggested and ground away the fences but he also said even better results could be had by reprofiling the bellmouth of the PWK to that of a PWM carb, Since I do not have a PWM to study I googled some images of PWM's and they appear to have the similar bellmouth profile to an old PJ carb I had left over from when I had a NF4 Honda RS125. Studying the PJ bellmouth it appears that either Keihin or HRC had in a lathe turned away the part of the standard dirt bike PJ bellmouth that clamps to the aircleaner rubber boot to give it a trumpet like shape over the cone like shape of the standard carb. The only way I can think of to do it would be with a CNC lathe (but if someone can suggest a way to do it on a manual lathe I am all ears) So the question is there a engineering outfit (preferably in Christchurch or surrounding area) with a CNC lathe that is able and more important willing to something like this?

A quick look on the internet suggested Mark Sutton Engineering maybe the sort of small outfit that would do this type of work, Also Automatic lathes and Arcore Engineering came up but these look like bigger outfits that maybe not be interested in this type of piece work?

[timg]

Since my thread on Alloy wheel repairers was so successful at getting a result, I thought I would start this one with the hope of similar results. So in the ESE thread Wobbly said that Keihin PWK Airstrikers have fences on the bottom part of the bellmouth to get better response at low throttle openings but these fences badly affected the airflow at high throttle openings (the one I am really interested in on my bucket) but since I want to keep using the PWK with it's Throttle position sensor,Solenoid Power Jet and short over all length, I did as Wobbly suggested and ground away the fences but he also said even better results could be had by reprofiling the bellmouth of the PWK to that of a PWM carb, Since I do not have a PWM to study I googled some images of PWM's and they appear to have the similar bellmouth profile to an old PJ carb I had left over from when I had a NF4 Honda RS125. Studying the PJ bellmouth it appears that either Keihin or HRC had in a lathe turned away the part of the standard dirt bike PJ bellmouth that clamps to the aircleaner rubber boot to give it a trumpet like shape over the cone like shape of the standard carb. The only way I can think of to do it would be with a CNC lathe (but if someone can suggest a way to do it on a manual lathe I am all ears) So the question is there a engineering outfit (preferably in Christchurch or surrounding area) with a CNC lathe that is able and more important willing to something like this?

A quick look on the internet suggested Mark Sutton Engineering maybe the sort of small outfit that would do this type of work, Also Automatic lathes and Arcore Engineering came up but these look like bigger outfits that maybe not be interested in this type of piece work?

Wallace @ Marshland Eng?

[diesel pig]

Wallace @ Marshland Eng?

Does he have a CNC lathe? Last time I was there it appear his lathe was manual one.

[Michael Moore]

You can do that on a manual lathe. Keep changing the angle of the top slide and nibble away at it, checking with a template frequently.

Another way would be to plot XY coords for the curve and come in straight in the Z axis after setting the cross slide to the next coordinate.

At the end you can smooth things out with a sanding roll in a die grinder or hand drill whilst spinning the carb in the lathe.

That looks like a larger radius than you'll be able to do with a form tool, but you might be able to use a form tool to do the final smoothing cut where you are removing minimal material.

Yet another way is to mount the carb on a vertical mill table and mill the contour. If you've got a manual mill you'll want to use a rotary table and keep stepping in Z and X or Y. With a CNC mill you can do a 3 axis operation to generate the curved surface.

ETA: use a ball end mill or a mill with a corner radius. The larger the radius the smaller the cusp between cuts will be for a given step-over.

This should be of use to you:

http://www.eurospares.com/graphics/engine/RET_Bellmouth_Sept.pdf

cheers,
Michael

[seattle smitty]

Others have made any suggestions I'd have on the machining (my notion, based on my fairly limited and entirely manual machining abilities, would be make more than one form-tool for different parts of the shape). I just wanted to say that you might want to look at David Vizard's 1996 book, "How to Build Horsepower, Vol. 2, Carburetion," for the page with
drawings of various intake shapes that he tested for flow. The best shape might surprise you.

--Smitty

[Michael Moore]

I hope one of these are readable, the PNG may be the best one after you download it to zoom in.

[Grumph]

Thanks for posting that Michael. I agree with what Michael and Smitty have said. Looking at your pics there's enough meat around the carb entry to get pretty close to a good shape. Biggest problem is probably holding the carb body securely. I think I'd do it in a mill.
My recommendation is to go and see Dick Chapman at ProTurn down Sydenham. He's got the right gear to do it - but you need to specify the profile you want.
Won't be cheap either...

[Kickaha]

Does he have a CNC lathe? Last time I was there it appear his lathe was manual one.

Yes he does, would it be worth looking at 3d printing as well?

[timg]

Does he have a CNC lathe? Last time I was there it appear his lathe was manual one. I don't know what the difference is :( I give him stuff to do and he does it :) If CNC means you punch in some numbers and the machine does what you just told it, then yes. Cheers.

[husaberg]

Since my thread on Alloy wheel repairers was so successful at getting a result, I thought I would start this one with the hope of similar results. So in the ESE thread Wobbly said that Keihin PWK Airstrikers have fences on the bottom part of the bellmouth to get better response at low throttle openings but these fences badly affected the airflow at high throttle openings (the one I am really interested in on my bucket) but since I want to keep using the PWK with it's Throttle position sensor,Solenoid Power Jet and short over all length, I did as Wobbly suggested and ground away the fences but he also said even better results could be had by reprofiling the bellmouth of the PWK to that of a PWM carb, Since I do not have a PWM to study I googled some images of PWM's and they appear to have the similar bellmouth profile to an old PJ carb I had left over from when I had a NF4 Honda RS125. Studying the PJ bellmouth it appears that either Keihin or HRC had in a lathe turned away the part of the standard dirt bike PJ bellmouth that clamps to the aircleaner rubber boot to give it a trumpet like shape over the cone like shape of the standard carb. The only way I can think of to do it would be with a CNC lathe (but if someone can suggest a way to do it on a manual lathe I am all ears) So the question is there a engineering outfit (preferably in Christchurch or surrounding area) with a CNC lathe that is able and more important willing to something like this?

A quick look on the internet suggested Mark Sutton Engineering maybe the sort of small outfit that would do this type of work, Also Automatic lathes and Arcore Engineering came up but these look like bigger outfits that maybe not be interested in this type of piece work?
The best way i believe is as Greg suggested with a mill.
You could do it by hand to 90% of what you want with a pipe reamer, a plumber or pipe fitter will have one that goes to 3 inch.
They are used for deburing pipes
From memory one of tyhe dodgy old pricks on KB is a plumber in Canterbury.
Not in Canterbury but you could ask Neil Hintz aka Fletner.
He could make it into what you want.

[seattle smitty]

I had to laugh (first at myself) when I saw those findings of Vizard's. Since the Fifties, in fact longer than that, rodders and racers, and even manufacturers, have intuitively chosen some variety of trumpet-shaped funnel as the best entry shape for a carb or injector body . . . when it turns out that just a big flat disc with radiused edges has the best flow. No doubt all those funnels were of some help in containing stand-off, but the flat piece sure is simpler to make (you only have to machine one side) among other advantages.

[F5 Dave]

I had machined surrounding fitting from acetyl about little finger size that clipped and glued on giving no real extra length based on RS125 aftermarket parts. Gave minimal (measurable) improvement on the Dyno but made me feel better.