Bike carb conversion; folding the manifold tubes?

Evenin' chaps. Just browsing your threads tells me I've found a forum where I can talk to the organ-grinders instead of monkeys. Here's one for you.

Bought an old 3rd Gen Honda Prelude (the flat square retro one) for not much at all and on the understanding that the owner and anyone who'd looked at it couldn't resolve the persistent low speed/idle misfire. Turns out that the 20yr old twin Keihin carbs were totally ronnied and Mr. Honda wants £1265o for the service kits! (no I haven't misplaced the chuffing decimal point either; that's actually what they want to invoice for a fistful of brass 'n butyl!)

There's none left in the scrappies now and even if I could find a pair they'd likely as not be in a similar condition = back to square one in a year or two,... so... in a fit of lager I decided to have a crack at fuelling it via these puppies....



WOOF! a set of mint quad carbs from a Yammy YZF R6, will flow enough gas for 129bhp in the bike so with judicious jetting corrections should be more than adequate to restore the poxy 109hp that the Honda was originally shipped with. It's worth it for the go kart handling believe me.

So the only issue is fabricating the fanimold and induction pipes to mate the carbs (38mm int. bore on equal 72mm centres) to the rectangular offset ports of the Prelude's cylinder head. I had a new aluminium flange jet cut from 12mm plate and tried to go with mitred sections of 40mm diam. ally tube for the runners. The idea was to machine a stub at the end of each one to make the final transition from circular tube to rectangular port. I went for ally thinking it would be easier to deal with having nowt but a Black and Decker Workmate to work on. (I'm not joking here) Well I handed the finished sections to a 'welder' who pretty much wrote the lot off. B*gg**! Odd as well considering his stainless work was immaculate??? I mean even I know that ally is a completely different metallurgical kettle of fish when it comes to welding it. Anyway the whole shebang was as ugly as sin and a gas flow nightmare no matter how much die grinding I might have applied. Pah!

So on to 'Plan B' then, trouble is I can't find anyone who can swage and bend some solid tubing into shape so I thought "can it be folded or rolled up from stainless sheet and simply seam welded? Gotta be the way to go surely, I mean how hard can it be? we've been bending metal since year dot there's got to be someone in this city who can take this simple job on. Er... it seems that there isn't. Well not for the money I've got spare anyway. Looks like I'll have to do it m'self then. I've now arranged access to a workshop/tool room but first of all here's a few CAD screenshots of what I'm trying to achieve.



the moody upskirt shot




The Yamaha carbs are equally spaced but the Honda's ports for the inner cylinders (2 & 3) are offset towards each other requiring the inner pair to slant inwards whilst the outer pair (1 & 4) of runners need to splay outwards slightly as this shot shows.



This is an added dimensional complication but CAD says it's do-able or I just might make them all identical and 'fettle' them into their allotted place and angle of splay. Overall runner length isn't too critical so long as they're all within a millimetre or two of each other.

A few more...

The original ally flange... I want a jet cutter for Crimbo! Note also that the OEM gasket is 'siamesed' as is the original cast manifold. This conversion will restore individual induction tracts which may require some jetting trickery to compensate on the inner pair, s'going to be difficult to determine without four individual Air/Fuel ratio probes welded into the exhaust manifold.??



And a few 'hidden line' renderings...



and...




You get the general idea. So that's where I am at the moment, so I'm sitting here scratching my swede wondering whether it's best to start making templates 'n such. Is there any benefit in milling up a pair of mandrels (it's symmetrical about its centre line) from some engineering plastic I've got hanging about, does it need cone rolling or something? I read Chunkolini's thread and his 'ghetto' folding bench technique and thought aye aye.., that might do it, dunno... where to start or do I just wing it? Any thoughts, ideas, gratuitous ridicule? all is welcome at this point in the bl00dy epic, Christ I knew it wouldn't be an instant fix but sheeesh!

(Carter pulls out last handfuls of prematurely greying hair)

Gentlemen, I am as Prince Charles; .... all ears.

We have progress; found a neat little application called "Litio2" that works as an add on to AutoCAD and can be called up from its command line. Once it's running you can select a mesh object and it will unfold it into a flat sheet = instant cutting template. 'kin brilliant.
http://www.softpedia.com/get/Science-CAD/LITIO-3D-Sheet-metal-unfolder.shtml



Just the job and should save the ******ation of nibbling a template from a Carlsberg can which I'd already slit and squared up in preparation!

The program produces the 'contoured' image above and as a bonus gives you a closed polyline that can be fed straight into the jet cutter which is the intention.

I'll be having the stock delivered tomorrow morning (Metal Supermarket)...
for the new flange (1pc - 350mm x 100mm x 12mm)
for the runners (5pcs - 150m x 150mm x 18g) four for fabrication and one to balls up while I get the hang of it Nearly dropped me bacon sarnie when he came back with the prices though, £26 for the slab and £15 for the sheet? Reckon I'll be buying a stack of old catering fittings unless I get some serious discount for quantity!

Why spend a fortune on something that might not work, to test it i'd have just welded round tube straight onto the flange and put a radius on the square edges. A firm that makes exhausts should be able to produce bends for you, but it might be easier to split the carbs up and run straight tube.

outofthefire said:

Why spend a fortune on something that might not work,

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Do you mean the folding up part of this...? You may be right. One local company makes a big shout of their 7axis CNC bender but can't turn out something as 'simple' as this...? What's all that about then?


at 2009-10-04

The above is not a million miles away from what's needed so: In the words of Clarkson; "I mean how hard can it be?" even the ruddy Vikings could have beat that into shape over a log fire!!

outofthefire said:

...to test it I'd have just welded round tube straight onto the flange and put a radius on the square edges.

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Had that idea when I was going the ally route but when casting around was told that I'd have to have some mandrels made up for the swaging machine. £1000 plus! Yikes I see what you mean about riffling out a circular manifold hole to a rectangular flare. It's a variation on the individually machined stubs/transition pieces I was dreading making.

outofthefire said:

A firm that makes exhausts should be able to produce bends for you, but it might be easier to split the carbs up and run straight tube.

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It's trying to find one here that's the issue, I'm lacking running transport at the moment so I have to source everything within walking distance if poss. Plus I really don't have a lot of spare buntz to punt at anything really. This is most definitely a low/no budget operation I'm afraid.

I was tempted in the early stages to knock one up out of mild steel just to test the concept but was reassured early on that the concept was sound, well tested and do-able which prompted me to put the effort into producing a decent finished item ie to use design to get it right first time. My thinking was that it is going to take as much effort (although maybe less expense) to produce a piece of cr@p as it will to make the pukka item.

As for splitting the carbs? Well apparantly men have tried, and men have died... All advice was to avoid that if at all possible. Once you do that it's can open, worms all over the shop. I'd then be into precision turning and slotting of extended butterfly spindles or shaft linkages, it's got the 4 choke valves commoned onto a complicated slide bar mechanism, there're fuel passages and delivery tees that would need to be redesigned, some air passages that common up the CV slide chamber pressures. There's shedloads in fact not to mention the full carb strip and the potential for introducing a lot of backlash and float which would throw the synchronisation out. Maybe not immediately but as all those individual linkages start to wear.

Perhaps it would have been easier to source a pair from a larger capacity machine like a VMax or something and then mate them to the two flanges left vacant by removal of the OEM Keihins. At least the manifold could have been retained but I was looking to do something a little different, give a mild performance hike, quicken up the throttle and mop up surplus bragging points if there were any going about! Other factors? I Can't resist a challenge and I've already had emails from 3 'Lude owners asking if I can knock something together for them as their twin Keihin rigs are approaching a similar state of decrepitude but the cars are so worth saving.
If I can nail this I'll be doing my bit to keep these little whizz bangs on the road and might even be able to turn a few quid.... yeah right!

Carter, there's a good article in the August edition of Practical Performance Cars about transplanting bike engines into cars. Obviously can't post it on here 'cos of copyright etc, but PM me if you can't find a copy in the shops

Have you spoken to Bogg bros? They do this sort of thing all the time.

drjim said:

Have you spoken to Bogg bros? They do this sort of thing all the time.

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Yep but they have a long lead time on it, they're getting pricier and ultimately I just don't have the dosh in the required amounts. I did have a very helpful email from Steve Bogg which assured me I'm on the right track with it so props to him for that one.

I also like the idea of doing it m'self after all... how hard can it... erm... very hard as it goes.

Why not hydroform them? You could stick a square section tube over one end of the flat welded form, pump it up and have a circular to square transition formed automatically. Practice on sheet steel, then knock 'em up out of stainless or aluminium. Make 'em longer than required and trim to suit. Cheap but a little practice on cheap material required.

Hmmm. hydroforming? Interesting. I've checked out a few videos of folks doing it with a domestic grade jet washer but it still requires access to a welder, vice bench (and a pressure washer!) Interesting technique but being honest Angus, not one I'd like to try without someone who's done it before.

As a layman my first instinct is to fold/panel beat it round a former, or use a folding brake of some sort to nip the four radiussed corners and hope that the circular form can be persuaded to form at the other end.

All the brakes I've seen thus far act on the whole width of the workpiece, I don't want that I just need to fold one rectangular end and hope that something fairly circular emerges at the carburettor end. My question is; does such a machine/press brake exist?

I think the picture above with the folded square section runners is a start. The dimensions of the ends would need to be right, at one end to suit the square cylinder head ports, and the other would have to have a perimeter to suit the intended diameter (38mm??).

Could you not form them into the folded square section tubes, and then weld them to the manifold flange first of all (to fix that end shape) and then drive a conical former up each runner to beat the free end into the required shape? The ideal former would be steel, but I suppose you could even use a wooden one if necessary? The reason for welding them into the flange first is so that you have something to grip them with.

just a thought, interesting project though!

http://cgi.ebay.co.uk/Custom-Build-...49966221?pt=UK_CarsParts_Vehicles_CarParts_SM

Could you not use one of these? not had experience of these tools myself, but been think about getting one for swaging an exhaust.

Blimey, nice one krall! Y' see, I had no idea such a thing existed but that looks like the very feller. I'm imagining that it works on the same principle as the humble Rawlbolt? Need to google that right now, any idea who the manufacturers/stockists are?

Right, it's made by Sealey and the bumf says...
http://www.justoffbase.co.uk/Exhaust-Pipe-Expander-Set-3pc-Sealey-VS1668

Exhaust Pipe Expander Set 3pc Sealey VS1668
Ideal tools for fitting aftermarket components to standard exhausts. Used to expand a length of pipe in equal proportions so the pipe is the same diameter and not tapered. Ensures a perfect fit with no leaks and also removes dents and clamp marks. Supplied in carry-case.

• Ideal tools for fitting aftermarket components to standard exhausts. • Used to expand a length of pipe in equal proportions so the pipe is the same diameter and not tapered. • Ensures a perfect fit with no leaks and also removes dents and clamp marks. • Supplied in carry-case.

• Ideal tools for fitting aftermarket components to standard exhausts. • Used to expand a length of pipe in equal proportions so the pipe is the same diameter and not tapered. • Sizes: Small; 30-44mm, Medium; 38-62mm, Large; 54-87mm. • Ensures a perfect fit with no leaks and also removes dents and clamp marks. • Supplied in carry-case. • Model No. VS1668

Sealey's warranty on this product is for 1 Year

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So the small and medium one will do it. OK so I'm thinking the following sequence; fold 'em up on a finger brake. seam weld, then drop one of these expanders down the carburettor end to create the circular end form. Does that sound the correct order of things?

Krall, I like it. + much cheepniss so the missus should too! She loves that car (and 'Ludes in general) as much as I do bless 'er.

Carter said:

Blimey, nice one krall! Y' see, I had no idea such a thing existed but that looks like the very feller. I'm imagining that it works on the same principle as the humble Rawlbolt? Need to google that right now, any idea who the manufacturers/stockists are?

Click to expand...

bergen make them but thats a good price on ebay i dont think u will get any cheaper err heres one a few quid cheaper item no 250679818302

Yeah Sealey want the thick end of £65 for it.

Gareth_16V said:

I think the picture above with the folded square section runners is a start. The dimensions of the ends would need to be right, at one end to suit the square cylinder head ports, and the other would have to have a perimeter to suit the intended diameter (38mm??).

Click to expand...

Yep, with you, the LITIO 2 'unfolder' takes an AutoCAD mesh and opens it out exactly to size, 'sall taken care of so the unfolded jet cut blanks I get back will have one end exactly the length of the radiused rectangular port and the length of the other end will be/is equal to...
pi x D = 3.142 x 39 = 122.5mm, Apparantly it has used the median line of the sheet material for its calculation so the final dimension of the unfolded template including the bending allowance is 123.3mm.
A millimetre here or there really isn't much of an issue as I'll be using a silicone joiner which will easily compensate. If I can use that tool to create a 10-15mm parallel 'land' for the rubber joiner then the job's just about there.
I'm assuming also that a decent weld will cope with the expansion process? ..?..

Gareth_16V said:

Could you not form them into the folded square section tubes, and then weld them to the manifold flange first of all (to fix that end shape) and then drive a conical former up each runner to beat the free end into the required shape? The ideal former would be steel, but I suppose you could even use a wooden one if necessary? The reason for welding them into the flange first is so that you have something to grip them with.

just a thought, interesting project though!

Click to expand...

I think krall's cracked it with a variation on that theme but yeah I can see how that would work.

Glad to be of help, just happy that I know something

Sit. rep.

OK then, got the new stainless manifold flange and the paired blanks for the intake tubes. The surface finish on the flange had taken a bit of a hit (over-spray?) probably due to the additional time it has to spend under the jet compared to the aluminium one which came back unblemished.



They also came back unmarked as to which was which and when overlaid on top of each other (although it was possible to determine very slight variations in curvatures it was real 'fingertip' stuff, so alike were they) it was impossible to tell by eye/feel. I had to spend a good hour with the CAD drawings, straight edge and digital calipers trying to sort out the pairings.

However whilst the drawings were opened I took the opportunity to print out some 1:1 copies on 120gsm paper and spend a few hours with scissors and frigging sellotape folding up four paper runner pipes to verify that the reality matched the theory...

Yeah, yeah, yeah, I know, this should have been the first thing I did but such is my faith in the power of CAD. Shame I can't match its reliability though as we'll see!



and...



and offering up the carb set...





Cocked up the plane angle at the carburettor end which as it stands will pitch their float bowls backwards from the horizontal. Wrong! If I'd remembered to crop it correctly in 'model space' it would have been perfect, nicely pitching the carbs about 5degress forward. Hey ho, s'nothing that can't be rectified on the saw/linisher and I need to weld a 15mm wide land onto the ends anyway to recieve the carb clamps.



Doh!

So now it's back to the problem of forming the tubes?? Several options but the few I've spoken to are saying that the small size of the workpieces will rule out most commonly available workshop plant and processes. Either way I'm looking at having tooling manufactured or employ some one who remembers something called an English Roller... Eek! Carter offers up his polite thanks and backs nervously towards the door clutching emaciated wallet.

So back in front of AutoCAD and a mug o' tea I knocked up the following sketches to fix the idea of what I think would do the job.

The Task to recap...

To fold a pre-cut developed piece of 18g stainless back into its orginating tube shape having a rectangular form at the engine end (rectangular intake ports) and a circular section at the carb end.





Should be obvious to you guys where I'm going with this I hope. The workpiece rests on the two fixed bars and the sliding central arbor carrying a length of 8mm rod is brought down between them forming the bend from one end only, i.e. there will be no need for the pressing action to be applied down the whole length of the job as with a conventional 'full width' press brake.



Then it occurred to me that if the fixed formers were to be splayed outwards it would then create the diminishing fold I'm after. ?

There's no reason the fixed bars can't be substituted for two lengths of 8mm angle iron between which that central finger can pass. This would in fact make it a lot easier to set up the correct splay if one end is fixed and the other able to be clamped at the optimum angle. I can see me having to run a good few test pieces through this to get it cock on but if I end up having to do ten or more of these (as is my intention) then it's effort well expended.

What say you fellahs/ladies? Does this sound like a workable device? Does such a thing exist already? (and so far no-one I've spoken to can recall seeing such a thing.) When I first heard the term "finger" brake years ago I actually pictured something like this in my mind's eye.

In its simplest 'ghetto' incarnation I don't see why the pressing force couldn't be derived by simply arranging the device to sit in a bench vice and then winding it in jaws. On the face of it this seems a very controlled (and controllable) way of doing it.

Still thinking around the folding issue; can't use a conventional bench folder, I need something that will allow me to fold a tube in situ as it were and from one end only. The 'trick' to this device (apart form its monumental cheapness) is that the largest dimension of the central radiussing knife is less than that of the tube I intend to fold so when it's done it will simply pass over the handle.

So here's a lateral development of the above idea I've come up with that takes advantage of the theory that splaying the fixed formers whilst the central 'knife/finger applies its force will produce a diminishing fold... again tell me if I'm wrong here.





All it has to do is 'crimp' the corners of a piece of 18g stainless and I figure it can be effectively assembled from some 8mm angle, "Unistrut" accessories, 10mm 'zebedees' and for a bit of bling I can nick a handlebar grip from a small child's tricycle or something.



Does this look like an effective means to the end? Again it'll require playing around with some test pieces to get the feel of it but as a muck cheap way dong it I think there's mileage in it. It also seems to be a very controllable method.



I'm hoping I can score some 7mm x 30mm stock for the central guillotine/finger to allow for some spring back? or should I go for 8mm?



What do you reckon then guys? In your experience, would this have a cat in hell's chance of doing the job? If absolutely necessary I suppose a circular former could be clamped in place to ensure the carburettor end can't do anything other than form a circle.
For clarity I've not shown any materials clamping, mainly because I haven't crystallised it yet. All and any contributions are more than welcome.

-CARTER-

Interesting problem, I do like your last drawing and I would give that and the pipe stretcher idea a go in combination. Lots of times I find that I start with methods that should work in theory - start to bash or push metal and the most practical way becomes apparent as you work. That and I am crap at CAD!!!! I look forward to seeing your solution. Cheers

paul33 said:

Lots of times I find that I start with methods that should work in theory - start to bash or push metal and the most practical way becomes apparent as you work. That and I am crap at CAD!!!! I look forward to seeing your solution. Cheers

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Thanks Paul, and I know what you mean. You think you've thought it through to the 'nth degree and last detal, you roll y' sleeves up set to it with the tools only for events to take a completely different route. Unfortunately I don't have any workshop,shed/garage space to let me tinker so AutoCAD has had to become my virtual workshop where I test the theory as far as I can before the metal munching starts and for that it's been a godsend. In fact in many ways it's preferable...eg

Took me about 3 hrs. to figure this mk2 version in cyberspace and frankly I doubt I could have done it any quicker in 'real' space even if I was given the run of a fuly stocked machine shop.
This way I can pull welds apart, reposition, shift datums and heights, fill in misplaced holes and 're-drill', I can bend a bar one way and if it doesn't work just erase it or hit the "MIRROR 3D" command for an instant fix to bend it the other way.

Works for me big time. Can't deny that there's a bit of a learning curve involved but once you get the basics you can pretty much teach your self by mousing around the tutorials. Well worth the expenditure of a few weeks.

Thinking on, I can see me being able to source all the parts for this from any bulding site skip including a few Unistrut brackets. Right that settles it, scavenging mission tomorrow then.

Smoke me a kipper; I'll be back fer breakfast!