Twin carb heads

Picked up some interesting heads recently - the gentleman I got them from says these mods were factory. Whatever the story behind them, an interesting piece of workmanship nonetheless!

"Factory Mods" is just a standard term used for attempting to inflate the asking price of any oddball item of unknown provenance

looks like some interesting kit though - what carbs would fit those manifolds?

I'm hoping someone here can answer about the carbs, although I'm assuming there would be a spacer or additional manifold of some sort. The bolt holes pictured are narrower than the Tillotson width, for example.

Anyway, factory mods or no, I'm looking forward to trying them out. They look to be based on XLR heads. How's your project coming along?

I am going to venture a guess, based on the angles of the bolt holes, that those manifolds once carried a pair of Amals, probably Concentrics. It looks to me as though they would be in the horizontal plane when the heads are on the bike.

It's a bit speculative, but it would be right for the period.

My project? errmmm.. much delayed by a long stint of work offshore, mainly. I have now got another set of crankcases, because they were available, and all the stroker crank stuff is at the builders' workshop. I have someone working on the fitting-up of seat, an 80s Paughco Mustang tank I picked up along the way, footboards, foot controls, oiltank, battery tray and exhaust, now we have got the belt drive sorted out. The No 2 engine turned out to be pretty good inside on inspection, so that has some new #6 heads and maybe WLDR cams, depending on exactly how I feel, and it is going in the bike for now.

I have got so much stuff collected that the no 2 bike is going to be an all-on vintage sprinter and hillclimber, seeing as there are a couple of Indians running round in the VMCC.....

A pattern begins to emerge:
1. a subject is raised, inviting useful comment.
2. some information is absent.
3. I post it.
4. someone else did not know enough to post it, or even understand what I wrote, but knows I'm wrong.

No reply needed.

I would doubt that GP carbs were ever there, they are large units with external needles and separate float chambers. It's not impossible but I doubt it. I know from experience also that tuning twin GPs is a nightmare

It does appear that the holes are app. 2" centre to centre. What are the Amals like? I've never run one myself.

Will check on the valve length later when I get the chance, but great idea on the lift to coil bind check.

It does appear that the holes are app. 2" centre to centre. What are the Amals like? I've never run one myself.

Neither did the factory

LOL - that's a good one.

Don't worry guys, I'm as sceptical as the next bloke when it comes to these things. I've always loved "History Detectives" though, so...

One thing's for sure though, I have to master this Imperial system...was born a few years too late to have grown up with it in any form bar referring to people's height in feet and inches. All these fractions you guys use!

45Brit, I want some pics once they have progressed to your satistfaction!

Amals are good reliable carbs, easy to tune and much easier to live with than Linkerts. Unlike Toiletsons, they were made for motorcycles not chainsaws....

The Concentric was, for its day, a very advanced unit. I've seen knuckleheads with twin Amals occasionally in magazines. Never a panhead though

I've got a pic of my 45 in my gallery on this site. It's the first fit-up, when we were just trying all the bits to make sure it all went together. Gives a good impression of the overall size and proporttions of the bike, though

I've never understood how to put pics in posts this forum...

oh ok, here it is... this is where we were gettin g the transmission lined up

current stage.. different frame, seat base fitted, some old front wheel to keep the frame off the ground, the more I see that mudguard the less I like it

original fit-up.. shovelhead bars not used at present... don't like those drag pipes, more on this later

The Concentric was, for its day, a very advanced unit

The Concentric was a method of reducing the cost vs. Monobloc, with reduced airflow, fewer tunable bits and fewer sizes.

I was using Concentrics on grass bikes back then, the Monobloc never had it as against the DT carb but the Concentric was a different animal altogether

Thanks for your comments guys. Sounds like I'm going to have fun with the mock-up. Might save these heads for a future iteration of the bike.

Hi Suicideshovel, hi guys, was away for a while!!!

Theses Sporty heads are in fact derived from XLR racing heads, see deeper 3/4 inch spigot for race spark plugs. It then follows these are indeed factory made, 1970 Daytona Iron 750 XRTT, which were fitted with twin Tillotsons... just after Daytona, they dumped that set up and started experimenting with longer inlet tracks, rear carb facing forward on the left, front carb backwards on the right, see Allan Girdler H-D racer books for the whole sorry saga and photos of the 4 complete Iron carburation/exhaust set ups!!!

The std Tillotsons for these heads had their flanges cut, rubber hoses atop head nipples and carbs, plus wide clamp band. Haven't seen any period photos of the set up, so don't know if they were mounted "normal", diaphragm underneath, or mounted "à la KRTT", rotated 90 degrees, diaphragm on the side... The KRTT fairing was moded with a huge extension on the right to make room for these.

Patrick

Hi Patrick, yes I haven't seen you post for a little while. Hope you had a nice time!

May I ask, how do you mean the flanges were cut down on the Tillotsons? I have an HD5A and an HD1D here for comparison and I'm not sure what you're referring to. From what I can see the bolt pattern on the Tillotsons calls for roughly 75mm distance from centre to centre, mounting into the fibre spacer block.

The bolt centres here are roughly 50mm, in a machined spigot which slips inside and is welded to the tubular manifold section. This in turn is welded to the original port (unsure how much of this remains). If you removed the round spigots, I guess you would be left with something that looks very similar to the 2nd type of heads pictured in the lineup in Girdler's book.

FWIW the actual diameter of the restricted manifold section in the spigot section is around a mere 30mm for an inch or so, opening up then as it progresses in a curve back into the original port area of the head. If not designed for Amals as suggested, then I would think the Tillotsons would need to mount on their sides to bolt up to these, with a modified fibre plate or spacer between manifold and carb. Alternatively one could remove these spigot portions quite easily and have others machined to suit different carbs. It'll be a while before I go doing things like that to them though.

It would be interesting to try the twin Tillotsons - surely a bugger to tune? I have nil experience with Amals, and wouldn't even know where to begin.

I can confirm that they are based on XLR heads - they have provision for the longer reach plugs and various other differences. No stampings anywhere bar the usual foundry/moco crest and numerical part/casting numbers.
In the head I was just looking at one of the guides looks to be a custom, machined guide.

One thing to be said - not all performance mods are pristine ones, and these certainly look to have been evolved over time. There appears to be some sort of hardened and smoothed modelling putty or similar adding shape to some of the curves, as well as the smoothed metal braze/filling that one often sees in porting jobs.

Hi Suisideshovel,

Indeed, your outer round spigots look way too small for Tillotsons... As you say, this probably started as a factory moded set and was "improved" by later owner... Tapering up from carb to inlet valve is also the wrong way to do it... Any experiment on a flow bench will show the reverse is much better to optain laminar flow, i.e tapering down towards the area just after the inlet guide, then opening up to the valve. This is to accelerate continuously the inlet flow which then tends to be more laminar. I also don't think these heads would respond kindly to being undercarbed, you'll need at least 36mm carbs...

Flangeless Tillotsons: put in a lathe, holding the choke side in the chuck, then turn out the bolt flange till it's round and same diameter as what's on the heads. Fit big diameter but short rubber hose over both, wide clamp over both, et voilà!!!

Std Sportster inlet track, and obviously XLR derived one as well, arrives on the valve seat at the wrong angle, too shallow... They cast them that way cause of restrictions in height for the std XL engine to fit in std frame, heads had to be as thin as possible, so they "squashed" the inlet port. See more on that here, discussion between Herbert Wagner, others and me about K, KH, OHV KL and XL development:

http://knucklenutz.com/tech/viewtopic.php?t=1422

One of the race tricks to rectify the port was to build up the bottom of the track so that gases are forced to hit the valve at 90 degrees... can be done with putty or brasing...

Patrick