Author Topic: Amac 1919-20 (ish) Main Jet Holder - to stop leaking . . . contact me quickly!  (Read 1217 times)

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Offline pvn06

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Hi Folks, hope you are all well,

It has been a while since I have logged in and given any updates, but I wanted to quickly let other veteran Douglas owners know - I am just in the process of making a small batch of Amac main jet holders - for Douglas HYD Amac (and similar Amac types), as shown in the photographs . . . and due to the massive recent cost increase in the cost of brass, and setup costs. .  . I wanted to check beforehand, if anyone is actually interested - if so, I may consider making a slightly larger batch than I am currently envisaging.

If you do want to reserve one of these - it is very important you email in the next couple of days - paul.norman@racingvincent.co.uk, and let me know you are interested - otherwise I will continue with the small batch I am planning, and you will be fighting with other customers on my online catalog for the few I do actually make!

Background:
I am not an expert on the early Amac carbs .  .. as fitted to Douglas 2 3/4hp models, as well as 4hp - but know there were numerous variations over the period 1912-25, as well as other types fitted.  However, I believe the type shown in the photos were known normally as the HYD type, and I know the jet holder and carb body shown in the picture were listed in the Douglas catalog as definitely being fitted to 1919-21 models as 1644D part number (please correct me if I am wrong - anyone with more knowledge).

However, the point being, this type of main jet holder also secured the Amac float chamber . . . but unlike later Amal carbs with seperate floatchambers, these early carbs seemed to rely on an almost knife-edge chamfer between the bottom of the floatchamber and the main jet holder. 
As you can see in the photographs - I show two original jet holders - one in very good original condition (although part of  the spare carb I currently have reserved for another build) and the one that is fitted to my current build . . . and that one is not in the best condition - and inevitably is p*ss**g fuel!!!
Having looked at it closely - I cannot believe this is not a common issue .  .. and actually the floatchambers I have seen have a machined bottom face that lend themselves to being secured by a flat, lipped face main jet holder, and red fibre washer.

Therefore - for my own project batch I am going to make an 'improved' jet holder, with a large (0.875") bottom lip  and red fibre washer, and mill the hex at the bottom of the jet holder, to the same dimensions as the original Hex.

You can see in the second photo - I have now written the CNC programs for the first and more complex operations . . . and am just doing setup . . . the jet holder showing has many small changes to make, and then the hex to be milled, before satin nickel plating . . . but hopeufully you can see what I mean.

I have paid special attention to ensure the height of the main jet will go into the exact position the original jet holder puts it in, providing the correct (high quality) red fibre washer is used . . . but I will be supplying these with the holder, ready to fit.

The price of brass has gone through the roof in the last 6 months (a 4 fold increase on a year ago seeming quite normal!) . . . so I definitely dont want to make more of these than there is demand for - so let me know quickly if you might want one . . . I have not calculated setup time and material etc yet . . . but on a small batch I am expecting circa @30 each plated with washer (current CNC prog writing time and CNC setup time has already eaten any profit I will make on first batch!!).

Final pictures are my own Douglas 1919-20 (loosely termed a TT Rep!) . . . not getting much time recently - but now fettling ready for the first trip on the road!

Best wishes
Paul
www.RacingNorton.co.uk
« Last Edit: 04 May 2022 at 11:46 by pvn06 »

Offline Hutch

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Hi Paul,

Information on these early carburettors does appear to be scarce. The cross section of a 1919-20 Amac HYS carburettor (single control rather than dual) may show some useful information ? It came from "Hints and Tips for AMAC users" AMAC carburettors 1914-1920 from the excellent Barnstormers website in NZ;

https://www.barnstormers.co.nz/barnstormers/wp-content/uploads/2016/09/AMAC.pdf

The originals I have rely on the chamfer being a good fuel tight fit to prevent leaks.

cheers

Hutch

Offline cardan

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I have paid special attention to ensure the height of the main jet will go into the exact position the original jet holder puts it in...

Hi Paul,

The main jet in this AMAC is completely submerged in fuel, so it's height is unimportant: it just meters the fuel. The carb has a "bar" of sprayers that do the vapourising, so the fuel height is set (by the float level) to be just below these. I've had no problem with AMACs of this type leaking, but if they do leak at the taper of the jet holder they can be lapped with Brasso with no effect on the function of the carb.

Cheers

Leon

Offline pvn06

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Many thanks Leon and Hutch - both very useful and I appreciate your expertise both, as always.
 
I agree in theory with what you say Leon - about the lapping in, etc, but I know the holder that came with my original bike had a hex and chamfer in very poor condition, and would not have cleaned up properly (i.e. the chamfer seat on the hex jet holder, not the floatchamber) . . . and I cannot believe that given the design, there are not many carbs out there with similar 'chewed' hex's and buggered chamfers.  Past experience of working on many Amal racing carbs tells me - the favourite place to use a crappy adjustable seems to be on the main jet holder of a carb!

So anyway, finished all the details of setup today and am now chomping through a small batch.  I can see quite a few 'reads' but no emails . . . so no probs, will stick to my original small batch size (the 0.875" brass round bar I am using was originally reserved for Norton CS1 1920's one way valves . . . so dont want to use any more than necessary!).

Will upload a picture when final op complete, and the jet holders have lip and milled hex complete.

chrs
Paul
www.RacingVincent.co.uk

(ps - made a new batch of 2 3/4hp petrol tanks screws last week, and am doing a new batch of gearchange gate screws next week, both in stainless)

Offline pvn06

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Hi Folks (and Leon and Hutch of course),

Well here is the finished main jet nut . . . and I am quite pleased the way they have turned out.
As you can see - rather than relying on the tiny chamfer on the original nut (the nickel plated one on the photo is an original Amac item in good condition), they now have an 0.875" wide lip, with the hex milled underneath that, and rely on a red fibre washer and flat face of the floatchamber for sealing.
The small main jet fits into the threaded/recessed top hole as per original.

The one I have fitted to the carb in the photo feels like it gives a really good firm seal, but proof will be in the pudding - and will try my own one on the Amac on my Douglas 2 3/4hp in the next week or so.

As an interesting (anal?!) aside - when milling the first hex (having very carefully replicated the original Amac nut dimensions) - I was confused when I came to do the normal sanity check - and try fitting a ring spanner . . . it would not fit on.
It took me a few minutes to realise what the reason was - it seem's that Amac when they made the original nut had used an 'AF' (American) spanner size!!
I cannot believe that Amac were as bad as Douglas at that time - i.e. of making everything different to normal sizes!, and would have expected them to have used a BSW spanner size . . . but then I realised, if they had used the corresponding 5/16W (Whitworth) spanner size, it would have meant the 'across flats' diameter being 25 thou less .  . . and then the crappy small chamfer seal would not have worked at all!!
No wonder my original was chewed . . . I wonder how many people at the time had AF spanners as well as Whit? . . .so guess using an adjustable might not have been unusual.  Unless Hutch or Leon know more . . . would assume Amac did not make a habit of using AF sizes . . . so as I was no longer bound by the same constraint - I have amended the CNC program for the Hex, to reduce the across flats diameter by 25 thou - so it now a nice crisp fit for a 5/16" BSW spanner!

For anyone interested - as you can see, the small batch is now complete . . . but it may be a few weeks until they go for (satin) nickel plating - but will add them to our Douglas section in the near future

Best wishes
Paul
www.RacingNorton.co.uk

Offline Hutch

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Unless Hutch or Leon know more . . .

Leon might but I don't ! :-)

I can hazard a guess that it might be something to do with brass threads and pipe fittings but could be off track? I have come across  old fittings that use "AF" spanner sizes rather than BSW sized ones and maybe AMAC erred toward that as the jet holder is also part of the banjo style fitting to hold the float bowl on? Or maybe some old, obsolete and obscure C.E.I, S.M.M.T or S.A.E standard?

I dug around in the shed and found a circa 1915 AMAC float bowl pipe fitting which adapts from 0.558" x 26 tpi (9/16" British Brass Thread?) on the float bowl to a 0.515" x 19 tpi fitting for the fuel supply pipe and the spanner size is 5/8" across the flats - not a BSW size.

I found that a Douglas spanner marked "B&B" matched that fitting so this practice appears not to have been limited to AMAC? A quick look through the Douglas parts lists show a few special spanners being supplied in the bike toolkit for different carburettors - including Douglas's own device. I used to have a Douglas spanner with an "AMAC" size on it but swapped that a few years back so couldn't see if that was the correct size for the jet holder for the carburettor in question :-(  (a forum member might be able to measure what size it is?)

Your idea has some merit tho' in reducing the number of spanners one has to carry on the bike!

cheers

Hutch

Offline cardan

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Plenty of "AF" hexagons on AMAC carbs and their associated levers, and even the square heads used in different places (there was a square-head variant of the jet holder under discussion, and sometimes square heads on the screws for the attachment clamps) require AF spanners.

Even Douglas was not immune to the occasional AF hexagon - did we recently discuss 3/16W vs 7/16AF hexagons on the forum, maybe in relation to tappet lock nuts on DTs and the like?

[Edit: See e.g. https://www.douglasmotorcycles.net/index.php?topic=9034.msg35538#msg35538 ]

Cheers

Leon
« Last Edit: 06 May 2022 at 03:35 by cardan »

Offline Scomoto

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HI, apologies for not being carb related but I was looking at your TT rep and wondered where the levers (full bar set) came from?  Were they with it when purchased or are they made by you

Thanks

 

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