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D1 carburettor replacement with modern Dellorto PHBG 18AS
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Anderzander
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April 9, 2012 - 12:10 pm
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Phil, what was your old carb set up ?  261 or 523 ?

Stephen

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April 9, 2012 - 12:26 pm
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The Amal in use before was a 361 with settings as per a D1, 106 needle and 75 main jet etc, when I experimented with this carb to put it to D3 settings it always was too rich and the best combination was always as per D1 settings, bit odd as I have a 150 barrel, piston and head. Although I am never 100% convinced that the float needle and sealing was in the best of health on this carb. As we know its not easy to get parts for the 361 and despite waiting patiently no sign of any parts being re-made by Amal, one reason why I am seeing if I can get a result with this Dellorto.

Phil

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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Anderzander
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April 9, 2012 - 12:33 pm
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Thanks Phil.

I believe the 523 has a bigger bore (venturi)  - so whilst it has bigger jets it flows more air. The mixture balance would therefore be thrown out I expect by putting bigger jets in the 361.

Sounds like the D1 and D3 ran on a similar mixture but the D3 could take a bigger charge. That's good for me as it means I can spread the cost and put the D3 top end on with the D1 carb like you did until I can source an alternative.

Stephen

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April 9, 2012 - 12:39 pm
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Yes you are right, the D3 carb was a 11/16th as I remember and the D1 a 5/8th so like you say a bigger choke size, saying all that the bike runs ok with the D1 carb and settings as does a genuine 50 mph plus on the flat so probably not losing too much power with it.

Phil 

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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David Dale
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April 9, 2012 - 1:11 pm
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Getting down to basics, what do the jet sizes actually mean? is it actual flow rate in milli something? In that case is it corect to expect 3 different carburettor manufacturers to use exactly the same criteria? just an idle thought, david.

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April 9, 2012 - 4:17 pm
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Yes you are right about the flow rate, what the values are I don't know ? cc's or ml's per minute or something like that I should imagine, and also I don't know but would think most or all manufacturers would use the same system ? If I am at a loose end this evening I will do a bit of investigation. The new Dellorto carb I got recently is an 18 mm choke size with an 82 main jet which sounds roughly similar for that size of carb to the D1 which has a 75 main jet and the D3 carb which has a 90.

I'm sure I can find some info somewhere about what the jet size number refers to.

Phil

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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April 9, 2012 - 8:01 pm
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Been investigating how main jet sizes are graded, bit of a minefield this one, as yet I have nor found anything definitive but came across some interesting posts on several other forums.

From what I can make out Amal's are graded by flow in CC's per minute and it may have something to do with the "Renolds Number" which is a calculation regarding flow through an orifice, seem to recall this calculation from my college days. 

Mikuni's and Dellorto's seem to be based on the cross sectional area of the jet, so comparing jets sizes between different makers is maybe not a good way or compare jet sizes. Then just to complicate things even more the chamfered edge of the jet affects the flow again and some makers use a hexagonal jet which changes everything again size wise.

Another factor to bear in mind is that any main jet size is only relevant to that particular design of carburettor and the flow rate will not be the same in a different design of carburettor even if the maker is the same. This explains why if you change from say an Amal Monobloc to an Amal Concentric using the same main jet size in either carb, most likely will not produce the same effect ,, does that make sense ? I know some of us on here have changed over carbs and had some problems setting them up maybe this is part of the reason.

Found some interesting info too on emulsion tubes, difference between four-stroke and two-stroke tubes, that info is on the Ducati site I posted the link up before, apparently even with a two-stroke emulsion tube different heights may be available giving different characteristics through the operating range. Anyhow all interesting stuff, well keeping me occupied on a wet evening.

Phil 

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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CharlieCeng
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April 9, 2012 - 8:58 pm
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Guys,

Orifi, orifices, small holes or jets are described by hydraulics engineers using a 'coefficient of discharge', aka Cd or sometimes Cv, particularly to americans.

Lots of things affect this, shape, roughness in addition to the diameter and the length of the orifice, but essentially you are describing a flow rate at a given pressure drop. Obviously the fluid properties are key to the measurement as well.

To understand the relationship between Amal jet sizes and other manufacturer's jets, you'd need to understand how they measured them - what pressure drop for what fluid, and what units they use to describe the flow rate.

Hope that gives you an idea of what to look for.

cheers for now,

Simon.

 

'42 Ariel W/NG, '58 Ariel Huntmaster, Masquerading D3, '51 Ariel Square Four Mk1 http://ariel-sq.....ogspot.com

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April 10, 2012 - 1:24 pm
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Flip up choke lever now fitted, seems a bit easier to use, also raised the needle another notch, time for another test !

http://i1125.photobucket.com/albums/l585/bsathunderbolt/Dellotoflipupchokelever.jpg

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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April 10, 2012 - 1:43 pm
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I think there maybe quite a few of us D1 owners avidly following this topic, wanting to know what the final verdict is!  Good luck and keep the updates coming.

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April 10, 2012 - 3:58 pm
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Thanks for the encouragement, hopefully I can get to the best combination of settings to get good overall performance with this carb and we can keep the info on the forum. Then if someone else wants to do this they can order a carb from Dellorto with the jets, needle etc in that spec, as they can send you a carb made up to you specification and it would save the time and expense of experimenting with various combinations like I am doing. 

Anyhow tried another test today and its really starting to fly (well for a D1) pulling up hills a lot better in top gear than before and generally holding the revs a lot better than with the Amal. Had a bit of a red herring with the full throttle tests as I found some of my old friend the rusty sediment getting through the fuel tap filter and the inline filter so I have renewed the inline filter. The test after went very well, starting to get there I reckon. Next stage is to order some bigger main jets and different needles as I still reckon its a bit weak at half to full throttle.

Phil

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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April 10, 2012 - 4:04 pm
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And going to have a go with this, never used one before but might be worth a try.

http://i1125.photobucket.com/albums/l585/bsathunderbolt/GunsonsColourtune.jpg

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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April 12, 2012 - 10:00 pm
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The Colourtune seemed to indicate it was running weak especially at the larger throttle openings, ordered two more needles with steeper and longer angles plus three more main jets, an 82 is fitted at present now have an 85,90 and 95 so will try a few more tests tomorrow and see how it goes.

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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April 13, 2012 - 3:44 pm
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Needle and main jet change today then another test, blimey flying now must be getting very close ! The bike managed to get up to 35 mph in second gear before this with the original Amal I never saw more than 30 mph, anyone else got a 150cc D1 or a D3 to compare ? getting up to around 50 mph in top which is about right, seem to remember the period road tests of the 150's getting a top speed of about 52 mph. Had to throttle back down a hill on a dual-carriageway to keep it at 50 mph or so, but flew up the hill on the slip road when I turned off and holding the speed much better in top gear, also going down to about 25 mph in top gear so can't be too bad at the end either

This is the latest plug chop, looks quite good.

Phil

http://i1125.photobucket.com/albums/l585/bsathunderbolt/12.jpg

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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Anderzander
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April 13, 2012 - 4:27 pm
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Excellent ! Chocolate Brown thumbs-up

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April 13, 2012 - 5:08 pm
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Mmm Choctastic, hopefully just some fine tuning required now as it seems fairly close !

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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kevin
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April 13, 2012 - 5:54 pm
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Cornish Rooster said
Mmm Choctastic, hopefully just some fine tuning required now as it seems fairly close !

  hi could you send details which model carb you bought& all the extra bits also where you bought it from good job  regards kevinclap

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Anderzander
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April 14, 2012 - 9:11 am
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Phil, would you consider doing a seperate write up on your colour tune?

I'm heard of them but am not really sure how they work, or how you fit it with the petrol tank ?

Stephen

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CharlieCeng
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April 14, 2012 - 9:45 pm
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I read somewhere that colortunes didn't work so well with modern fuels. I haven't used mine for ages - I guess it worked ok for you Phil?

cheers for now,

Simon.

 

'42 Ariel W/NG, '58 Ariel Huntmaster, Masquerading D3, '51 Ariel Square Four Mk1 http://ariel-sq.....ogspot.com

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April 15, 2012 - 9:17 am
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Sort of successful but to be honest I think it's better to use the normal tried and tested methods. On the early test runs I could tell the engine was running very weak on larger throttle openings by the white color on the plug and it was not pulling full revs, then when I changed the needle settings and main jet just riding it showed it was getting better . I did recheck yesterday with the colourtune and now the colour does look more like it but I'm not relying on what it shows make further changes.
As you have a 150 D1 what's the maximum speed you can get in 2nd gear ? On the old Amal mine would never do more than 30 mph now with this Delloto I can get it up to 35 mph !
Phil

BSA Bantam D1 "150" in use regularly often as general purpose transport, quite a few other bikes as well. Cornwall Area Rep. 

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