404-5 carb jetting differences

[GlennBurnage]

According to the 404 - 405 manual the centre carb is set up differently from the front and rear on 404s and early 405s. The differences are in the main jet and the air bleed. There is also a difference in the carb designation numbers -
1705 (centre) and 1704 (f & r) I wonder if this is for the actuating mechanism attachments? But this is not the case on the later 405s which have the same designation for all 3 carbs -1763, and the same set-up, jet etc wise.
The only other Bristol engine details that I have are in the Bristol 100C, D & D2 series manual downloaded from the ACOC, this states all carbs to be the same designation and all jets etc the same.
Can anyone shed any light on why there are these differences and why things changed during 405 production?

[Julian Caples]

Glenn

The different jetting for the centre carb is theoretically correct but there are also differences in the pilot air jets which change how the pilot circuit interacts with the main jet circuit. The Air correction jet and pilot jet is also different.
I suspect the change was simple rationalisation as there is sufficient adjustment in the carbs for most purposes once the basic settings are right.
The carbs for the D/D2 (also 110) engine were actually different in that they have accelerator pumps - normal 405 carbs don't

401 - main 3x 110 air corr 230 pilot air 3x 150 pilot 50
403 - main 115/120/115 air corr 190 pilot air 120/100/120 pilot 45 (also for 404 and early 405)
Late 405 - main 3x 115 air corr 200 pilot air 3x 100 pilot 45
100C engines - main 3x 130 air corr 230 pilot air 100 pilot 50 (sports engines)

Your current set up is highly idiosyncratic - see my comments on your other post

I'd suggest you read the solex carb book for a detailed description of how they work and how the jets interact

Julian

[GlennBurnage]

Julian, many thanks for the prompt and comprehensive reply.
Some background - the engine in question is in a Werner Oswald Frazer Nash Le Mans Replica (WOK7) which uses, almost exclusively 405 components. The exception being the engine which, I'm led to understand was a new-old stock 85C (?) block totally rebuilt by Tony Byford to give circa 130 BHp using a Piper 'High Torque' cam (of which I can find no info) The Main jets used seem to be of comparable size to the AC Bristol that I owned (similar output engine) and my Frazer Nash TT rep (328/Bristol head, same carbs) by Steve Stanton circa 120BHP.
Doulble checked the 44 pilot jet, and it is plainly marked 44. This is stamped on one flat of the hex, not on the end as with the other two. I'd attach a photo, but can't work out how to do it!!
Regardless of the carb to carb differences, the thing I want to change is the emulsion tube on the centre carb as this is the one spitting back. I removed the emulsion tubes from the TT Rep (all marked 10) and, regardles of the different number, they look remarkably simillay to the L24. would you consider trying the 10 tubes (all of them) to see if it cures the spitting back a rasonable plan??
The question still remains - ARE EMULSION TUBES AVAILIBLE????

[JoeF]

Are they similar to these as advertised on eBay ? https://www.ebay.co.uk/itm/262467467...IAAMXQTghRI8eH

I found a jetting chart ( from an old Bulletin or Newsletter I think ) which gives emulsion tube 13 as being used in the BSI engine, and L24 in the BSI mkII.

[GlennBurnage]

Joe, that's very welcome, many thanks for taking the time to reply.
I think I need to find a rolling road who has a large stock change parts for the carbs!!

[Julian Caples]

Yes emulsion tubes are available - 10s are no problem the others are a bit harder - Classic carbs (classiccarbs.co.uk) have had them, classic carbs here in oz have 10s (classiccarbs.com.au) as do Rocky SRL in Italy (ebay store)

I'd have a chat to Tony Byford before mucking around - he's very good

Yes a rolling road is the way to go

Not withstanding my comments about the idiosyncratic parts bin approach, if the car was otherwise running well, then the spitting back may well be something simple and amenable to tuning - particularly check the throttle linkages - if one carb is not opening at the same rate as the other two you can get spitting back

Julian