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

[Geoff Dowdle]

Glenn,
You have mentioned all the jetting sizes but what are the venturi sizes.
403 - 406 have 26mm ID
Most Bristol Sports engines , C,D,D2 have 28mm or 27mm
The size of the venturi determine the size of all other jets, then compression and other factors have a bearing as well.
The pilot jets #44 seem too small to me.
I have found over the years that many jets have been drilled out but they don't necessarily flow at the same rate as expected.
Do you have an inline filter between pump and carburettors as small particles can block or partially block the pilot jets.
Are the throttle bases all the same , most Bristols have two progression ports but other bases from Standard Vanguard, Landrover, Jaguar etc mostly have 3 or 4 progression ports .
Have the float levels been accurately checked, I have found an external sight tube from the main jet holder is the only way to do it accurately. Replacement carburettor tops are often machined differently where the needle valve is fitted, for different applications . Fitting a new needle valve with the standard 1mm washer can give quite different float levels depending the machining of the top.
I suggest you start the tuning process with three #10 emulsion tubes and have each carburettor jetted the same.
The emulsion tube holders can be different lengths which can raise the emulsion tube in relation to the float level , check the heights of the emulsion tube holders above the top of the main carburettor body, that they are all the same.
Remember you most likely have 70 year old carburettors that have been through many owners and so called "Carby Specialists" who have modified and exchanged parts from other carburettors which might not match the original part.
It might all sound a bit depressing but this is the accumulation of problems I have found in the past trying to get three Solex running together correctly, but on the other hand an original matched set in good condition mostly work very well and are easily tuned.

When you are happy you have a matched set in good condition take the car for a rolling road session to check the mixture through the rev range under load and calculate the ideal distributor advance curve needed. Then build that curve into the distributor .

Geoff

[David C]

I just had a look in my spares for 405 and 406 emulsion tubes.
I have what look like L29 with 3 holes and DL with 4 holes and a bit shorter than the L29

Neither of these seem to go with the numbers below and I think they came off carbs on stock engines.

Any clues?

[GlennBurnage]

'Carburettor Hospital - Specialists in carburettors and spares'
contact details via the web. They had all I needed to get my carbs similarly jetted. How it all goes will follow!

Geoff, many thanks, will let you know the progress

[GlennBurnage]

Having got all excited about finding parts to get all 3 carbs set up equally, I find that there are differences in emusion tube diameters!! The ones supplied by the Carburettor Hospital were 4mm diameter and the ones in both cars that I have with 32bi carbs are 3.5mm diameter. INRacing have some of the right size on their way to me. Why I didn't try them first is a mystery to me as they have sorted out several carb related problems in the past and are always very helpful (and expensive, which I'm happy with).
One lives and learns -aparently!!!