Engine: 2.1 Carb B21A: Adjusting the Stromberg 175 CD-S

[AidanC]

Hi,

My '77 B21A with a single Stromberg 175 CD-S seems to be running lean. When I tried to adjust the mixture, I couldn't see any way to do it. There's no nut at the base of the float chamber and my trusty L shaped allen key type carb adjuster doesn't find anything to grip in the dashpot oil chamber.

How do I adjust the mixture on this carb?

Thanks in advance,

Aidan

[loki_the_glt]

According to the Haynes manual (usual disclaimer about other manuals being available) requires a special tool for major adjustments; Haynes quote Volvo Part 5159 or "Sykes-Pickavant carburettor adjuster 660480".

The drill is:

Screw the volume control screw fully in. The screw is by thetemperature compensator.
Remove the damper piston.
Fit the tool, engaging on the lugs
Connect a CO meter and adjust the needle - up to increase the CO content, down to reduce it.
There is a 4-turn range - if the adjustment can't be made then you'll ned to re-set the needle.
Remove the tool and top up the dashpot.
Run the engine at 1500rpm then let it drop back to idle and adjust the volume control screw to bring the CO reading into range.

Hope this helps - I suspect that a Colourtune would do for measuring the CO, but am not certain.

[AidanC]

Thanks Loki,

I have the tool that they mention but its not doing anything....

I remembered that I have a CD150 in the garage - tried that and the tool grips and works perfectly. I cleaned out my dashpot and the centre is circular so I can only guess that someone's worn the edges off the central allen key type received so that the needle can't be raised/lowered.

Guess I'll have to buy a replacement dashpot.....

Thanks again for your response,

Aidan

[Clifford Pope]

Early ones don't have an external adjuster for the needle setting.
There is an air bleed fine adjuster on the side of the carburettor, but if that does not give enough range then you have to take the cap off the dashpot (4 screws) and then move the jet up or down manually after releasing the grub screw.
The basic setting is with the shank of the brass holder flush with the the surface of the piston. After setting that, you then reassemble and make fine running adjustments with the bleed screw. If it is still not possible to get the mixture right then you take the piston out again and press the needle holder in (richer) or pull it out gently (weaker). It's a bit hit and miss, so move it only a tiny amount and then try again.

But check other causes of mixture upset first - eg air leaks, leaking or perished diaphram rubber.

Also make sure that the needle is not worn from rubbing on the jet.

[AidanC]

Thanks Clifford,

Is the air bleed fine adjuster what Haynes calls the volume control screw? Looks like that's my next port of call....

I checked the diaphram and it looks fine. I was able to push the needle up into the dashpot slightly so hopefully that'll help.

Would a worn needle not result in a richer mixture?

Aidan

[loki_the_glt]

If push comes to shove, I've got a Pierburg 175CDSU from a B230A. It should be a straight swap for the Stromberg, though not so cheap to maintain as needle/jet assemblies are (or were) expensive. It's complete and ready to bolt on.

[Clifford Pope]

I forgot what the screw was called - volume control screw sounds right. I converted my 1989 DL with the awful Cisac carb to an old Stromburg Zenith like yours a few years ago, so I found out then about the adjustment method.
From memory you start with the screw fully in, then wind it out to weaken the mixture by letting more air past.

I suppose logically a worn needle would let more mixture through, but if it was worn because it was scraping on the jet then a sticking needle might do anything. I can't remember how you centralise the jet - possibly it is pre-set and needs a special tool to press the housing into place. The test is to lift the piston with the oil damper plug removed - it should give a satisfying thud onto the bridge of the intake, without any feeling of binding at any point in its travel.

I have read that these carbs wear at the throttle spindle, letting air in. A semi-permanent easy cure is to run silicone sealant round the shaft at the bushes.

[AidanC]

Originally Posted by loki_the_glt

If push comes to shove, I've got a Pierburg 175CDSU from a B230A. It should be a straight swap for the Stromberg, though not so cheap to maintain as needle/jet assemblies are (or were) expensive. It's complete and ready to bolt on.

Cheers Loki - let me see how the car runs after today's changes - I've a funny feeling I'll be getting back to you...

Originally Posted by Clifford Pope

I can't remember how you centralise the jet - possibly it is pre-set and needs a special tool to press the housing into place. The test is to lift the piston with the oil damper plug removed - it should give a satisfying thud onto the bridge of the intake, without any feeling of binding at any point in its travel.

I have read that these carbs wear at the throttle spindle, letting air in. A semi-permanent easy cure is to run silicone sealant round the shaft at the bushes.

Cheers Clifford - I've centralised the jets on my Amazon's SUs - I'll check the piston on the Stromberg and see how it drops..

That's a good idea about the spindle - I'll check them and see if there's any play

Thanks again to you both,

Aidan

[Clifford Pope]

I've just remembered something else. There is a temperature compensator bolted on the side, with a bimetalic spring which opens a valve to let more air down the air chanel, same as the volume screw I think. It would be worth dismantling that and freeing the fibre plunger valve and resetting the adjuster screw. I think Haynes describes it.

This site gives a good description of the Stromberg and how to set it:

http://www.buckeyetriumphs.org/technical/technical.htm

They don't all have quite the same features, some just have blanking plates over bits that don't apply.

[AidanC]

Thanks again Clifford - we have a bank holiday weekend coming up so I think that I'll remove the carb, give it a good cleaning and check out the temp compensator.