Amazon: Jet-setting

[HRA]

No, I'm not looking for holiday suggestions, this is a more mundane question about carburettors.

For a few years now I've been running my B20A Volvo Amazon on the "wrong" Stromberg carburettor. When I bought the carb I didn't realise quite what a variety of 175 CDs there were for different applications. I think I've actually been running one of a pair of carbs meant for a Triumph or a Lotus. On the positive side the car ran, and ran better than OK. However, I was never totally happy with the needle-profile. The car should have had a B1AN or B2AF needle, but both of these are non-adjustable and wouldn't fit my carburettor. I did manage to reshape a B1G needle (which did fit) but although it ran well enough it can't have been quite as good as a "proper needle".

Anyway, fast-forward to last weekend when I finally fitted the "right" Stromberg; a 175 CD-2SE with a B1AN needle. It had been stripped and cleaned by the previous owner and new parts had been fitted from a Burlen kit. Those new parts included a new non-adjustable jet.

The car started after a couple of attempts - it probably took a few turns to fill the float-chamber for the first time. It idles well and it is certainly driveable, but it seems to be lacking a bit of "sparkle" that I was getting from the "wrong" carb.

Back in the garage I removed the air-cleaner and, with the engine idling, I used a screw-driver to lift the air control-valve (or piston) a tiny amount. I can't swear that it was exactly 1/32" as specified in the manual but probably near enough. The revs dropped noticeably which, to me, indicates lean-running.

Neither the needle nor the jet are adjustable but it is possible to insert the needle too far into the control-valve/piston. This should set the needle a bit higher and richen the mixture, but still the engine seems to be running lean at idle.

Before going any further I dug-out a tin of carburettor-cleaner and (carefully!) sprayed it around all the joints between the carb and the head. I also sprayed it around the the carburettor itself. I did this with the engine running but at no point did the revs rise. From this I take it that there are no air-leaks making the system run lean.

So that leaves me wondering if the new jet has been set at the correct height. The previous owner said that he'd got a local garage to insert the new jet, but didn't have any more details. In various manuals I've seen that there is a special set of tools for setting non-adjustable jets that comprises two screw-presses to push the jet up and down, and a calibrated tool to set the correct depth below the bridge. I'm sure these tools used to be two-a-penny on eBay when I didn't need one, but now they're either like hen's teeth or brand-new and priced accordingly/eye-wateringly.

If I take the carburettor off again I'm pretty sure I can move the jet up and down - either in my vice or on the fly-press at work. There's also a lathe at work so I can make my own depth-setting piece. So, to the big question: does anyone know what depth I should set the jet too?

I've looked online but not come-up with much. Most sources recommend the special tool and indeed that would be preferable since the jet can be set with the carb still on the car. One Triumph site suggested 0.100" below the bridge, measured at the outer (higher) edge of the jet, but this might not be the correct number for a Volvo B20A. My jet certainly looks higher than that. As ever, any advice or suggestions would be much appreciated!

[Derek UK]

The adjustment tool is about £15 +vat from Burlens and about the same from other sources.

[Burdekin]

Have you spoken to Burlens?

My first thought is is the needle and jet centred now? With the plunger in and also out does the piston drop down and hit the bridge when you lift it and then drop it? Not sure how you would go about centering the needle if it isn't centred with the non-adjustable carb.

I've mentioned it previously, you can get a conversion kit to make the jet adjustable and should also make it so the jet can be centred I assume also. I have a kit but not fitted as yet so no actual experience on fitting and adjusting.

[Ron Kwas]

HRA;

It certainly sounds like you understand the important, and finer points well...a couple of notes, including the reminder that I am not a ZS expert!...the mixture at idle is only valid at idle!...Reminder: if you are missing "sparkle" under load, adjusting at idle may not address the load range operation (adequately, or at all!), although I am not familiar with how the Metering Needle is secured into the Dashpot, and if its location can be varied for a little mixture adjustment OR if the jet can be lowered, and how simple this might be (although I warn about applying any major forces with a "fly-press at work")...

Good Hunting!

[HRA]

Originally Posted by Derek UK

The adjustment tool is about £15 +vat from Burlens and about the same from other sources.

Thanks Derek. By the look of it, the £15 tool from Burlens is the tool for adjustable needles. It's a long allen key inside a sleeve that keeps the piston from rotating and damaging the diaphragm. The 175 CD-2SE has a fixed needle so there's nothing for the allen key to engage with.

[Burdekin]

Also try swapping over your diaphragms just in case the new carb one is damaged.

[HRA]

Originally Posted by Ron Kwas

HRA;

...the mixture at idle is only valid at idle!...Reminder: if you are missing "sparkle" under load, adjusting at idle may not address the load range operation (adequately, or at all!)

Thanks Ron Kwas. I agree that mixture at idle is only valid at idle, but I'm hoping that the original designers would have specified a needle-profile and return-spring-constant such that the mixture is "correct" throughout the range of engine-speeds and loads. Provided that's a valid assumption then lean at idle suggests lean throughout the range. But you're right; it is only an assumption. All I can really do is set the jet-height to "standard" and trust Volvo engineers from the 1960s.

Neither the engine nor the car are new so I'm not looking for "high performance" as such. I'm actually more concerned that running lean on the motorway for a few hours might cause some serious damage. I'm thinking of taking a holiday up to the Scottish borders but that'll involve 350 miles of motorway driving to get there. I don't want to find myself at the road-side with a seized engine...

[HRA]

Thanks Burdekin. I haven't had much luck from Burlens. I e-mailed them a few weeks ago asking about metering needles but still haven't received a reply. I expect that Covid has left them under-resourced and snowed-under with enquiries.

With a fixed-jet and fixed-needle there's no way of adjusting the centering. The needle is also spring-biased to one side so it must always rub inside the jet. But the piston drops down on the bridge with a reassuring click so it must all be aligned.

I'm certainly tempted by the adjustable-jet conversion but was hoping to learn from your experiences of it. I couldn't find much in the way of detail about the kit online. Will it be the right jet for the 175 CD-2SE and will it cope with a biased needle? £85 is reasonable if it works but a dead waste if it doesn't.

On another note, I've often wondered why there can be so much play in a carburettor that the jet would need re-centering. Does my 175 CD-2SE only get away with it because the jet is press-fitted?

Lastly, I'll try another diaphragm on the off-chance, but the one that came with the carb looks brand-new and is still even lightly powdered.

[Burdekin]

Only other thing I can think of is the idle trimming screw, wind in fully and then two full turns out is the starting place for it.

I assumed maybe wrongly that the jet is just pressed into position and the needle locates in its position and that was it. If that is not the case then sounds like you are on the right track.

[HRA]

Thanks again Burdekin. The idle trimming screw is another of the Stromberg's mysteries that I'd like to understand better. As far as I can see, it allows a small stream of air to bypass the bridge and emerge just up-stream of the throttle plate. This should reduce the suction at this point and therefore reduce the amount that the dash-pot piston rises. This, in turn puts the metering needle deeper into the jet, making the mixture leaner. The air from the bypass should itself also make the engine run leaner. The flow of air in the bypass stream is quite small, so it only makes a significant difference at idle.

I've read that this adjustment was only used whilst the engine was new and "running in". If a new engine started to run a little rough at idle the idea was to screw the bypass screw in a bit until idling was even again. When the engine was fully run-in the screw would be fully tightened and there would be no bypass air. I wonder why a new engine needed a leaner mixture?

At the moment I've got my idle trimmer screwed right in. I did briefly try it a couple of turns out, and then some more, but it really didn't seem to make any difference to the idling, even with the screw completely removed! I screwed it back in again mainly on the grounds that less air is less lean.