Stromberg mixture adjustment question

[dasadrew]

Hi guys,

went for the bi-annual emissions test a couple of days ago. Set up the Strombergs to give me around 4% CO using my emissions tester.

When the test centre (like MoT) hooked up the car it started reading nutty values like 7%, then dipping for a second or so to 1%. This went on for a couple of minutes until I said I'll try to reduce the idle mixture. Basically nothing worked as planned and the emissions values just kept dancing around no matter what I did to adjust the needles.

Anyway, the tester just made a note Emissions:OK so I got the "MoT" certificate.

Having fiddled around in sort of panic mode, I lost count of the needle turns which I'd set.

Is there a quick way I can just get back to a basic setting on both carbs by turning the needle fully in or out or whatever to then counting the turns as I enrichen or weaken the mixture? (I must have done this years ago when I set them up but have completely forgotten it having not done anything on the Stag for about 4 years due to house-building.)

Cheers in advance

Drew

[AndyG]

When I had a similar issue I wound mine fully in then counted the turns to fully out then went half way between the two. That gave me a starting point and the gas analyser did the fine tuning.

Not sure if that's the best way but worked for me. Hope you get it resolved

[Lingen]

Drew.
I think this is the recommended procedure for each carburettor, but it does mean lifting the pistons out.
Having removed them, turn the adjusting key one way or the other till the needle carrier is flush with the bottom of the piston. Then turn the key 1 1/2 turns clockwise.This is a good starting point.
Replace the pistons, making sure the tab under the outer edge of each diaphragm fits in to the depression in the carb. body. Then adjust each needle by the same amount to get your 4% reading.
As each carburettor feeds both heads, and there is a link pipe behind the silencers, either exhaust will do for taking measurements.
Have you also got the idle setting equal on each carb. That is important as well.
Mike.

[dasadrew]

Thanks guys.

When I rebuilt the carbs (about 6 years ago but only about 1000km!) I was meticulous in all aspects - especially the idle setting. I'll start off agin with the full needle job like Mike says. When I set it up with my exhaust gas analyser it seemed to work ok (although the occasional sudden "hiccup" made the values jump up and down). Can't understand why the MoT test showed wild values. My Exhaust Gas Analyser is the same Bosch equipment which the MoT testers used to use, and the Bosch service agent who calibrates the MoT equipment was were kind enough to overhaul and calibrate it a year or so ago.

[Philip Wardle]

We took our car for an MoT test two weeks ago with the mixture on the Strombergs set to what we thought was right, essentially following Lingen's procedure. The MoT tester's equipment was showing 8%, but afterwards we realised the measurement was taken after the engine had been idling for around 10 minutes, and we didn't think to suggest that the tester first blipped the throttle. I wonder if the measurement might have been lower if we had made sure to do that.

[Lingen]

Phil. It could well have been. With the engine at full working temperature, raise the revs considerably for say about 10 seconds with the sensor removed from the exhaust. Then let it tick over, replace the sensor, and then start taking a reading.
Mike.

[DJT]

It would. The MOT tester who did mine for years always gave it a good clear out by revving it up to about 3,000 rpm before checking the readings. My Stag doesn't need an emissions test - he was just nosey and it was good information for me. Sadly he has now retired and the guy who does it now doesn't bother.

[Philip Wardle]

Thanks for the advice guys, our Stag similarly doesn't an emissions test but the MoT chap offered to measure for us, we thought this would be a backup for our tip test for mixture. Not sure if anyone has researched the accuracy / repeatability of the tip test, I might do a search.

We have acquired a Hawk No 766 analyser which appears to be in good condition, it works as expected but don't know whether it has drifted from its factory calibration. When we have the opportunity we thought we might cross-check with another analyser which is calibrated.

[dasadrew]

I think I might have found the reason for the MoT shenanigans. Put the carbs together and set up like Mike suggests. Just as I was putting the breather pipes and tubes back I noticed that the distributor vacuum pipe wasn't in its (bloody awfully made out of rubber which loses its elasticity and tension as quick as you can say Bob's your uncle) rubber elbow. Popped the tube back in and I've just been out for a celebratory run. Car hasn't been as silky smooth and energetic for a year or two. I'm guessing that was maybe the reason why the values at the MoT were jumping all over the place?
(I did notice when driving over the last few months that she didn't really want to accelerate and I'd put it down to sooty plugs or WHY.)

[Philip Wardle]

Good news We've noticed that some sources of the rubber elbows aren't the best (what a surprise) but now have a good pair with a tight grip on the pipe. Some apparently abandon all hope with the elbows and just use a length of 1/4 inch vacuum hose.

[stagerooney]

Your initial problem with the emissions test was probably something to do with automatic advance and retard. As Philip Wardle has correctly stated, the currently available rubber elbows connecting the pipe are a loose fit and should be replaced. Personally, I would bin those awful Strombergs anyway and fit a Weber. So much smoother with better pick up and improved fuel consumption. Back in the day we all went to Carburettor and Ignition Services Ltd on The Birmingham New Road for SU bits for our old Sprites, MGs and Minis. The guys there used to tell us they had more trouble with Stromberg carbs than all the other makes put together!!

[dasadrew]

I'm afraid you won't get me converting to a Weber! I've got an Escort RS2000 'Group 1' with two twin downdraught Webers - a nightmare! Additionally I'd lose the advantageous "Historic Vehicle" status and priviliges in Germany with such a major modification - especially if it wasn't a current upgrade at the time of production.

[dasadrew]

Good news We've noticed that some sources of the rubber elbows aren't the best (what a surprise) but now have a good pair with a tight grip on the pipe......

Hi Philip, is the "We" here yourself or SOCTFL? Supplier where I could get some decent rubber elbows and T-pieces?

Cheers

Drew

[richardthestag]

the newer elbows I have found to be poor quality, short lived "service items", a bit like the float chamber vent elbows and t-piece.

For distributor vacuum look for a quality 6m vacuum hose and lose the elbows.

[Philip Wardle]

Hi Drew, "we" is myself and a friend with BL and Ford history who helps with fixing problems and keeping things working - especially heavier jobs I can't manage thanks to inherited neurological disease which kicks-in as you get older - not life shortening but physically limiting. His reward is that he's a named driver on the insurance and gets to drive often

I was involved with SOCTFL for a long time, not recently. However I research parts myself, much of which is trial and error and ends up with a proportion of scrap. The elbows I'm using now came from LDPart under part no 12B062 which is listed for some AH / Mini / MGB / Morris / Lotus cars (apparently equivalent to Triumph 128262). Lots of places sell 12B062 but I found the LDPart ones to be good after first trying another of the usual suspects.

A length of vacuum hose is arguably better (half the number of connections, but I'm stubborn and don't like to be beaten - this is perhaps the reason I haven't given in to my friend's efforts to persuade me to ditch the Strombergs and fit a Holley carb or fuel injection (which he does all the time on lesser American cars of the same era). But I think he secretly likes the Strombergs ... just won't admit it.