Setting Mixture on Strombergs

[andrewh]

Just a quick question for those that have set up their quad Strombergs, I am actually referring to my XJ12 Series 1, but its the same in most respects. I wonder whether you use a Colortune device in banks of three to set the individual carburettor mixture? I have totally rebuilt the carbs and she runs much better but just a little rich somewhere. I dont think the inlet manifolds are connected between packs of three. (air balance aside)

[MarekH]

Use the same position spark plug each time, e.g. the middle plug. That is because a different length runner will have a different air pressure in it, i.e. the longer end runners will be run leaner than the shorter inlet runners.

Sadly, Colourtune only measures at idle, whereas you really want it matched at running rpm.

kind regards
Marek

[andrewh]

Thank you Marek. I shall make sure I use the middle plug. Not sure how i can measure the running mixture other than at idle.

[vee12eman]

I’m a bit late to this post and I hesitate to disagree with Marek who I know is far more knowledgeable than me in all matters. However, I use the guide from Colortune to check mixture on acceleration, which definitely does mean it’s checking the mixture at other than idle speeds. It should go rich (yellowish) during the acceleration then settle to a bunson blue at a settled speed, usually a bit brighter and possibly just a little more yellow (rich) than at idle, but not greatly so. Don’t run at higher than idle speeds for a prolonged period wth the Colortune fitted.

I use two Colortune units, matching a front/rear carb on one side, using the mixture screws through the dash pots, then match two carbs across the engine (usually two matching rear carbs and only adjusting the carb on the “new” side) then matching the front carb to the newly adjusted rear carb so that colour is the same all around. My “apprenticeship” so to speak, was on twin carb (Stromberg and SU equipped) Triumph cars where I always used two Colortune units, always trying to select two cylinder locations that had similar intake runs.

Marek may disagree with my ideas about higher rpm tuning with Colortune, but I have followed the instructions for years with good results. I’d be interested in an opinion? I still trust his judgement over most others!

[andrewh]

Thank you for the reply Simon. I am sure Marek will be along in a moment to comment. I seem to recall somewhere over the years reading that you can melt a Colortune by running it over idle. I see you refer to that. Also , as you refer to, you will get a surge unless at steady revs.
I must confess to not having tackled this job yet, as the car runs prefectly well just a tad rich on perhaps one carb. Also, some confusion as to whether the carb mixture can be adjusted through the top on these particular model of Stromberg. Presumably you can?

[MarekH]

When I said "at running rpm", I should have said "under load". If you are on the driveway, you are at idle loading, no matter what rpm you are running. You are probably loading the engine less by holding it at slightly higher rpm when on the driveway as you'll have some advance.

There are two types of Stromberg. One has its jet pressed into the body to a fixed height and you use a tool into the dashpot to rotate the needle up or down. The other has a needle fixed into the piston and you rotate the jet from below.

kind regards
Marek

[andrewh]

Thanks for the reply Marek. I am familiar with both types of Strombergs, early Range Rover has the mixture on the bottom and the later has the type accessed through the dashpot. I have not looked yet on mine but did read some comments on Jag Lovers I think suggesting that the XJ12 had non adjustable mixture and if it was desired to weaken or enrich, then moving the needle in its housing was the only way. Personally I cannot see that being the case unless it was on an export emission controlled vehicle?

[vee12eman]

Hi guys,
Marek: thank you for a very useful reply as always, I would not even have thought of the effect of advancing on the engine load, which is why we all respect your input, much obliged.

Andrew: yes, I am also familiar with both methods of adjusting Strombergs as I have had Triumphs with both types. The through dash pot adjustment has the advantage of accessibility, but I think gives less adjustment range. It also has the disadvantage that the seal around the adjustment nut at the bottom of the reservoir is prone to failure, which relatively quickly, drains the oil from the dash pot damper. Fairly easy to replace if you can get them, they come in the carb rebuild kit and I think you can buy them separately or with replacement adjustment nuts from carburettor specialists. Problem is that if you disturb them to replace them, you have to set the mixture afterwards. The V12 E has them and so do early XJ 12 sedans, I’ve worked on both. My GT6 had them, but my Vitesse (Triumph, not Rover) had those which adjust from below, similar to many SU carbs.

[vee12eman]

Hi guys,
Marek: thank you for a very insightful and informative input as always, I would not even have thought of the effect of advancing on the engine load, which is why we all respect your input, much obliged.

Andrew: yes, I am also familiar with both methods of adjusting Strombergs as I have had Triumphs with both types. The through dash pot adjustment has the advantage of accessibility, but I think gives less adjustment range. It also has the disadvantage that they MUST be adjusted using the correct tool (to prevent tearing the piston diaphragm) and also that the seal around the adjustment nut at the bottom of the reservoir is prone to failure, which relatively quickly, drains the oil from the dash pot damper. Fairly easy to replace if you can get them, they come in the carb rebuild kit and I think you can buy them separately or with replacement adjustment nuts from carburettor specialists. Problem is that if you disturb them to replace them, you have to set the mixture afterwards. The V12 E has them and so do early XJ 12 sedans, I’ve worked on both. My GT6 had them, but my Vitesse (Triumph, not Rover) had those which adjust from below, similar to many SU carbs.