SU twin carb setting
 

What's the best method for equalizing the fuel settings on twin SUs. When the carbs have been set up for vacuum I usually screw one jet up to the top counting flats than back to original setting then screw the other carb jet to the top and then back down the same number of flats as the first one. Is there a better method? How do y'all do it?

You should set up the carbs to have equal "suck". A vacuum method is usually the most accurate. It's rare that the jet settings on both cabs are exactly the same and maybe there will be a couple flats between them to get a smooth idle and best general running. As you've had your carbs rebuilt you shouldn't have the idle setting problems caused by worn spindle bushings so you can try a flat or half a flat even up/down each side to get the best result. Use the instructions in the Haynes regarding the lift pin. Not everyone's cup of tea wheb tuning but it does give you a bit of an idea. I 1/2 turns down is 12 flats. You might end up with 10 1/2 and 11 3/4 or something like that. Not usually above 12. Someone who has tuned a lot of pairs may say that the front carb tends to run weaker than the back, or vice versa, as far as the number of flats go. Comments welcome on that.
Good luck George. Being in a hot clime, if you never need choke to start, you can probably come down at least 1 flat.

Only reason I ask is I took the PV in for it's inspection yesterday and the emissions were way high. after weakening the mixture a bit I got it to an acceptable level for them but still high. So, now the carbs are unbalanced I will need to start again. I usually check plugs for color after a run, colortune is all right for a ball park setting but not terribly sensitive. Emissions setting of carbs is alright with a single carb but not very helpful with two,One can be weak and the other rich and still give acceptable emissions reading. I usually finish off with the vacuum carb balancer.

Hello George,

I don't know why, but I love setting up twin carbs!!

Firstly, for the mixture settings, I think the Colortune is great. As above, I always find one runs differently to the other, so setting them up on number of flats turned doesn't always do the job, so using a Colortune at least means the mixture is the same on both carbs. However, I do it by ear, then a very sensitive rpm gauge that I have: the mixture is spot on at the point the the rpm is at the highest.

The absolute best tool I've used for blancing air speed through multiple carbs is a little wind speed gauge, otherwise called an anemometer.

http://www.amazon.co.uk/439/dp/B007P...nd+speed+gauge

The problem with a vacuum gauge is they inhibit induction through the carb as you hold it there. That means that, in effect, you're slowing the air speed by just holding it there! Also, they're fiddly to use and the little ball gets sticky when hot resulting in inaccurate readings.

With the wind gauge, you can instantly get a digital reading holding the "fan" at the carb opening. Firstly, you set the throttle flap so that the airflow at idle is the same. Then, once you have tightened up the throttle linkages, not only can you re-check the air speed at idel, you can check the airspped is the same at partial throttle openings really quickly and easily by opening the throttles with one hand and using the air speed gauge with the other.

On Amazon, wind speed gauges are cheaper than Gunson carbalancers and in my opinion, much, much better.

One last thing, on an old car I would disconnect the crank case breather (if fitted) if you are using a probe up the exhaust.

Come over here and do mine:)
Might give your wind gauge a try. Not too much money.

Use the vacuum tool first. Uni-Syn or what ever you have. That way the carbs suck the same and the pistons should rise together. Then do the mixtures. Changing the mixture shouldn't affect the suction at tick over and when running at steady revs. Because of the balance tube in the inlet manifold a change to the mixture on one carb will affect the 2 cylinders fed by the other carb. The change is small but that's the reason you have to back and forth between the carbs to get the mixtures the same.
Valve clearances and timing should be spot on before you tune the carbs.

Different SU's but similar techniques

https://www.youtube.com/watch?v=ASeMfXfjNpw

https://www.youtube.com/watch?v=4nvGLgO6pj0

Almost.....If you set the mixture for maximum idle speed then you will be running too lean on the road and won't have the power you need. The published procedure calls for a slight rise in RPM followed by a return to normal when you lift the piston pin.

Of course, I like to have a bit of G-force when I drive! :-)