Balancing 1 1/4 inch su carbs

[2009poolesparky]

I don't have a carb balance meter or know anyone with one, I preferably don't want to have to buy one for a one off job. How can I balance my carbs, I notice the rear one, the piston doesn't rise the same rate as the front one, despite being cleaned throughly.
No vacuum leaks either. Any ideas?

Thanks in advance

[ferny]

If it ain't moving properly then it ain't right. I've always found lifting the piston as you tighten the screws helps locate it so that it moves freely.

For balancing I've always used a petrol can hose held up to my ear. This has often improved cars set up by someone who "knows what they're doing" and has "all the correct tools".

[2009poolesparky]

Ok I will try the hose method. They both move freely, just when I Rev the engine the back one doesn't lift up as high

[ferny]

Check dashpot oil level. I always used atf fluid as well. It could have different spec springs as well. A quick comparison will tell you

[Nobbie]

From the reading I've done, it sounds like the linkage between the two carbs needs to be adjusted

[2009poolesparky]

The dash pots have new oil in and the linkage is adjusted correctly too

[piman]

Hello 2009,

operate the throttle and observe the two carburettor spindles, they should both move simultaneously, if not the linkage is not adjusted correctly. To contradict ferny, the correct oil is SAE 20, or 20/50 is very close.

Alec

[RobPearce]

As Alec said, used SAE 20 or standard 20/50 engine oil. ATF is too thin.

[2009poolesparky]

I have double checked. Both throttle spindles are operating at exactly the same time.  The dash pots have new 20w50 oil in also

[don cook1]

Slacken the throttle spindle (on the non cable operated carb) enough so that the cable will only open one throttle, set the tickover a bit faster on this one. Listen to the hiss through a tube or use a guage, now adjust, by the throttle-stop, the tickover of the other carb until the hiss is the same or the gauge reads the same as the first one. The carbs are now balanced. Tighten up the throttle spindle and check hiss/gauge reading is the same. Now slow down the tickover to what revs you want.

[ferny]

There's always room for improvement, which is why I started trying atf. I did find it a better fluid. Or, maybe I just tinker too much...  

[piman]

Hello Ferny,

S.U.s are quite adaptable; a book I have on tuning Jaguar engines briefly mentions how S.U.s can be modified for increased performance by leaving out the spring and the damper oil but he does say it takes a lot of work to get the needles correct.

He has written a book that goes into it in detail, :- How to build and power tune SU carburettors by Des Hamill.

Alec

[RobPearce]

Quoted from piman

a book I have on tuning Jaguar engines briefly mentions how S.U.s can be modified for increased performance by leaving out the spring and the damper oil but he does say it takes a lot of work to get the needles correct.

I really don't see how that could make an improvement. It sounds like some "pub wisdom" that used to be common back in the day, on the theory that, if you want to get faster acceleration, you want to lift the piston faster. This is plain wrong, and completely misses the point of how SU carbs work. If you lift the piston too fast then you don't get any acceleration enrichment, the engine goes lean and hits a flat-spot every time you accelerate. Yes, it is possible to compensate for this tuning error by also cacking up the needle selection so that it runs pig rich at steady state (as was done on Solex Vitesse 1600s when the factory found the accelerator pump to be too problematic to use) but it's in no way an actual improvement.

[piman]

Hello Rob,

yes,. I agree with what you are saying but this is for competition; he claims near DCOE Weber performance, at some cost to fuel economy. His book on Tuning The Jaguar XK engine is very thorough and he sounds to know what he is talking about. I mentioned it merely to show how good and versatile a carburettor the S.U. is.

Alec

[nang]

Des Hamill does state in his book that for a standard engine the factory needles and settings are the best. Modified needles etc are really only for modified engines.
Tony.