SU Carbs set up

[asam_x]

I think its time for me to learn how to set up the carbs on my 2500s. Basically, how should I start and in what order, ie, fuel or air? What do I need to do? I don't have no specialist tools so just want to learn the basics. Any info appreciated. Cheers, Asam

[johnnydog]

I would just follow the information that is available in the Haynes manual. It will inform you how the SU carb works, and how to undo the top covers to remove the pistons for cleaning before any adjustments. You will then be able to see what condition the needles and jets are in before making any adjustments for the initial basic settings. If there are any signs of wear on the needle, or the jet is slightly oval, then they need to be replaced.
Before you make any adjustments, it is important to ensure that the throttle linkages at the carbs are balanced and opening simultaneously. The air flow (intake) needs to be equal on both carbs too. Without doing this, any mixture adjustments will be futile.
There are some relatively inexpensive tools available from Gunson that will assist in balancing the airflow (Carb balancer) and ensuring the mixture is somewhere near (Colortune)
When the basic settings are achieved, it will outline how refit the tops to ensure the pistons rise and fall freely which is essential. Don't forget to top up the oil level in the pistons after they have been apart! Fine tuning on the road is usually the final adjustments.
Practise and patience are the best way to understand carburettors. Setting them up by ear is an art, and one I haven't been able to master!
Good luck!

[Clifford Pope]

One frustrating thing about setting up a carburetor, and even more so two, is that all the adjustments interact with each other, so having been round once you often find you need to tweek something you have already set once.

You set the linkages to start with, but later on after you have adjusted one of the throttle stops you have to re-set it.
If you adjust the mixture on one carb it alters its idling setting, so you have to re-set that too.
Then finally you do a road test, check the plugs and conclude that one carb is perhaps slightly rich. The moment you alter that you alter its airflow. You alter its airflow so you need to re-check the linkages. etc etc.

But slowly you get it slightly better each time.
My tips are:
Check the plug appearance after your first run. That is the best indicator of mixture. I've never found the pin-lifting business helps much.
Listen to the exhaust note. The straight six is perfectly balanced, and although it can be thought of as two engines connected together, they work in harmony. The exhaust burbles blend together, not discordantly. (A V-8 burbles too, but differently, because the two engines always sound slightly out of sync)

The key is understanding what each adjustment does and recognising why you need to make it, before fiddling.

[asam_x]

Thanks for the replies. For no apparent reason the car seems to misfire as if it's not running on all 6cylinders. I removed the plugs and they were soothed up. Cleaned and refitted and still the same. When it feels like it's good. Only happens now and then. I have checked the floats and they are clean. Going to change fuel filter next. Cheers

[Clifford Pope]

And of course before you do anything to the carb set up you need to be absolutely certain that other things are correct - plugs, ignition leads, distributor cap, capacitor, points, advance and retard. Also check the float valves. If they stick, open or shut, you get fuel starvation or a sudden glut of petrol.
Another basic is to check that the choke really does release properly, not just at the linkage but that the jet carrier isn't sticking. I've noticed the flexible fuel pipes from float chamber to jet get less flexible with age, and cause the jet carrier to stick.

[poppyman]

The first thing i would do is slacken off all the links between the carb's and then adjust them. The gunson unit is very good for this, but if you can access a co meter all the better. Once all balanced retighten the links and it should be fine. This is what i did on my 2500 S.

Tony