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Originally posted by Andyinthegarage
OK, there is loads of great advice out there (and in this post) which I have gratefully used myself, but I tried to tie it all together to give my own experiences and at the same time give a step by step guide to setting up the Weber34 ich - I include it for what it is worth and tho I don't imagine it is the only way...it is One way - hope someone finds it useful - sorry it is rather long:

The Weber 34 ICH is a common direct replacement for FoMoCo carbs, is relatively easy to connect to the throttle linkage, and bar a few pinch points against the servo-vacuum pipe, drops straight in. When correctly adjusted it will give better fuel economy and increased power particularly when combined with an improved exhaust system but if you dive into adjusting stuff without a plan, you can be chasing your tail with no benefits.
So….before you attempt any adjustment:
Make sure your carb has the magic “27” size venture as stamped on the casing – it isn’t impossible to set up a different carb, but 25 won’t give you best performance and 29 is very difficult to get right with most un-tuned engines.
Make sure that your in-line fuel filter is clean, you have adequate clean fuel in the tank and that all electrics are in good order.
Check that the carb is in good condition with the internal filter recently cleaned or replaced. Ideally give it a full service.
Set the timing to standard 6 degrees, and if possible, pre-warm the engine to make starting easier – not hot, just warmed.
After changing anything, replace the air filter before final adjustment of tickover and fuel mix as the type of filter will slightly affect things.
Keep your tools clean to avoid adding dirt to the confusion!
Start settings:
Most agree the standard settings for a 1.5 as being close to those listed below but individual engines, carbs, filters, fuel, altitude, opened manifold passages, exhaust choice etc will all make tuning worthwhile. So, I set up as follows:

Main 155
Idle 55
Emulsion Tube F6
Air Corrector 175
Pump Jet 40

Now, the fun begins….

Start with the idle circuit which takes care of 80% of the performance improvement available and whose fuel is controlled by the Idle Valve.

Back off the tickover adjustment until the screw no longer touches and the throttle plate is closed. Slowly adjust it back until it just touches – watch very closely (smoker's trick is to slip a leaf of hand rolling paper between the surfaces to check clearance)…and then count it in ONE AND A HALF TURNS more. Don’t go beyond this as the throttle plate will clear some very small apertures inside the body and start using a different cycle.
Screw in the mixture screw until it GENTLY grounds and then back out 2 full turns.
Start the engine – it should run, but may sound quite rough, certainly very slow but as long as it is running and not “hunting” it is fine.
If it won’t start, close the throttle tickover screw half a turn just to get it started and remember to open it the same amount once it is running.
Now, by ear, adjust the mixture screw a quarter turn at a time until you find the sweet spot right in the middle between the point where it stops increasing the revs, and the point when it starts to sound a little rough.
Once you have found the best setting, turn off the engine and turn the mixture screw in until it gently grounds again. As you do this, count the turns as this will indicate if you have the right idle valve. Opinions vary (as do engines) but if the sweet spot is between 1 ½ to 2 turns then it is pretty much there. You could try a different idle valve just to see what changes, but otherwise you are fine.
If you count less than 1 ½ then your idle circuit is drawing too much fuel. This is very unusual, so check that your tickover is at 1 ½ turns and repeat the mixture screw adjustment. If you get the same result try a “50” idle valve, but if this doesn’t correct the setting it is unlikely to improve by going further. So strip and clean the carb.
If you count more than 2 turns, you need more fuel in the idle mixture. Go up a size in idle valve (increments of 5). Most engines seem happy in the range 55 – 65.

Repeat your idle mixture screw testing until you are happy, then adjust the tickover to about 850 - 900 rpm with a decent electrical load such as main beam. Replace your air filter, check the settings are unchanged and move on to testing the next circuit and so setting the Main Jet.

Unless you have access to a rolling road, the best test is for the “Kill” method – sounds dramatic. You will need a good long hill, steep enough for you to hold about 1/2 to 3/4 throttle without taking off, with safe places to pull over, no traffic and a spark plug spanner. (That's you, not the hill). Engage free wheel (makes stopping easier). With a warm engine, and fresh or at least clean plugs, set off up the hill on a steady ¾ throttle. After at least half a mile and assuming it is safe to do so, kill the ignition without first changing the throttle, and coast to a halt dropping the car out of gear and releasing the throttle once the engine is dead.
Take out at least two plugs from opposite sides and examine. Oily plugs suggest that the main valve is too big and vice versa for hot dry plugs. Adjust and try again. Again, engines vary, but I have seen preference range from 155 – 185 for a 1.5 engine.

Finally recheck timing hasn’t wandered.

Hopefully by now you have a fairly good setting which you can then gently tweak to get the best average setting for all temperature, load and altitude conditions.

You can also try advancing the ignition a little, but only change one thing at a time!.......Have fun!

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