I just posted this on the cobra forum, the guy is having issues with his webber 500:-
During the tests that I've done using a lambda probe I noted that the carb runs way too rich in the cruise mode but does not run too rich up on the power step and when running flat out. The cruise mode is either controlled by the jet (which will effect both modes to a degree, but the secondaries also have an effect) or is solely controlled by the thicker part of the needles, 65, 67, etc. In fact going from a 65 to a 67 is only a 2 thou increase in size but leans of the cruise I think by 6% from memory according to the chart.
My tests have confirmed that it leans off nicely with the AFR going to about 13.5:1 in cruise mode. Des Hammills book reckons that you can go as lean as 15:1 in cruise but my 3.5 and Grahams 3.5 both ran like bags of crap when we ran them leaner than 13.5:1. I've not tried going leaner than this on my 4.6 and have no intention of trying!
The out of the box setting is not too rich for the acceleration mode and WOT, in fact its a bit leaner than what is required for max BHP according to "Des". He reckons 12.5:1 which is similar to other general gumph that I've read so I do believe the book even though it's wrong about the cruise mode.
The 55 part of a 67x55 needle will have the acceleration mode at round 13.5:1 which I don't think is too lean to harm the engine (It's still classed as a rich AF ratio as it is richer than 14.7:1).
The needles that I've made are a set from the standard range available but are 67x52 (The 52 bit being the part that I thinned down to get this figure). Even these needles only get the acceleration down to 13.1 on my 4.6 lump. (but the WOT still stayed around 13.5:1)
I could not get the WOT settings richer than 13.5:1 with just my modified needles so I've increased the secondaries. I've gone off the scale with the ones supplied in the calibration kit so I bought some bigger ones (104's) Even these are only allowing the WOT to go to 13.1. I would like to point out that I've not noticed the car going any better as the mixture has richened up
As you can see it can all get a bit complicated if you let it!
What you do with your carb really depends on what you want from your motor. If you want every last BHP and need to run it rich because its tuned right to the limit then you need to do quite a bit of fiddling and you would need either a probe or a rolling road. (I don't think that any of our road lumps are what you would call "race tuned" as that would be horrible on the road, not to mention the fact that a race engine needs a re-build every 500 miles!)
If you want a degree of accomony whilst knowing that you are not washing the bores or running way too lean than its dead easy to setup the carb, bung it all back to standard and lob in the
67x55 needles.
You'd think that its not posible to make the above statment as each engine is different (in size and tune) but I was surprised to find that it is has proved to be true so far. My tuned 3.5 ran fine with just the needles, Grahams standard 3.5 runs fine, and my tuned 4.6 ran OK with them.
(Other people on the V8 forum have reported that their cars run OK as well).
I then posted this up:-
You are quite right, 95's are the standard secondaries. (My memory ain't what it used to be!) They will be fine if you want to run WOT at 13.5:1
When you put the new needles in I doubt that you will feel any more BHP but you will "smell" the difference between them and the standard ones with 86 jets.
You will also need to set the pilot screws. Get the engine up to temp, open each screw out to about 2+3/4 turns (if it does not like running this rich go for 2 + 1/2 turns) Once you have set them turn each one in slowly and listen to the engine, it might speed up at some point or it might just stay the same however there will be point at which the engine just starts to slow down (only just mind you). Stop turning screw and open it up 1/4 turn. Do the same with the other screw. This will set the AFR at about 13.5:1 and the CO at about 2.5 %. You can go in an 1/8 of a turn to get the AFR to 14:1 if you like but I run mine at 13.5:1 at idle.
I bet the screws end up between 1+3/4 and 2 turns!
The low floats will cause flooding as the fuel level will be too high.
Edlebrock only screw the carb together to make shipping it easlier, they don't seem to set the things up at all!
HTH
Pete
Was thinking about measuring engine vac? What would any of you gents rec as far as needles and springs for the above? The car drives fine, no bogging down or owt.
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