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yozzer74:
I put 140 mains in and I'm running velocity stacks pump is working  cheers

K2-K6:
Here's what I believe is happening, if I post it we should get some input as to how you can optimise it.

When you start with the original intake system it has a manufacturer set draw / restriction on which the carbs operate. If you open the throttle slides fully, then with the engine flat out, the draw through the main jets is controlled by that fixed process,  so the original jet size is set to supply demand under those conditions. They then use the shape of the throttle slide in combination with the needle taper to get a linear delivery of fuelling as you move away from wide open throttle all the way down until you get to the idle circuit.
This relationship is vital to jetting and throttle response being accurate throughout the engine's range. It's important to note that as the throttle slides get more closed, then the vacuum characteristics are progressively handed over from the airbox intake design to the throttle slide design as you move away from absolute maximum flow. It's this area I believe that's most changed by removing the original intake.

When you remove the original airbox, you remove a restriction which causes the carb when wide open to exercise less vacuum on the mainjet resulting in less fuel (running lean)being pulled from it. It's this effect that you're responding to when upping the main jet size just to get back to the same mixture for the engine, you are not running the engine at a different mixture from standard at wide open throttle. It's this point that people commonly don't consider.

So you've got the maximum throttle mixture something like correct by using bigger main jets. Now this is where it gets more interesting. Dropping away from wide open, you get much more into how the vacuum through the carb was controlled just by the throttle slide shape, which hasn't really changed. So you've now got very similar vacuum throughout made range but pulling fuel through a too large main jet. Unsurprisingly it runs rich.

If the original development engineers where doing this for that maximum flow they'd have had a number of different throttle slide shapes (different cut aways cause different vacuum gradient) along with a variety of differing needle shape to bring the Jetting and response into acceptable range. Without those options I doubt anyone would get close to optimum jetting over the whole range.

I'd start by just considering dropping main jet size and being careful to see how close you can get the wide open setting to optimum without running too lean at that point. This will get you closer too being right in made range.

You've got an additionally problem with accelerator pumps as well, as they're adding just where you've got it richest (they do nothing when the throttle and engine are at a constant flat out state). You may be needing snookers to truly get it right. It's also very difficult without previous experience or something like a dyno and exhaust gas analysis.

I'd start by trying something like a 120 main jet to give you a little better look at what it's doing.

yozzer74:
Wow thanks for all that information k2. I will give it a try .

onethumb:
Watching this with baited breath. When I rebuilt my carbs I used 120 jets because of the pods.
I saw somewhere about restricting the flow using an old tshirt.


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NigelWilmshurst:
Hi Yozzer
I have not been on here for many months, just watched your you tube stuff on the F2 it looks and sounds the bowl lox! Hope you get the carbs sorted out soon...

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