In terms of richening the mixture, aren't they the same or is there a difference?
There's a difference for sure. Think of it this way: the main jet allows more fuel at all times when the engine is not idling..... except the needle is in the way. With washers under the needle, you'll also get more more fuel when the engine is not idling.... but this time the main jet is metering the amount of fuel.
So when you're at half throttle, the needle position is MOST important even though gas is coming up from the main jet. When your at wide open throttle, the main jet is most important because the needle is pretty much out of the way.
So the needle comes into play ~half throttle? If so, I rephrase my question, is there a difference b/w the mid main and shimming the needle?
No, the needle comes into play before half throttle. It's actually being used the whole time the engine is running, but least of all at idle. Each circuit (pilot jet, mid main jet, main jet) of the carb is designed to work in a pretty specific way, but they all overlap. And the needle is what allows each circuit to overlap so smoothly.
Adshed, what exactly are you trying to do? If I had to guess I'd say you're looking at jetting your carbs the least expensive way..... If you only needed a minor change in air/fuel mixture, shimming your needles might work fine. But generally speaking, changing jets will give much better results.
Needle is an obstruction in the emulsion tube. Needle is tapered past a certain point.
The main jet is in the hole @ the bottom of the emulsion tube.
You can sort of richen the mix - a bit - by lifting up the needle with a washer - but that mainly affects where the taper comes into effect. Like 1/4 throttle vs 7/32'nd throttle vs 9/32'nd throttle. So it controls the throttle position of the needle's ramp. Not the fuel mixture @ any position.
Bigger main jet would let in fuel all the way across but not have any effect on where the taper comes into effect. Controls the fuel mixture, but not where the ramp of the needle taper begins.
So if you're fine @ 1/4 throttle, but lean @ 1/2 and continue to be lean @ 3/4 (where needle is out of the taper) lifting the needle aint gonna help you cos 3/4 throttle is outside of the needle's taper. You would go up a main, and if that makes for a too rich 1/2-3/4 throttle, you can lower the needle.
Just an example.
Cool.
Buddha.
@Big Rich
Thanks for the responses, I was just trying to better how the fueling works in the carbs
@Buddha
It makes a lot sense now, appreciated