MB750's MKIV Roadster build thread

[MB750]

One of the things I really love about this part of the project is being extremely granular when it comes to understanding how something does what it does. Topically, everyone knows what a carb does (meters fuel into the engine). But there's TONS of design and testing that goes into flow rates and whatnot that all happens behind the scenes during the design phase that nobody sees. That's the stuff I love. It's the "why" aspect of taking a deep dive.

Case in point, here's how I'm tuning my carb. I know there's a relationship between AFR and jet/needle size. At cruise, constant throttle, the metering rod is down and there's only two sources for fuel into the engine (the idle mixture screws, and much more applicable is the thick part of the metering rod and main jet). If the jet and rod have a relationship where the more area available to flow fuel, the richer my burned mixture will be. I discovered there's a sweet spot, and that seems to be around .0028 in^2.

My notes below:



Jet, rod, clearance between the two, and one thing I was missing was the difference in area between those two. The clearance might be the same, but as you reduce the diameter of an orifice you're inherently reducing the area as well so that clearance measurement wasn't the most accurate.

Now, knowing this I can see what "delta area" works best to get me the best cruise AFR. Then it's just swapping jets and needles to get me there.

Here's more notes. It's me calculating the area of each needle and jet, and also at the bottom proposing a few different combinations that are in the "sweet spot" of flow area:



I also made a spreadsheet in Excel for quicker math:




For what it's worth, I'd be doing the exact same type of stuff for an EFI conversion.