I'm not sure if what I'm about the explain for a car exists - I haven't seen it so I doubt it does, but anyway.

Why isn't there a carb made that has "default" sized pilot fuel, pilot air, main, etc jets but has tapered seat screw that screws into and out of the jet bore. So the jets in the carb are replaceable (if you damage the seat, etc) but to tune the jets, rather than change the jet you can adjust the screw to regulate the flow through that particular jet.

That would making tuning carbs that much easier in one off situations, so for a manufacturer jets are the more viable option for mass production as once they size them its just a matter of using the same jet numbers in each carb.

I dont know about you guys, but I see it so much easier to just throw this hypothetical carby on any engine you want then use the screws to tune it (with out even taking it apart or off the car) and in a day you would easily have the car to a very drivable state that would only take a little bit more fine tuning.

So, just wondering - does such a carby exist; one that has basically externally adjustable jets?

Sounds like a chainsaw carb. Now to answer your question, I don't know but I can guess;

1) Fixed jets never change their flow and so are less likely to need resetting ever

2) I would imagine when manufacturing cars for a particular location all the cars would get the same carb jets then a quick run on the rollers to make sure the mix is within spec. Having to tune every single one would be much more expensive.

There are probably more reasons however there's no reason why you couldn't make your own adjustable jets and fit them to a carb. If your design can reliably lock the jet position you might have a nice after market product.

Just my thoughts

Just my thoughts exactly.

For a mass produced carb this would be bad but as a upgrade to a previous engine (attach bigger carb) it would make tuning so much easier to joe average. I guess having fixed jets makes sharing jetting easy on forums but at the end of the day every engine is different, compression ratio, cam type, inlet flow, exhaust restriction - so even having fixed jets and telling people your jetting doesnt really help.

Me having access to a full machine shop (and a lathe and mill at home) this looks like a interesting project - hell I'd even make the carb from scratch.

Locking the screw can be done by low viscosity loctite that wicks or better yet, a small double nut to lock the screw.

Those Walbro type carbs spring to mind when thinking of all this but they are far too small with their 1/2" venturis.

What about SU's? Single variable main needle jet adjusted by turning a screw.

I just learned something new. I've never owned one of them.

http://www.wind-drifter.com/technical/WalbroServiceManual.pdf

At around page 8 they show diagrams of what I'm thinking.

Low speed jet and a High speed jet to tune the fuel air.

Why cant this idea be used and ditch the pulse membrane pump for a float chamber fed from the fuel pump?! Keep reading and it even shows you how the accelerator pump works.

Its kind of like a weber, but why open and fiddle with jets when a small 3mm flat head screw driver is all you need to tune it? Biggest walbro make is 25.4mm (venturi) and these carbs are used on 100cc 2 stroke motors, so on a 200cc four stroke (x4 cylinders) gives us 800cc - needs to be a bit bigger for A series motors.

The problem is simple enough - it's about economy and practicality for manufacturers. With a fully tunable system like that, they'd have to individually set up each carb to find the right flow for a given engine. With fixed jets, once they find the 'right' ones for a particular engine combination, they can just get any monkey to assemble the carb using the jet/bleed/choke sizes they specify. And it's easier to keep in tune.

The SUs don't technically count, because the real a/f curve is set by the needle and the spring, the moving 'jet' is only to get the initial setting right so the needle profile works across it's range. They essentially only have to alter one setting to get them all to line up (so to speak) once the right needle/spring is found.

Of interest the Reece fish carb, and indeed the 'predator' (was hailed as the replacement for holleys etc when first made, but never really gained massive popularity) carbs are all more or less infinitely adjustable. Which is good if you have the time and patience and skill, but most potential buyers were put off by it because it's just too much to have to do to get to run well. Things like Holleys might be 'fisher price' in design, but generally if the carb size is well chosen, tben the out of the box metering blocks and air/fuel bleeds are close enough that with jet/powervalve/acc pump circuit optimisation, it's usually satisfactory for most users. To get really technical the hardcore tiinkerers will fully optimise even the holley, if it means fitting new high speed air bleeds, modifying the emulsion holes in the metering block, you name it.

SO the very short version - yes they do in fact exist,and their main drawback s that they are so inifitely tunable that some potential buyers would get lost trying to dial them in at all.;