MOPED ARMY

The VM20 is going to be a bear to tune, and horribly inefficient for this application. The SHA is more than adequate, even with a circuit pipe.

The size of the vm20 on your 50cc setup. You already have to tune them way down for a 70cc bike.

Get a bigger Sha tho and have some fun. To me, a Sha is so much easier to tune since there are fewer "stages" to it so less tuning options in a sense which makes it a little less daunting to get tuned up.

Grab a 15sha and accompanying intake. Get some jets in the high 50's thru mid 60's range.

Upgrading the pipe will help a lot as well. Ask questions as you have them.

Dellorto makes the SHA, and on a setup like this you don't have to muck with slides.

You can attempt to put a VM18 on this, but general rule of thumb is you want venturi size to be at most 50% of bore. So, on a 38mm~ cylinder you wan a ~18mm carb max (and that's theoretical maximum, so including a pipe and porting). Aka, a stock port and intake, and a 12-15mm is more than adequate and youll get better response from the smaller carb.

You don't need to dick around with slides. I run plenty of kitted and piped bikes with SHA's with standard slides and they are fine.

And you can have a carb that has swappable parts, but probably will never tune well for your porting/bore size; or a carb that will function just fine for a stock port map and pipe that requires you to buy some jets.

I sold a bike once, 50cc with a 15 bing. It came back to me, still 50cc with a nice proper VM20 and intake all done up. It was almost $150 and it was distinctly worse with the VM20 just cuz it didnt really pull enough to be useful. you gotta really scream in the RPMs to need a carb that big on a 50

sha is easy, and sure it's less tunable, but that means it is easier to get to "good enough" with less tuning required...

You must like the working on way better that the riding on part of this hobby . LOL

Go ahead and squeeze the pulp , but , it'll still be just a moped . ;)

I mean, stock (+) mopeds run right with a SHA, without modding slides.

You wanted opinions on whether it was a good idea, everyone seems to think the SHA is going to be fine. If you still want to go Mikuni for your own reasons, roll with it.

You ride one of the bikes I have a Sha on and tell me what it is missing.

But the question didn't sound like it was coming from someone who tunes mikuni already, it sounded like a question about how much carb should I buy for this stock cyl bike.

Go big and tune up that mikuni if you have the chops. I personally would go with a Sha cuz I can tune them really easily and really well.

SHA's respond really well to a properly tuned airbox, especially on a stock moped

I really like the PHVA's and have had a few real awesome Amal setups in the <18mm on a 50cc range.

If you wanna do it right right tho, make it USE that VM20. port the dicks out of it and get that pulling 5 digit RPMs, cuz on stock, to your original Q, yea it's kinda too big

I think the point most folks are making tho is the SHA is just an easier reliable setup if you're jusst building stock +

And swapping slides is pretty easy, if you're doing it in detail you're swapping needles in the Mik too

But I absolutely feel tuning what you've got. I'd say at least 4/5 builds just wind up with whatever carb I have, or have jets for. my comments are more considering/comparing buying new intake & carb - if I was gonna buy one

I've swapped equal size bings and SHAs and never felt like one was any bit better performing in the end. I like the click action of the choke on the SHA better for starting, and filters are cheaper and more available. SHA idle issues I've only ever had problems with the tiny little idle hole plugging (not the jet, the circuit under tha main bore), and that's easy. The bings maybe have less blowback? but yea just for having a needle vs slide i've got no hard prefs

So many ways to start making and modding air boxes. You can just use the black cap they come with when bought new. And remove some screens to allow more air flow. Even add some air filter foam in for additional tuning options.

A few boost holes in the slide and some screens out of the filter pack and they really wake up.

And really, 'tuned' is relative, it's mostly just about damping out the pressure fluctuations in the carb so the tuning isn't all fucky. You can go to the next level and actually make it 'tuned' to displacement, rpm, intake porting, but the biggest gains are just from having anything on there at all to give the carb a consistent pressure signal.

Graham, it does seem that Sha carbs work better on reed setups for that reason of the reeds making a better consistent and contestant pressure.

Yeah exactly, the air in the airbox acts like a spring recovering that energy and smoothing out the reversion wave. If you actually tune it you can recover that pulse like an expansion pipe, but it often makes the packaging difficult on a moped. Even in a full size moto it ends up taking up major real estate in the frame.