Modern Vespa

I've been pondering the differences of these carbs, and their pros and cons.

SIP is selling a V-One cases with a VMC244 kit for $5,200, with what looks like a 24/24 style carb, and I'm wondering "why"? Just for Autolube?

And I've been pondering going with a PHBH28 (or 30) but am having a hard time losing Autolube. Not a big deal really because I premix for the 177, and all Lammys are premix.....

So Pros and Cons. Please express your opinions, thoughts, etc.

24/24 Pros
Autolube
Low profile
Been around forever
Super adjustable with the bazillion jets

PHBH30 Pros
More modern?
Easier to jet?

24/24 Cons
Been around forever
bazillion jets and PITA to dial in

PHBH30 Cons
No Autolube
Taller profile - might hit frame

So can anyone wax poetically on these two modes of fuel delivery?

Want to have your cake and eat it too? I've wondered about modifying an automix airbox to retain just the oil pump and the base w/ injection channel and fitting a side draft adapter to it. Seems like as long as you inject the oil somewhere downstream after the carb adds gas like a 24/24 does it might just be possible?

If you wanna give this a try I have a 24/24 automix airbox setup complete w/ pump I could donate for you to chop up to give it a shot. I know, I KNOW - this just makes fitting the carb in even tighter , but maybe...

I am not a mechanical engineer but my common sense tells me that the 244 cylinder kit with its nice engine cases would be performance hindered/limited by the 24/24. I mean, I get it, it bolts into a stock location and retains the oil lubrication but when you go nuts with a big block I think at that point you go with a bigger carburator and perhaps just do your 2 stroke oil mix.

For me, I am the type of guy that likes simplicity so the 24/24 with a average aftermarket cylinder kit would just do it. To answer the question, yes, I would have prefer a bigger carburetor if I am going with such bigger and more powerfull built.

Don't forget the 26/26 and 28/28 too. The SI has no manifold and sits right on the crank, hence punches above it's weight.

The PHBH is also an old carb been around 50 years or so. Has a round slide and was designed for 4 stroke engines. We use them with 2 stroke conversion parts. Better 2 stroke carbs are available.

If under 20bhp an SI carb is ideal, tricky to jet in well but it is possible. PHBH is cheap and very easy to jet but often end up with poor mpg.

Not going to lie but when I saw you post the VMC 244 my jaw hit the floor. Jealous yes, do I want one, absolutely!

I've tuned many round, flat and CV carbs without difficulty but I don't know jack shit about these SI style carbs. We'll see how that goes on my 221. How about a Lectron carb?

Needed a carb for my KTM since the Mikuni was destroyed by ethanol for sitting unused for a few years by the previous owner. Decided to try the Lectron. Easiest and quickest carb to tune by far. It starts easy, idles nicely and flat out rips through the whole rpm range. I really like it and would put one on my build. Just something for you to ponder.

I noticed you stated the SI was a "pita to dial in" and a phbh was "easier to jet".
Did you mean the opposite?

An SI is pretty easy to set up, especially if you know the base settings; and you can easily access the jets on the fly if you need to fine-tune later.

On a phbh, there is way more overlap between idle jet, needle height and size and seat size, and main jet. In my experience, it takes ages to get it in the right ballpark.
And each time you want to try a new jet you have to empty the &€#%¥ bowl first, which usually means pulling off the carb. And don't forget you'll never get it right if the float height ain't tight.

If you want a big carb, look at a keihin PWK instead. Never tried a Lectron, but everything I read about them seems dithyrambic.

On a modified engine based on my experience setting up an Si is much much harder to do than a phbh or similar. Also I think if done right a phbh is much smoother. On the other hand if you figure out the Si it's extremely satisfying. It would be interesting as V oodoo inquired how to integrate the autolube system with a modern carb that could be a cool combination.

As Jack said, SI has an advantage in that it sits directly above the crank. Shorter distance for fuel to flow, so higher velocity and better fuel economy. On a mild tune this is a great choice. Even up it to 26 or 28 per Jacks recommendation. Con for me is the numerous jets and air correctors that need to be dialed in. Stock+mild kit + pipe: upjet and a bit of tweaking and its done. On a HP build, there are many different components to fine tune, and can be alot of back n forth. Thats a big con for me. Although, there is alot of info around for jetting options that work.

PHBH has just three main components to tune. Main, needle, idle. Idle is easy, working the main and mid range needle is fairly straight forward IMHO. Start rich and work down per usual. Round slide is CON, as a 30 round is actually more like a 28 flat. Long distance of fuel travel is a CON, and larger size is a CON as well, but minor downsides IMHO. Fuel economy is not great, ask me how I know. I really like the advantage of simple design for tuning. Dismounting and disassembling the carb to change jets is not the most fun, but its quick.

There are other 'better' modern carbs out there. All follow basicly the same principles of jetting with a slide/needle/main/idle circuit. PWK as mentioned, Mikuni, Dellorto VHST. Many of these are used in the 2T kart racing world, and 2T dirt bikes of course. Smart Carb is another options of going full hog. If going big HP with a reed, modern carb is the only option.

Had to look up that last word
I have Keihin PWK and Mikuni TMX on my old lists of bail-out-of-the-SI options.
There's also the $mart carb.


https://www.sip-scootershop.com/en/product/carburettor-smartcarb-26-billet-sc2-flat-slider_618426SC?cr=e993ab67-6807-4109-bbed-f6374ebf9f9b&gad_source=1&gclid=Cj0KCQjwzby1BhCQARIsAJ_0t5PG93Nm1YztzAqPnt2cAQiR0KyhHboPXdFtP3xfdPx1Ze3rHSnfCRYaAihXEALw_wcB

The SI is pretty easy to set up if you have a re-worked carb*, and enough jets and slides to sort it. That, and as you say, a baseline to work off of.

* BGM (channel to the MJ needs drilling to 2.5mm) or a Pinasco out of the box.

Original or Spaco carbs have more fuel flow restrictions (in the float assembly) than just the 1.5 channel to the MJ, that will add jetting complications to a kitted bike*.

*Fuel line in the bowl drops, lowering the fuel line in the well of the MJ stack under sustained load. You can compensate for this with jetting, but you'll be jetting for that point and not the rest of the ride, or do damage.

Thanks for all the info everyone. I learned a bunch and have put together a better gameplan for my next build.

I see no reason to go with anything bigger than a 26/26 carb. And I'm going to stick with Rotary vs reed. And I think a Malossi MkII Sport would be the ticket, with a 57mm crank. Before I pull the trigger on it all, I'm going to start a new thread and ask what y'all think

Thanks again for all the replies and info!