Adapting a PWK 30 Carb to a '48 4G Square Four

[DaveD]

I have a 1948 Ariel Square Four 4G. The original Solex is junk. Couldn't get it to more than start and run poorly. I’ve tried maybe 8 or 10 different carb setups on it. Even a tiny 24mm Mikuni. Remember—the early 4G has the dynamo way up high so there’s precious little room for a larger carb.

I have made a 90-degree elbow manifold and have fitted a new PWK 30 carb that faces out the left side of the machine. I tried various Mikuni and Amal Concentric carbs there but the vacuum of the four cylinders pinned the slides to the bore of those carbs, holding the throttle open. The PWK flat slide (with a heavier return spring) doesn’t have that problem.

The bike will start easily but it had a flat spot transitioning from the idle circuit to the midrange. I dropped the needle clip to the #4 position (raising the needle one notch and enriching the mixture). The flat spot was gone.

But at idle the bike blows out thin black smoke and runs lumpy, indicating a rich mixture. I replaced the original #38 pilot jet with a #32 jet to lean it out. That did not appear to fix the problem. The pilot air screw adjustment seems to have no effect whatsoever on the idle. It can be all the way in or screwed all the way out. No difference.

Considering that this is a quartet of 250cc cylinders cycling in quite fast sequence (twice that of a twin), it seems that the idle setting for a 250cc single would be the key for this application, unless there are other dynamics in play, such as the continuous vacuum signal of the four.

Generally, the bike starts fine, can be made to idle (although with the idle speed adjustment screwed well in) and runs fine through the rest of the ranges, but the smoky, lumpy low end needs to be corrected. It seems the idle circuit isn’t in the game at all, since adjusting the pilot air screw has no effect.

Any experience with the PWK (it’s like a Keihin) carbs out there? I feel like I’m this close to getting it right, and the installation is clean and logical (although there’s no room for an air filter as it would foul the battery box).

[simon.holyfield]

What's wrong with the Solex?

[will_curry]

Found on the forum a while ago:

"The 1BJ is the post war designation for a bottom feed horizontal carburetter float
chamber which is only found on one application paired with a 275 - 275BT/1BJ.
This is a 1952 specification for an Ariel Square Four assigned to the Rich Child Cycle Company
of Nutley, New Jersey.

The specification uses a 275/414R body which our records show was created from a
275/154R body (from a Triumph 3T) modified with the Pilot Outlet opened to 0.028" and
the bolt holes elongated to suit a 48mm stud spacing. The carburettor used a 130 Main Jet,
.1065" Needle Jet and a 6/4 Throttle Slide. The needle position is 3."

The specification may be of some use.

[nevhunter]

Don't slide Carburetters tend to Jamb on a Sq4? Nev

[Chris Sealy]

It seems to me that you need a PWK30, not a square four, expert; to answer your questions.
The fact that the idle mixture screw does nothing suggests that the throttle slide is already too far open at idle; and that most of the idle fuel is actually being supplied by the needle jet.
It may be that a 30mm carb is simply too big. The original Solex was a 26mm, reduced to 23mm.

[DaveD]

Simon - The original Solex is all worn out and I couldn't get it to run right. Dragenfly had no parts for it.

nevhunter - The Amal slide carbs all jammed from the constant vacuum, but the PWK with optional stronger spring seems to work fine.

Chris ~ I believe you're right. But I can't find one, so I'm developing my own understanding of carburetion theory. I have a new 24 PWK. I'm going to try that on it. If and when I figure out the carb size, jetting, etc., I'll share it with the site so we all have a quick and easy "go-to." You'd still need a 90-degree adaptor though for an early 4G as the dynamo is so high up that there's no room for a float bowl.

[DaveD]

"By Jove, I think I've got it!"

I think I've solved the carburetion project on this bike, not using the old Solex. I've adapted a 24mm PWK flat slide, spigot-mount carb (copy of a Keihin, jetted for four-stroke) on a 90-degree intake adaptor I fabricated. I've been experimenting with carb size (the 30mm was too large to maintain vacuum), jetting, needle position, etc. So far I've got a bike that starts, idles, and runs up through the range cleanly, on a 32 pilot jet, a 125 main, and the needle all the way down in the lowest position (to prevent constant-vacuum siphoning past the needle jet and thereby bypassing the whole pilot circuit and flooding the idle circuit, a problem I discovered through trial-and-error).

After I take it on a few longer rides, I'll see if it needs a few tweaks. But as a cheap alternative to a new or rebuilt Solex, it might interest a few people. Fabricating the 90-degree elbow is the only aspect that will require some light machining, or perhaps you might find something that works. With the Mk I Square Four, having its generator lowered significantly from the high position of the 4G, you might have sufficient clearance to not even require the elbow that I used to re-position the PWK pointing to the left side as in my photos.

These carbs are available new online as cheap as $30-$50, or from JRC Engineering, for around $135 and already jetted for four-strokes but still requiring different jets as I discovered. https://www.jrcengineering.com/air-fuel ... rjet-carb/

Hopefully this is useful information to someone.

[Roger Gwynn]

Good and interesting, I am sure this will be very useful. Please keep us posted on any further developments. I have posted recently about a French specialist or does good restoration work on Solex carbs, recommended to me by a continental member.

[DaveD]

Roger: I probably have maybe $100 invested in the current carb, cable, jets, parts to make the elbow, etc., and many weeks of trial-and-error testing, removing, dismantling, trying a different jet here, needle setting there, reassembling, re-testing, etc. While tedious, it's part of the fun of being resourceful and keeping these old beasts alive and on the road!

Of course I've conveniently forgotten about all the failed tests with a few different Mikuni carbs, several Amals, that first 30mm PWK, etc. OK, it's been months, not weeks.

~ Dave
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[Roger Gwynn]

As long as you enjoy doing it, which you would be unlikely to be doing it if your didn't, I suppose.