This article addresses in detail the very common Dellorto SHA carburetor. Much of this was originally by Graham Motzing on his blog Fast, Cheap and Out of Control; http://outofcontrolmopeds.blogspot.com/2012/03/dellorto-sha-boring.html.
Dellorto SHA carburetors are common, cheap, easy to tune. They contain probably less than half as many parts as are in the carb in your leaf blower, but because of that they are highly modifiable, robust and easy to work with if you know some of the tricks.
SHA carbs come stock from a few different manufacturers. Mostly Italian brands, Minarelli, Garelli, Morini, Tomos A35 (very few A3) and some of the later model Motobecane and Peugeot.
There are Chinese clones available. By the time you finish tuning the carb, it really won't matter if it is a vintage or new carb, and the clones have been found to be adequately functional. The clones are available for $20 and up. The nice thing is that all the seals and needle are a decent quality. It's common in used SHA's to leak at the float needle.
Dellorto Jets are 5mm by .8, which is a commonly available size.
O ring that thing
One of the most misunderstood design features of the SHA is the design of the clamp and the proper way to mount the carb. Nearly all criticism of the SHA in one way or another stems from a basic misunderstanding of this process.
When looking at the clamp, you will see the mounting flange is split in two directions.
This allows the clamp to compress around the intake when you tighten the bolt. This clamp is not designed to provide an air tight seal. If you file the clamp or over tighten it, it will snap.
Often times, but not always, you will require a nylon shim to allow the carb to clamp tightly. This shim can, in many setups, create an adequate seal between the carb and intake, but it's generally safer to add the O ring as well and seal it the right way.
To properly seal a Dellorto SHA carb you will need to use a rubber O ring between the intake and the carb.
Get a proper O ring that will just fit inside the diameter of the clamp. When mounting the carb, maintain some pressure pressing the carb on the intake while tightening the clamp so the O ring seals well. Properly done, this will make it nearly impossible for a SHA to leak at the intake.
The SHA can be tuned on the top end via the main jet, and uses standard 5mm round jets. Tuning at idle and low throttle is done by changing (or modifying) the slide. The slide cutaway refers to the carburetor circuit which controls the air/fuel ratio from one-eighth to one-quarter throttle opening on slide type carburetors. The height of the cutaway portion of the carburetor slide controls how much air is mixed with fuel, the larger the slide cutaway number the leaner the transition off of idle will be.
Ok so now we should probably discuss how this thing works a bit more. Fuel is flowing up through the main jet. When it is at idle and the throttle is closed, fuel is flowing out that tiny hole in the bottom of the slide slot. As you open the throttle you expose the other two holes and the fuel flows out of those holes. As air is passing the throttle slide it is going from high pressure low speed, to low pressure high speed, by Bernouli's principle. This is why the throttle cutaway matters, the cutaway will determine how much the air speeds up. When you put a slant on the bottom of the slide, known as 'cutaway' the air gets slowed down before it hits the emulsion tube. This has the effect of raising the pressure of the air at the tube, which reduces the amount of fuel that gets sucked up through the tube, in effect making you leaner.
This crappy drawing shows how the cutaway matters more at less than 1/2 throttle. You are in effect creating a nozzle, you go from a small opening to a bigger opening and as a result the air slows down. You can see above how when the throttle is mostly closed, the ratio between the front to the back is bigger than the ratio of front to back on the second drawing. This means that at low throttle, the air gets slowed down more, and the cutaway has more effect.
This is cool because 2 strokes tend to have a peaky power curve and therefore a peaky fuel delivery curve. When your pipe hits you are making more power and need more fuel (richer) than the linear curve that most stock-ish engines need. For most two strokes this means that the wide open throttle/ high rpm conditions require a dumping of fuel, but when the porting isn't flowing very well the mix should be leaner On more advanced carbs you can use smaller idle jets and an atomizer tube setup for progressive fuel delivery. On the SHA you are stuck with a relatively constant delivery of fuel.
Another interesting point about these is that reed valve engines (which as we know make more power in the low-midrange than piston port) will work better with the stock-ish flat fuel delivery curve. Thats why SHA's are popular among Peugeot and AV10 dudes, they run pretty good out of the box. For a piston port engine that has less midrange, especially if its a big kit or ported, you will need to modify the carb. This is also why, with PHBG or Mikuni carbs, reedvalve engines will work well with a 'four stroke' atomizer, which has a flatter fuel delivery curve, and piston port engines need the '2 stroke' atomizer.
So by mimicking that effect by changing the taper on the bottom of the slide, you can run a larger main jet so your top end is nice and fat, and your bottom end stays crispy. If you have ever ridden a Puch piston ported 65 or 70 kit with a unmodified 16 SHA you will notice the rich bottom end, this wont hurt anything, but it makes your throttle response on the low end poor and when you mod a stock small carb, it is really bad. Porting a stock cylinder can make this really bad, and the stock cylinder doesn't make enough torque in the low end to make up for it.
The size and placement of the emulsion tube holes also plays into the fuel delivery curve. Unlike carbs with needles, air bleed, and different emulsion tubes, the SHA is very simple: want more high-end fuel delivery, drill out the top hole, want more low-end drill out the bottom hole. Usually when modifying a 12mm carb to 16 mm or so, you will need to enlarge both of them some If you have a set of jet drills, which you should, just reach them in the intake there and drill it out, if you don't, you can use the corner of a file to slit them out bigger.
If you combine the progressive effects of cutting the slide, and drilling out the holes, you can play around with things enough until the carb works very well across the entire powerband. If you use jet drills you can stick the emulsion tube in once and forget it, if you have to keep pulling it in and out you will want to chew it up with some pliers on the seating area when you put it in for good, so when you push it in, it jams in there. It would be really frustrating if that thing was loose.
The Crazy Wayne Notches
From CW, to improve SHA performance and lean out the rich condition at idle:
Just use a #2 or #4 slide on the SHA or put two small V-notchs on the bottm of the slide, Make them .5mm to 1mm deep, with one on ether side of the atomizer tube like so. S L I D E l***T***l . l***u***l . l***b***l . l***e***l . ---^I^---
Follow up: Added 4/24
This took me a while to get to, had a lot of projects get in the way, but very slowly over the last month or so I've been getting the Peugeot back together. The variator got modified to delay shifting and keep the engine in a higher rpm power band. The SHA got installed along with the intake manifold, and the engine got mounted back in the bike.
At first, the best i could do at tuning was to get it running ok-ish by drilling out jets and swapping around air filter pieces. The float needle was leaking like crazy, so fuel was pouring out and making the low end impossibly rich. It took a week for the needle to get to me, and with it some appropriately sized jets in the 60's and 70's.
This would be a good point to remind you, of course, that its absolutely necessary to have a properly functioning carburetor. All the parts have to work. This would seem obvious, but i've ridden bikes for people 'hey, can you ride this and see if i'm jetted right?' followed up by, 'oh yeah it leaks a little, you've got to rev it to keep it running.' Well, shoot, there's part of yer problem right there partner!
I've done dumb tricky stuff to try to save old crusty needles like filing a new point on them and shimming them up in the float with some beer can, but to be honest, its just not worth your time or mine. You will spend more than $5 on the gas you loose when you forget to turn your petcock off, and nothing gets you kicked out of shop spaces or gets people mad at you for parking your bike on the sidewalk, like a puddle of gasoline. Something about fire hazard, blah blah, yeah that too.