Dellorto SHA carburetors are common, cheap, easy to tune, compact and complete junk. They are made about as cheaply as you can possibly make a operational carb, 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. Starting with a standard 12:12, you can create a very functional and precisely tuned carb that will easily perform as well, if not better than the stock 16:16 or equivalent, but dont expect to slap it on your kitted puch and have it run out of the gate.
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 also Chinese clones that are out there, possibly Indian as well. By the time you finish tuning the carb, it doesn't matter if it is an old one or a new one, so I've found the clones to be totally functional. The clone used to be available for $20 so I bought them, but now they cost more. The nice thing is that all the seals and needle are good, most used SHA's will leak and need a new needle. The bad is sometimes they need the jet tapped to the correct dellorto jet size. Dellorto Jets are 5mm by .8, which is a commonly available size. If you can find a bottoming tap that will help, otherwise do the tapping with the emulsion tube removed.
|Lever Choke Carb w/ filter- $50 from treats (Jets do not fit)|
OK so, briefly, what we're going to do here is pretty simple, first dissassemble the carb completely, then we have to pull out the emulsion tube (brass tube thingy in the middle of the venturii), then bore out the inside of the carb. The emulsion tube goes back in, the carb gets reassembled, back on the bike. The carb is then tuned by modifying the holes in the emulsion tube and the slide cutaway. Fun!
Part 1: Boring
Dissassemble the carb completely. You can leave in the idle screw if you want to, maybe the choke if you're good. Clean up everything as well as you can, tooth brush and carb cleaner (and safety goggles).
The emulsion tube is pressed into the carb underneath the main jet. When you remove the mainjet you might be able to see the brass lurking down there. You have to slide it out the bottom through the main jet and its fit snug. The first few of these i did, I used needle nosed pliers on the inside grabbing the tube, then someone told me this trick and it blew my mind.
Find a very small screw, I believe the one I use would be considered a #4 wood screw. You need a #2 phillips head to drive it, so its pretty small.
Screw it into the hole in the bottom of the emulsion tube. You'll have to press down firmly to get the thread to catch, but it will. The smaller the screw the better the thread catches. Tighten the screw reasonably tight, I'm not sure how to describe it but you'll feel the threads grabbing in the brass and stop before it strips the threads.
Grab a blow torch, propane is fine. You could probably put the whole dinghus in the oven too, but it might smell like varnish gas or kill you or something. Here I am using team overkill to very lightly flick the bottom of the jet snorkel thing with some heat. The carb is some bogus zinc/aluminum/matchbox car crap metal so it will melt pretty easily.
Now you'll grab those diagonal pliers you see sitting there pinch up against the head of the screw. This is why we put it in the vise upside down, you can give the joint of those pliers a quick tap with your right hand while you hold the pliers with your left hand, and that screw will yank the tube right outta there.
BOOM! TUFF ACTIN' TINACTIN
Its ok if you're excited, this is mopeds after all.
So now you're looking at this:
Now the carb is ready to be drilled out, see no tube in your way!
I didn't document the whole drilling out process, lots of other people have talked about it and discussed it on the forums, but its really up to you to figure out how you want to make that hole bigger. Personally I do it on a lathe with a drill bit and a few reamers, then I use a taper reamer to give it some venturii effect. You can just use a large drill bit and do a pretty good job if you go very slowly, I would definitely recommend a drill press at the very least.
This is a picture of one i did a few years ago, I'm pretty sure we're doing 5/8 here, just under 16mm. If you have a mill you can offset the reamer slightly so you dont break through that thin edge on top, but normally the biggest you can get these is somewhere around 15mm.
Some people with very steady hands even do this with a dremel and a sanding drum or stone or something. Crazy! If thats all you have, go for it, just be careful. I've never done that so i dont know how well it works.
Now that your carb is bored out, you'll have to clean up the burrs. The most important one is where the throttle slides in, if you dont file that off your throttle plate can jam and leave you stuck on full throttle. Not good. Also the emulsion tube hole in the bottom will have a burr which should just knock out with a small drill bit, and the inlet and outlet of the carb might have some junk. Small files work the best for this, if there is a lot of grungus you can get a dremel in some of those places.
Once everything is cleaned up, we'll have to re-insert the tube. First, it makes it a lot easier to sand down that press fit a little bit so you can get it in and out without having to heat it up. The shoulder at the bottom of the tube is what seats in the carb so get some emory paper or sand paper and chuck it in a drill. Be careful because it goes fast, you just want to take the tiniest hair off. Try it in the carb as you sand it down and see. You want it to go in about half way then get snug.
Dont install that tube just yet, there is another tricky part we have to check on. If you look closely at the bottom of the slide groove, from clamp side of the carb, you'll see a tiny hole. That has to line up with the tiny hole on the bottom of the emulsion tube, or the bike wont idle. This is probably the biggest reason people who drill dellortos cant get them to idle. In most cases the hole lines up with the other two holes in the carb, but sometimes that hole is slightly off. In this case it is way off. I'm pretty sure they drill these after the tube has been pressed in, so if Vinnie had a rough night last night, yours might look like this.
Here I've already marked with a black line the location of that hole, when you go to insert this thing you'll have to line up the black line with the hole you see sighting in from the clamp side of the carb.
Cool. The last step is to check that you lined the hole up right. You need a small strand of wire. I find that grabbing a strand from a wire brush and yanking it out works really well for this. Poke through that hole and make sure it went all the way, Not sure how to explain the feel of this, but you'll know it when its right. If the wire doesn't go in all the way, you have to play around with pulling the tube out and turning it slightly either way. Now that you loosened up that fit a little you should be able to do that just tapping it out with the pilers.
And now you have something like this, ready to be tuned.
Part 2: Setup/ Tuning
The SHA is tuned by adjusting 3 things: Main jet, emulsion tube holes, and slide cutaway. I could also include air filter in there because it makes a huge diifference on the operation of this simple carb, and you might want to think about changing it around if you aren't getting the performance you want.
Setting up the carb will require a shim. Most intakes are 18 or 19mm. The clamp size is 21mm (20.8-21mm) so a 1mm or 1.5 mm thick shim will be needed to fit your intake. Here i made one out of brass and pressed it onto this intake so i wouldn't have to worry about loosing it.
I also like to use an o-ring in the groove there to prevent air leaks.
Obviously match your intake and clean things up.
Oooh la la! That looks nice!
Install everything on the bike. As far as air filters go, its going to depend a lot on your mounting situation. If you can find a stock air filter that will fit that is probably the best. Because of the way the mid-range is set up on these things, they like to have a little resistance.
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.
|Note: This picture was wrong before, thanks to Todd Kingeshaft for fixing it!|
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 mimicing 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 dont, 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.
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.