Over-boring an SHA

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This is a How-to to overbore your stock Dellorto SHA carburetor to increase performance without having to buy a new one. 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, 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 14:12, 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 don't 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.

OK so, briefly, what we're going to do here is pretty simple, first disassemble the carb completely, then we have to pull out the emulsion tube (brass tube thingy in the middle of the venturi), 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

Remove the Emulsion Tube

(Here's an alternate way of removing the brass atomizer.)

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 dingus 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.




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 venturi 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 don't 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 don't 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 don't 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.


(Watch out for burrs.)

Watch out for burrs.

If you use a steel pick tool (looks almost like dental tools), you can use it to smooth out and remove burrs. Steel is harder than aluminum so just rubbing the edge of the hole can break off the burrs. You might have to work at it some.

Pick tool deburring.jpg

You can use this tool and technique to get into the impossibly-small space of the slide slot.

Inside the carb slide slot.jpg

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 pliers.

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.

For info on function and tuning see Dellorto SHA

Also See Modifying the throttle slide on a SHA for help tuning in that idle.

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-Dellorto SHA air filter mod