Dellorto SHA Carburetor & Others

Night and day, the difference between the stock carb and a Dellorto (a real carb). Now my bike idles smoothly and I can set it for a lower rpm for idle without fear of it dying. From idle to top speed the mixture is really good and lets me accelerate without hesitation or sputtering. I thought the Dellorto 12mm carb was equal in flow volume to the stock carb so that is the one I bought. (Actually the 14mm is equal). Even though the venturi is only 12mm I gained 1mph top speed on level ground. I bought it for use with my homemade reed valve which is why I didn't buy a larger carb. Surprisingly it doesn't act "smaller". The Dellorto Direct sent mine with a 15mm inner diameter (where it clamps onto the intake manifold) although the site says it comes with the needed 19mm for our manifold. So for a stock 48cc Grubee engine you could look for a 12mm Dellorto carb and a 19mm insert. If you have a 66cc engine then there's a 14mm Dellorto copy, the $60 Performance Carburetor from JNMotors. Just be careful not to let it have an air leak. I finally figured out that you have to remove the plastic piece from the carb (while prying open the outlet) and push the plastic piece onto the intake pipe first and push it 1mm farther than being flush with the pipe end. That way the pipe will touch the smaller o-ring in the carb outlet, and the plastic will touch the larger o-ring in the carb outlet. Once I did that it worked fine. The part you need to push the fuel line onto comes pointing downward so you need to loosen the screw and rotate the "fuel banjo" to point upwards. Behind that part lies the fuel filter screen. Its not as good as using a separate paper filter in the fuel line. The throttle cable fit into the carb just fine but there was way too much slack in the cable and so I had to unscrew the carb and throttle adjusters out as far as they could safely go. That took out the slack.

You can buy the 14mm carb or 16mm carb from Dellorto Direct for $66 and $63 respectively. Hopefully you will get the 19mm insert with it.

  The 12mm carb came with a #58 main jet which worked just fine for me although I was at an altitude of 8500 ft. Using the altitude/jetting chart it is best to also order the 60's main jets kit so you can install the #63 jet if you are at sea level, or the #60 jet if you are closer to 4000 ft above sea level. This carb comes without an air filter and so I bought a lawn mower filter and cut and silicone sealed it together to work with this carb with a hose clamp securing it. (Don't forget to oil it.)

Also, the throttle cable comes out from the carb straight up, not at an angle like the Dellorto site page shows.

This carb utilizes only one changeable jet, the main jet. I thought I would most likely have problems with it, not being able to change the idle or needle jet, but it self adjusts by the main jet size which also affects the other two fuel regulations in the carb. It is an amazing design and perfect for someone who has no inclination of being a carb tuning expert. Eventually I figured out that having the perfect size main jet for top rpm running left it a bit too lean at low rpm's and so with a micro drill bit I added another hole at the bottom of the orifice column after making a flat spot there with my rotary tool.

For the rpm's I am able to achieve with this carb it looks like the benefit of having three gas exit holes allows much more perfect fuel atomization and let's you cheat by 2mm carb size for the top rpm you want. (see chart on my reed valve page for carb size and rpm). So far I really like this design although it lacks full adjustment capability.

Air Leak
Jan 2013:  I moved down to the coast and then started having problems with the carb. I discovered that there is a small crack of space between the front of the slide and the body of the carb that allows a small air leak which leans out the idle mixture which is why I had to drill that third hole. If you put the slide in its slot and shine a light into the exit of the carb (while looking from above the carb) you can see this slit of light where air can enter into the mixture path. This can be corrected by putting a thin layer of JBWeld on the front of the slide and after an hour insert it into the carb slot and move it up and down to scrape away any excess JBWeld. Let it dry all day. That should stop 90% of the leak.
Then I also noticed that there is a gas exit hole for idle speed. The picture below shows it.

So now my opinion of Dellorto is that it's just right for use at high altitudes but problematic otherwise. At lower altitudes it's OK for someone who's not concerned about getting the best performance but these modifications will have to be made to really be happy. Dellorto sells a good adjustable carb, the 16mm PHBG for $95, and it is fully adjustable unlike the SHA models. But its clamp on size is 24mm.
Here are the hole sizes for the
Dellorto 14mm carb that I settled on for use with my piston port intake 48cc, starting from bottom to top: .7mm, .52mm, .8mm, 1mm. Main jet is .64mm. I use a micro drill bit set to make holes and, along with a digital caliper, to measure holes.

Alternative Carburetors
Right now I have a 18mm Mikuni carb for my 55cc and I had the local machine shop make a custom intake for my Rock Solid reed valve to match its 23mm clamp-on size. It only costs $70 from Treatland. Once jetted right it worked very good. They also sell a 16mm Mikuni with 21mm clamp on size for $85 which I would of got except for the fact that my engine revs to 8300 rpm and so is thirstier. For lesser engines another option is the $80 15mm Walbro carb from JNMotors with a $25 intake manifold for non reed valve engines (32mm bolt spacing) or $25 intake manifold for engines with a Rock Solid reed valve (also 32mm spacing). If you have a 55cc cylinder with 40mm spacing JNMotors will be carrying that intake adaptor size also. JNMotors also sells a $190 kit that contains the Rock Solid reed valve, intake adaptor for the Walbro carb, 15mm Walbro carb, velocity stack, and air filter.