Converting from stock Gurtner AR carburetor to a Dellorto SHA carburetor
The Gurtner carburetor is an oft-maligned piece of equipment that can function well and supply adequate performance for the motobecane user. It has the additional benefit of being designed to fit and work with the other systems on a Motobecane moped, such as it's controls, and so installation and maintenance is fairly easy. That being said, Gurtner AR's can be difficult to clean after being ravaged by time and the elements. NOS replacements can be difficult to find and pricey, so a number of riders over the years have looked to replacing the carb with a readily available and adaptable Dellorto SHA style carburetor. There is the additional benefit of swapping out a small, fuel efficient, slow carburetor for a larger, 12mm (Dellorto SHA 14/12) or 15mm (Dellorto SHA 15/15) body.
- Use the "bendy" 14mm intake, available from Handy Bikes or 30MPH parts engines.
- Fitment to the manifold can be simple or complicated depending on which style SHA you use. Each model uses a different but predictable intake clamp bushing. The two new carburetors available, the cable-choke style and the click-choke style. The cable choke will mount on to the bendy intake with no modification, however the click choke will require a thinner bushing to be fitted. You either grind down the existing one with a rotary tool, or remove it and insert a homemade spacer. If you fit your own spacer, be sure test for airleaks at he manifold when you are done.
- Many prefer the click choke model SHA carb body, as the choke slide on the cable choke model never fully closes. Furthermore, the click choke version means that you have one less cable to deal with, which takes up room, requires periodic adjustment, and can be a hassle to deal with whenever you reattach the carburetor after cleaning it.
- Cut your throttle assembly so you can pull the cable enough to open the carb all the way and let it close enough to idle. See guide on Adapting Your Motobecane Throttle for a Larger Carburetor.
- Use a #57 jet with the metal screen filter and round air cap. Use a 63-65 with a uni-filter or other high flow filter.
- Tilting the carb clockwise when looking at it from the moped’s starboard side will keep it from hitting the frame when the engine is fully variating.
- Some say using the Gurtner cable guides will help.
- If you have a variator I recommend removing 1 or two ball weights in order to utilise the new-found engine speed capacity. See guide on Motobecane Variator Repair and Tuning.
if you want to use an SHA on a budget with no throttle modification the 14.12 is a good option. i used a SHA 14.12 click choke, it went right on the stock "bendy" intake with no issues. the stock throttle assembly and cable will work perfect, i also used the old Gurtner cable guide to gain some clearance between the frame and carburetor when the engine is fully variated. the carb may also need to be mounted on a slight angle towards the engine to make it work, but it will not hinder performance at all. also, i am running a #52 jet at just above sea level at about 70 degrees F with a screen filter and it runs mint.
used 14.12 click choke off a tomos- $25 five 50's series jets from 1977- $11 having a tunable, reliable and better performing carb to replace the Gurtner...priceless.