Many are familiar with reboring the PA50I stock 10mm Keihin to 12.7mm with the use of a 1/2 inch drill bit. It is possible to further enlarge the venturi without ill effects. There are a number of after-market carb options, but this can be a less expensive and more 'stock looking' alternative.
A bore of 13.5mm is about the largest than can be reamed before complications arise. At about 14mm, two portions of the venturi wall become thin enough to breach.
In addition to the thinness of the venturi walls, the throttle butterfly valve eventually becomes a problem. The cross-section of the butterfly valve minus the obstruction of the screw head, valve disk, and the rod (at wide open throttle) is equivalent to a 14mm open bore. In addition to dealing with new holes in the top of the carburetor, the throttle butterfly valve needs to be slimmed down if it is to be bored to 14mm or larger. The carburetor has a limit, however. Anything larger than 14.3mm starts to dig into the idle air tube (green arrow below). As 15mm is approached, the area occupied by the emulsion tube gets opened up.
How to make a 14.3mm Keihin
Despite a few problems, this is totally do-able. Here's how:
- Completely dismantle the carburetor. The entire thing will need to be cleaned when this is over.
- rough up the areas indicated by red arrows below. If the carburetor body weren't aluminum, we could build up the walls with bronze, but in this case JB weld works well enough. Build up the waist area about 3mm or more. Be careful not to plug the float vent hole.
- When everything has fully cured (12 hrs plus), drill it out with a 9/16 inch drill bit (14.2875mm). Use a drill press if at all possible. Start at the flange that would bolt to the intake manifold, use oil, and go slowly. It's a good idea to hold the carb with something more stable than your hand because the bit could snag and end up doing terrible things to your poor hand.
- After the drilling is completed, the walls of the venturi should be polished to a mirror. Use fine sandpaper taped to a dowel or something. A Dremel polishing wheel would be good, too. Make sure that the areas of the venturi that have been opened to the JB weld are smooth.
- Remove any burrs that may be in the emulsifier tube opening of the venturi.
- To maximize the flow rate and to properly atomize fuel, the tip of the emulsifier tube needs to be flush with the side of the venturi (blue arrow above). Install it, mark it, and then carefully file it to that length. Check your work often. Make sure to remove any burrs from the filed area. It's also a good idea to lightly champfer the inner and outer edges.
- The throttle butterfly valve needs to be slimmed or else it becomes a sort of bottleneck for the flow of air. The rod can be safely slimmed to about half of its thickness. File away the rounded side of the rod making sure to stay parallel to the flat side. When finished, it will look almost identical to the previously flat side. Make sure that the edges of the new flat area are square and are flush with the walls of the carburetor when assembled.
- Reassemble the butterfly valve and file down the end of the screw that now protrudes from the rod. A small amount can be removed from the head of the screw, too, but the flow-rate gains are minor and this attention to detail begins to border on madness.
- Disassemble and polish all of the areas that were filed.
- Clean everything very, very well. If you used a polishing compound, you may need to get in there and wipe it off with a cloth. At the very least, use compressed air and some kind of solvent.
- Note that an upjetting will be needed. I upjetted to a 90, but it depends on the setup and the bore of the carburetor. High 80's at the very least for a 14.3mm.
- Reassemble everything. Don't forget to do a plug chop. Enjoy your engine's increased breathing potential.