KEIHIN 35mm FCR Pumper Carb Tuning
by Dennis Arnold

Stroker Recommended FCR settings:
(with aftermarket header and free mods)
  • Main Jet = 168   Pilot Jet = 28
  • Needle = Yamaha OBDVR at 4th clip
  • Air Screw = 1 turn out   Fuel Screw = 1.5 turns out

BOTTOM SCREW... Sudco calls it the "Pilot screw". but FactoryPRO and Stroker call it the Fuel Screw Whatever its name, it's in the hole above the Main Jet plug, and turning it out richens the circuit. It's easier to get to with the bowl off the carburator.

Adjust the Fuel Screw so when you rev the bike in neutral and release the throttle, the revs quickly drop to idle again. If, when you release the throttle, the revs hang up a few hundred rpm above idle, then drop to idle, you are too lean. Turn the fuel screw out. If, when you release the throttle, the revs drop below idle, then pick up again, the idle mixture is too rich. Turn the fuel screw in. In extremely rich cases, the engine will die after revving the bike and releasing the throttle.

The FCR is very sensitive to pilot jet size -- the main culprit in hard starting. With the carb installed, the fuel screw is difficult to reach -- cut off a small screwdriver to adjust the pilot circuit while the carb is installed.

BACK SCREW... Sudco and FactoryPro call it the "Slow Air Screw" but Stroker calls it the Air Screw. Whatever you call it, it's on the left in the pic, and turning it out leans the circuit. To get to it, the air horn must be taken off the back of the carburetor.

In this picture you can also see the "L" shaped bracket on the right side that the black plastic pump actuator rests against. The end of the bracket has a split in it. You can widen or narrow the split in the bracket with a flat blade screwdriver to increase or decrease the throw of the pump actuator arm. This will increase or decrease the amount of gas that gets shot into the cylinder. The motor will bog off idle when the throttle is twisted if this is not adjusted correctly...

The headstay bracket was modified to allow the carb to fit -- the triangular piece of metal on the right side was ground off.

I used a piece of rubber cut from an old truck bed liner to raise the gas tank slightly so it would clear the top of the carb -- I attached the spacer to the existing rubber spacer.

The Taffy mod uses a collar that limits the travel of the shaft that pushes on the accelerator pump diaphram. By limiting the plunger travel, you can limit (at least in a shorter direction) the length of the AP squirt. Instead of the collar, Kevin made a screw to adjust the pump arm squirt. It includes a spring so it works like an idle screw, plus you can adjust it on the fly-no, tools needed! The adjustable screw limits the depth of the rod that plunges the diaphram, just like the collar mod does, but the screw controls where the plastic arm stops, which is what pushes the pump rod. Turn the screw in, the more the arm will travel, out, less travel.