Tech Tips

Make your Mikuni carbs work right

By Chuck Sherman

The glowing pipe is not unusual. It indicates the motor is a little on the lean side; a re-jet will solve that problem right quick.

If memory serves, the correct jet size is a 117.5 - this allows you to drop the needle up to the middle setting instead of keeping it all the way up.

Also, I believe most of the 28mm Mic's ran best with approx one turn off of a lightly seated airscrew (+/- 1/4 turn). Normal settings for this style of Mic are 1-1/4 turns.

Having rode a Mic-carbed bike with the jetting set properly, I disagree with the need to go to Russian carbs. The solutions to a good running Mic-carbed bike are as follows:

1) Replace the compliance fittings with Mikuni pieces. The OEM piece is from Taiwan, and is prone to cracking. Mikuni compliance fittings are bulletproof, and run approx $20/pair. MIkuni fittings will say 'Mikuni' on the face of the fitting.`

2) Pull the brass tube the main jet comes in, and take it down to the local carb store.  Ask for an EXACT replacement (as per the #'s on the tube).  The replacement tube will be approx 4mm longer than the one in your bike. The reason? UA had to modify the tube to pass emissions.  The 4mm cut causes the fuel delivery curve to be lean in the midrange, which is why many bikes suffer from poor midrange performance.

3) Replace the stock jets with 117.5's

4) Drill 4 - 5/16" holes in the rear baffle of each muffler.  The combination of modifications will cost you less than $50, and will add about 15% power to your midrange. The bike will feel MUCH stronger at throttle tip-in; the result is the same as if you were running Russian carbs at a fraction of the cost.

Last Trick:

a) Buy a can of Copper Head Gasket sealant

b) Pull the heads

c) Discard the headgaskets

d) Follow instructions on can, and re-assemble motor sans headgaskets.

The modification adds about 1 HP, but it does so in the low end of the power band. When you come off the light, the motor is only making about 10 HP - the additional 1 HP down low adds 10% to your low-end torque.  You can feel it.

Chuck.