This is for tuning a SBN carb for a modified motor. If you motor is stock, you should be using the stock settings on your HS and LS screws. You can use this to make minor adjustments due to elevation or compression. If you have a stock motor and it is not running properly, this procedure will not help you.

Disclaimer from Bill O about these instructions:
“Generally speaking, most Mikuni's share very similar tuning instructions, with only minor differences.
I will tell you this, you will not be an expert carb tuner from reading someone's instructions. Only experience will give you real knowledge of tuning.”

This is a combination of multiple sources from this site for older models of two strokes. This is not a definitive all in one guide. Big thank to Bill O’Neal and all of the contributors on this site. If you really want to dig in, there are links at the bottom to the Mikuni manual and other good procedures.


Rich: Too much gas
rich hesitation is when the ski seems to go lower in RPM's and slowly comes back to life. BBrrrrrrrrrrraaaaaaaapppp!

Lean: Not enough gas
Lean hesitation is sounds like the ski dies for a split second and comes back to life. BB__rraapppp!

Bogequals a rich condition; symptoms of a bog are that the motor will not rev up right away because it needs to burn off excess fuel that is being put into the motor because the low speed adjusters are too far opened. If you were to let it idle for a minute or two, the sparkplugs would look a bit wet with fuel if you were to remove them without revving up the motor first. If the engine takes a second or two to clear out and then accelerate, the carb is too rich.

Hesitation. A lean hesitation happens when you stab the throttle from idle, and it acts like it is taking a huge deep breath before revving up, or it just completely dies for lack of fuel. The latter would be a very lean condition probably caused by the accelerator pump not working correctly, or the low speed adjusters are turned way too far in.
If the engine hesitates, the carb is lean.


Before you start.
Start rich!! start 2 turns out on both and see how the boat reacts. You are going to have to play with it because regardless what the book says you have a lot of different factors. Elevation, compression, etc. Every boat will tune different and a similar boat will have different ideal settings.

Before attempting to tune, always clip the sparkplug wires to make sure you have a good connection at the sparkplug boots on the wires. You also should know that the motor has good compression because you cannot tune a junk motor.

You cannot properly tune an engine that has low compression in one or both cylinders.

Carburetors are a compromise at best, and tuning them for anything EXCEPT idling and full throttle use is always a compromise.

Can I ruin my motor doing this? YES!

A rich motor will run terrible, but will not kill your motor.
A lean motor will kill your motor.

How can you tell if it’s too lean?
Most novice tuners cannot properly read sparkplugs.

A good indication of a lean motor is if the sparkplugs get so hot you cannot lay them on your opened hand without burning yourself. So when you pull your sparkplugs, do they feel real hot, or do they feel normally hot? Really hot is an indication of a slightly too lean motor and it takes no plug reading skills or special tools or instruments.

More plug reading tips below.