Power Jet Carbs

On the left is a stock Mikuni carb. On the right is the same carb fitted with a power jet. Image courtesy of: Mikuni American, Ginatsis Design Associates

 Getting the jetting right on a motorcycle is critical to the overall performance, both in terms of out-n-out power and fuel economy. Long before carburation was controlled by computers and fuel injection systems, manufacturers tried a number of different approaches to the age-old problem associated with carburetors: getting the mixture correct throughout the range of throttle openings. With hole sizes in the jets limiting the amount of fuel that could flow, variations in throttle position simply changed the amount of air that could flow.

It is easy to deduce from this that perfect mixtures (the ratio between the air and fuel) will only occur over a short range.

For street machines, the manufacturers were forced to compromise between power and economy. Typically they tended to favor economy but with a safety margin of a slightly rich mixture to assist with cooling—something that is particularly important on an air-cooled engine.

Jetting for Racing

For racing motorcycles, riders are more concerned with power and getting the jetting right is high on the list of any racer at the start of an event. This is particularly true with 2-stroke engines whose power output and rev limits are greatly affected by the jet sizes. In addition, while leaning out (less fuel, more air) the mixture on racing 2-stokes will increase the rev band and generally produce more power, so these engines are prone to seize as the cooling effect of the gasoline is reduced (this was a balancing act many of the older racers were all too familiar with).

The main problem with standard carbs (deploying a primary jet and a main jet) is that the main jet was required to meter fuel over too large a throttle opening. To remedy this problem, Mikuni introduced the Power Jet carb.

Operating Principles

The Power Jet Mikuni has an additional jet that is designed to operate in the higher rpm range and throttle openings; however, it must be remembered that all three jets (primary, main and power jet) all overlap each other to a certain extent.

In addition, the main jet needle controls the effective size of the main jet until around ¾ throttle openings.

With power jet carbs the main jet is typically smaller than on the equivalent stock carb as the power jet will add fuel to the high end throttle openings.

The main operating principles of the power jet carbs and its mixture are:

Idle to one-quarter throttle position is controlled by the primary circuit (occasionally a choke system will enrich this circuit for ease of starting)

One quarter to three quarters throttle positions is controlled by the needle jet initially which then transitions to the main jet hole size. In addition the slide cutaway affects this range too, as does the fuel level in the float chamber.

From three quarters to full throttle openings the power jet primarily controls the fuel flow.

Conversion Kits

A number of companies supply conversion kits to allow the owner to add a power jet to a stock carb. Fitting these kits requires the owner or mechanic to have a basic understanding and ability to drill and tap the stock carb. If necessary, a local fabrication or machine shop can easily do this work.

In a nutshell, when the power jet carbs were introduced on the TZ Yamaha Grand Prix racers (in 1979 on the TZ350F), they were a revelation and before long every 2-stroke used a variation of this design making the stock carbs obsolete until a kit was offered to retrofit them.