Factory Pro Power Jet Circuit tuning, GSXR750 - as installed on air cooled gsxr750 w/ 38mm Mikuni carbs, 90-92
edited 08 12 2012 Marc Salvisberg

Power jet carbs - Mikuni's great addition to a carb used in a high rpm application.

The power jet adjusts high rpm mixture, in the gsxr750 - from 10 to redline, in 1/3rd the step of a main jet change. Changing a main jet, in the 38mm carb, as installed on the gsxr750, adds or subtracts up to 2% CO per main jet change - when the CO% needs to be adjusted in in .2%-.4% for best power attainment.

Changing the power jet allowed much finer increments of change and, just as critically, happened to change the fuel delivery curve to what was optimum for the gsxr750 - something that would have required main air jet changes and other modifications to attain, but would still leave the main jet fuel delivery steps too coarse.

This Powerjet circuit works wonderfully when tuned on the stock airboxed gsxr750 (and it's pretty straightforward to tune on our EC997 Low Inertia Eddy Current dynamometers unlike simple inertia dynos. 

The method of operation is as follows.

At full throttle, as the rpm increases, at 10k, there is enough of a pressure differential between the float bowl and the airbox interior to draw fuel up the black hose on the LH side of the carb and exiting through the hole at the top of the bellmouth of the carb. If you have a broken powerjet hose spigot, an oil pump spigot from an RD400 carb will  replace it. They are pressed in, so they will twist out.

The fuel is metered by a jet that is located in the bottom of the float bowl. The jets are sized in increments of 2.5 or .025mm. Usual size for a gsxr750 with a stock airbox and air filter might  be between #58 to #67.5.

The power jet circuit, when properly tuned, adds the equivalent of 2-3- main jet sizes "on top" of the main jet, after 10k, so, if you were not using the power jet circuit, i.e. had a "0" or blanked jet installed with a #125 main jet, you would use a #117.5 with a #62.5 power jet installed.

Since this particular circuit works on the pressure difference between the float bowl and the airbox interior, it is absolutely affected by any change in the pressure differential. If the air filter is changed to less restrictive unit or the airbox inlet is modified, creating less restriction - the power jet area (size) should have to be increased above the usual size, though, a BMC or K&N, as installed for stock replacement, may only require 1-2 sizes increase in the power jet (in addition to +2-+3 on the main jet circuit).

I specified "this" application, as there are other "power jet" systems that do not rely on airbox / carb bore pressure differential. They are identified by having a tube that is long enough to reach the center of the carb bore (where air pressure is lowest at high velocities. In the Harley world, it's called a "Thunderjet".

If the airbox is removed on this 750, there is no longer a sufficient pressure differential to pull the fuel up the ~2.5" vertical rise from the float bowl to the outlet in the bellmouth and the circuit is no longer effective.

Why is the Power jet circuit difficult to tune on a simple inertia dyno and easy on our EC997 Low Inertia dynamometer? According to the former owner of Dynojet, the power jet circuit simply doesn't work because there is a lag in fuel delivery at 9.5k rpm - creating a flat spot there. It turns out that the reason that he saw that is that the dynojet dyno has insufficient load to simulate the Real World Loading (tm) that is present on the bike in 4th and higher gears on the road or track. There is a slight delay in the onset of Power jet fuel delivery, but it's only vaguely present in second gear in the real world, and not present in higher gears due to the slower acceleration rate that occurs when you are actually riding. If you were racing, as Yoshimura USA and other non sponsored, large US Suzuki sponsored teams (we lent them carbs for the Finals) verified, the kit outperformed anything dynojet had to offer.

How to tune:
1. Install the main jet that produces the best power at full throttle / 8k-9k. Don't care about 11k-12k+ right now.
2. Install the power jet set that produces the best power at full throttle / 10k to redline.
3. Raise or lower needle for best power at full throttle / 5k-7k.
4. Raise or lower float height to get best power at full throttle / 3k.
4. Recheck main jet and needle height if you needed to lower the fuel appreciably.
5. Adjust fuel screws for best idle.

Powerjet Benefits:
The size of the main jet DOES affect the low and midrange. Excess leanness isn't usually the problem on these carbs. Using a #117.5 vs. a #122.5 main jet (PJ equipped vs. using a #0 PJ ) leans and crispens the low-end and midrange for better off idle and corner exit performance.

There other applications on other motorcycles that use circuits that are called "power jet" circuits that work on different principles - some are electronically controlled and work in the midrange like RGV250, the RS250 for upper topend, where they activate and deactivate through different ranges and still others work for different reasons and by different principles.
"Power Jet" is a catchy sounding name and it gets used every few years or so...

Why did Suzuki specify that US and UK models, for example  would have a blank or "0" jet installed, disabling the circuit and other countries, like Canada, got the activated power jet circuit (though with pretty odd settings)?
Emissions? I don't think so. With the basic fuel level and needle settings virtually the same on both applications, using the larger main jet, as required with the circuit blanked, would only increase hydrocarbon emissions under measured conditions.

At any rate, the circuit works extremely well in dealing with the coarse main jet metering steps of the older style gsxr750 carbs - 1st through 5th place at the 1990 WERA Grand National Finals used our Factory Pro #CRB-S06-1.0 Carb Recalibration Kit. Pervasive kit use followed for the next couple of years -until 1992, the last year of the power jet.

 

 

 

 

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