Tuning the 91 ZXRR

Q. What’s up Ken? I just have a quick question for you, You seem to know your stuff.. I have a 91 zx7r I have thought about selling it, but I have decided to keep it and redo the whole bike this winter.. The motor still runs pretty good so I plan to just freshening it up, I was wondering if you had any suggestions on what kind of MINOR mods I can make to get a little more power. I don’t do any real racing other than a very rare drag race, I am new to biking but I really like my 7 but feel the need for a little more speed!!!

A. Is the airbox still on or do you want to use velocity stacks? I myself prefer to use the Airbox, but you can gain 5-7 Hp with velocity stacks. I need this info in order to tell you “EXACTLY” how to jet your flatslides, and ignition timing advance. I have worked out the PERFECT jetting for the Stock Airbox, again you loose power this way, but engine longevity is maintained, and “Bitchin” throttle response with the Airbox. I really believe in using air filtration on my street bikes, and there is NO K&N filters that will work on the 91-92 models. Yes, the air filter adapters fit the carbs, but there are no individual filters that will work.

Flatsides are “AWESOME”, when set up right, and that is where we will start with you, the question is, Airbox, or No Airbox, Pipe, or no Pipe, and when I know the country of origin, I can tell you about ignition timing to suit. There is also a Airbox mod I can pass on, if you go that route.

Then you just put it all together, ands say SH!T, that was so easy.

Jetting the 91-92 ZX7R (with flatslides)

WITH THE AIRBOX: The airbox must be 100% stock. This means that the intake snorkel must be intact and installed, and the white piece held into the snorkel by 2 screws must be. I know it looks like removing the snorkel makes for a bigger hole, BUT IT DOESN’T work like that. This is the most “confused” airbox I have ever worked with. I have tried for years to get it to work without the snorkel, with the white piece removed from the snorkel, holes drilled into the box. Every single thing makes less power except totally stock. I’m telling you, if you are to use the AIRBOX, you have to have it “ALL THERE”. However, you can take apart the air box and remove and discard the metal screen. Be careful when retightening the black plastic screws as they will strip easy. Also I did find a K&N filter to produce 2 extra horsepower, but K&N no longer makes this filter, because they say it didn’t work. On my bike it did. Make absolutely certain that you have a clean stock filter.

Now, these OEM flatslides are “NOTHING” like the aftermarket. Im not going to go into detail, you will just have to take my word on it. Also these 39mm OEM flatslides can be bored to 41 MM. Look on the engine side of the carbs and you will see a crescent moon shape chunk of metal that can be turned out by a good machinist. Once you look at the carbs, on the bottom of the throat, the metal casting is 2mm wide, and on the top, its 4mm thick and is crescent shape. This is the material that is removed. Presto, 41mm

These OEM carbs come with a different type of emulsion tube, or needle jet, the brass tube the needles go in. Remove these from the bottom of the carbs with a 8mm wrench. The OEM parts have a bunch of very tiny holes,(bleed nozzle), and the aftermarket ones have 4 large holes. (purge or primary nozzles) I recommend using the stock nozzles. These work in conjunction with the Main Air JET, located under the air box adapters. This is a standard slotted Kehin jet.

Nozzles, (stock or Bleed)
Main Air Jet 100

The pilot or mixture screws in these OEM carbs have a different taper than what comes in Aftermarket carbs, hence the big difference in what works.

Pilot Jet 45
Mixture screws 1 1/2 turns out.

Slow air jet(barrel shaped jet under the airbox adapters) 120

Main Jet, 135/138. And you must use the little screwdriver slot type jets, and NOT the hex jets that come standard in aftermarket flatslides. This is because they set the float heights at the factory to use this style of main jet.

Needles, OCEMP in numbers 1 and 4
OCFMP in numbers 2 and 3

The airbox has different lengths of velocity stacks inside, and I have tried every combination you could think of, and could not improve on stock. The two middle ones are longer than the outside 2, and must stay this way.

The needles to be set in position #7 or richest. I know there are those that will say that any needle not working in the middle position(4), and needing to use the #7 position needs different needles. Normally I would agree, but not in this case!

All of these parts are available from SUDCO.

That’s jetting, put it together, and it will work so good, You will love flatslides for life.

Also disconnect the fuel enrichner solenoid. No longer needed.

Spark Plugs, I have found that replacing C9Es with CR9EKs are good for a couple of HP. These plus were first developed for the 93 GSXR 750, but now come in some Kawasaki’s. Their very design stick the gap further out into the chamber, sort like built in ignition advance.

I found that the 91-92 likes about +2 degrees ignition advance.

I currently use Muzzy pipes, but I also like D&D.

So, Header, jetting as above, +2 degrees advance.

Valve clearances should be set at .006 on intake and .008 on exhaust.

Ken H2Os

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