The below information was kindly written by

Snoo ...

How do I Derestrict my 5 Speed Yamaha FZX750?

A: It is acknowledged that the 5 Speed version of the FZX750 has some restriction upon the engine. This is due to the fact that in Japan these bikes are used for training purposes. The 6 Speed version available to some parts of the world is to be considered a full power model.

The restriction manifests in 2 specific forms:
1. Top Speed restriction
2. Air Box Restriction

1. Top speed restriction can be bypassed fairly easily. You need to obtain a speedometer converter from a company such as M&P (www.mandp.com) they sell a gold coloured mechanical converter that fits down by the FZX front wheel. What this converter then does is move the speed restriction from say 200KPH to 200MPH!! Thus making it irrelevant.
2. The air box restriction is a bit trickier but nothing past the average home mechanics ability.
i. Remove the air box lid and you will see that there is a rubber bung that fits half way down the inside of the Air Filter Element. This has to be removed.
ii. Examine the lid of the air box and you will notice that one of the passage ways is blocked by another large rubber bung. This also needs to be removed. First remove the clip and then pull out the bung.

iii. Now the engine has doubled it's air intake which means there will be a lack of fuel, meaning the bike will not rev above tick over. To overcome this you need to replace the main jets in the Carburettor. The replacement sizes are:
Carbs 1&4 = 102.5
Carbs 2&3 = 105
The carbs are a bit tricky to remove. Take off the air box and then the fuel tank (Stop cock on lower left hand side). After disconnecting all of the pipes and cables to the carbs the carbs' need to be wiggled out sideways. There isn't much clearance so it takes some perseverance. I seem to remember that they exit to the right easier than the left!
To get to the main jets you have to remove the float bowls. Best done in a plastic bowl as there will be fuel still dripping from them (So NO SMOKING!!!). I found the screws retaining the float bowls to be very stiff. Have a friend on hand to hold the carb' block and use a good impact driver on them. Don't risk damaging the screw heads with bad fitting tools. It is advisable to replace these screws with Hex types on re-assembly.
The Main Jets can now be replaced (Exploded pictures of the carbs can be found in the Clymer manual or downloaded from the FZX Club site at Yahoo).

With the carbs out it's worth spraying some Carb Cleaner around and inside the float bowls to give everything a clean. Imports can be stood for some time meaning fuel residue can layer the units.

Now put everything back together, taking time to make sure you've fitted everything securely and turn the fuel tap back on.
My last suggestion is to drill 4 x 6mm holes in the ends of each of the silencers. This is just to provide a little extra flow through the exhausts for the extra intake. Possibly not necessary but it sounds nice at least!
 
Results.
Your FZX should now have gone from producing around 60BHP to around 90BHP. Opening the throttle from about 3000rpm results in a very addictive air box growl and after passing 6000rpm you should notice a large surge of acceleration from about 7500rpm to the redline (a la FZ750).
 
Other Notes:
In option to the above you may choose to fit a Dynojet Stage 1 or Stage 3 (Details from FZX club). This will release a few more horses but is a more expensive and time consuming option. Best done at a later date when you feel it's time for another performance boost!

You may notice a small flat spot at about 6000rpm. This is a standard emissions point and is the same with all bikes. Aftermarket exhausts can help with this. This is a performance hike though and not needed for de-restriction.

Rumour has it that there were some FZX's with restrictions in the CDi whereby a light would come on, on the display to indicate max RPM. However, none of these bikes seem to have appeared within the club so it could just be a myth. If however you do have this restriction people can assist on fitting an FZR1000 CDi to get around it.

Chicken shit Disclaimer:
Obviously I take no responsibility for any damage done to the bike whist carrying out these modifications or any future damage to your Driving Licence... 

Enjoy.