76 KZ900 Plug Fouling

This sounds like a good plan, I would add to that once the piston is where you want it select 1st gear to stop it rotating when the valve is pushed against it.
WARNING: with regards using compressed air - in the car word this is common practice where you can select a gear and firmly apply the E-brake to keep everything locked. In the case of a bike the air pressure is likely to cause the crank to try and rotate (normally at an inopportune moment when the keepers are removed) if you select a gear the air pressure could cause rotation which would kick the bike off it's stand if using the "centre stand" where the wheel is in contact with the ground. All possible but extreme care should be taken

wrote:

This sounds like a good plan, I would add to that once the piston is where you want it select 1st gear to stop it rotating when the valve is pushed against it.
WARNING: with regards using compressed air - in the car word this is common practice where you can select a gear and firmly apply the E-brake to keep everything locked. In the case of a bike the air pressure is likely to cause the crank to try and rotate (normally at an inopportune moment when the keepers are removed) if you select a gear the air pressure could cause rotation which would kick the bike off it's stand if using the "centre stand" where the wheel is in contact with the ground. All possible but extreme care should be taken

Good advice, but if the piston is at BDC on the power stroke, both valves will be closed and the crank will be locked by air pressure. Use a regulator to dial pressure down just enough to hold valves closed… 

Needless to say I hate making more than one change at a time but want to take a big enough swing to see a change. 
So I think my plan is to put the stock 17.5 pilots back in, reduce the main jets from 115 to 112.5 and raise the needle clip one slot.. Hopefully that will make a visable difference.

surely you want to drop the needles not raise them if you want to weaken it ?

Yes , Raising the needle clip one slot , lowers the needle. 

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Yes , Raising the needle clip one slot , lowers the needle. 

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Float height was set with clear tube method .Bike is still 900cc . No air correcttion  jets . Needles do show a slight bit of wear.
My clymer manual (I know not always the best source) shows Main jet should be 115, and pilot 17.5 . I'm at 115 and 15.
As far as guides and seals , there is no oil residue at all on the plugs just dry black soot. 

Apologies on the bad info on the pilots.  You are correct.  Stock was 17.5.

If the needles show signs of wear then the needle jets will be worn also.  They should be O-6.

You said the Design fuel level was 22mm, but the clear tube test is good. I’m hoping the 22mm was misspoken and the clear tube test true. The two can’t exist. Have you checked battery voltage or coil resistance at the point of plug foul? You say it happens after a 40 minute ride. Coils could be heating up ad failing, or your charging system isn’t working. 

  

Yes,
my manual says 22mm float height so thats what I set them at before putting the bowls back on to perform the clear tube method and made corrections from there.

If you can't figure something out you need to go back the the "basics" - Compression, Fuel, Spark, Timing

You seem to be glossing over the compression as two members already asked that question, and I believe I did not read a answer ( one member also asked about a leak down test)

If you don't compress it the fuel will go right out the pipes.

These engines will happily run on 120PSI if you let them. Cam timing also comes into play as far as compression goes as they will also run "one tooth out". Check your FSM.

Yes, float height is 22 mm.

Compression numbers.
1- 125
2-120
3-120
4-125
Dyna S ignition, and 3 ohm coils . timing is dead on , and a good strong visual spark. Charging system functioning well ( I have a volt meter under the seat to be able to monitor it periodicly)