missfire above 7,000 rpm

[yamazuki]

Hi, I hope someone can suggest what I should test next, as this is a long term issue that I've not yet resolved. The bike is an FZR400 3EN1 (1988). The bike runs nicely up to about 7k and then has a misfire and some mild popping from the exhaust and will not rev much beyond this. I've come to believe this is electrical and I have done quite a bit of work so far. Firstly, I worked on the fuel side and have done the following:
stripped and cleaned the carbs, all diaphrams are intact, infact I have tried 3 different sets of carbs, with no change in symptoms, each time, I've balanced the carbs manually.
new fuel tap, fuel filter and fuel lines. The fuel pump seems to squirt a healthy amount of fuel. I've tried riding with the fuel cap off incase the breather is blocked. New air filter fitted.
On the electrical side, I've fitted new plugs (twice), the exup valve is set up properly and appears to move as it should. I've tried replacement (secondhand) coils, plug caps and plug leads. I have an new battery and have fitted a replacement (secondhand) CDI unit. I've check the pick up coil and the resistance is correct. Appologies for the long message, but I'm running out of things to check. I feel that the popping from the exhaust indicates unburned fuel in the exhaust, which makes me think the problem is electrical. The speedo is wired up correctly. This is the second FZR400 I've owned and I also have a VFR400, I love the FZR, but I'm rapidly running out of ideas. I bought the bike as a project, so have never had the bike running properly.
Any suggestions of what to try next would be very gratefully received.
Regards,
Dave

[willandrip]

Two things I would investigate Dave, one not so relevant; it being the R/R output.

As you seem to have replaced the majority of both the fuel and ignition side, this leaves only one component really:
the engine itself.

Compression test and valve clearance check may yield some insight .

[Evilchicken0]

The exup valve is stuck closed.

Have a look back from where the downpipes join on the side stand side, you should see a chrome cover - take it off and that's the Exup pulley.
When you turn on the ignition you should hear a Weep Weep sound of the Exup servo - if you don't hear this then it isn't cycling through. So look at the pulley and turn on the ignition (you can flick the kill switch off / on) and see if the pulley moves. if it doesn't look at the cables and check if they're frayed. Label up the cables (upper and lower) then take the pulley off so you can disconnect them. Put the pulley back on and see if you can turn the exup clockwise / anti clockwise. If it rotates freely start up the engine and see if it revs

[yamazuki]

Hi,
Thanks to you both for your ideas. The exup valve is free and rotating correctly, so I think I need to be looking at valve clearances next!
Regards,
Dave

[Evilchicken0]

If it was valve clearances you wouldn't be able to start it

If the Exup is ok then look to your main jets

[Wraith]

Give your Inlet Rubbers a Very close Inspection

Give them a squeeze and run your finger around the inside at the same time

Small Split is hard to detect by sight only

[willandrip]

Double post !

[willandrip]

If it was valve clearances you wouldn't be able to start it

If the Exup is ok then look to your main jets

What? he says he had tried 3 different sets of cleaned carbs..

Two sets with the same symptoms perhaps.
Three would be extremely unlikely.

You need to be sure the valve clearances are correct before you can use the compression figures with any accuracy !