Carb Sync
 

Since buying my '93 CBR600F2 it's had a fluffy spot at just off tickover, apart from that the engine runs fine.

My initial idea was to strip the carbs and clean thoroughly as the bike had only done 30 miles since the previous MOT but, on advice and discretion being the best part of valour and all that, I decided to try syncing them first.

The problem came when I'd removed the tank and air box, rigged up the flow meters, warmed up the engine and then released the valves at the base of the dials - the needles on all dials covered about a 120 degree arc.

I tried spraying all of the joints with WD40 to check for air leaks but that showed nothing.

In the end I synced the carbs by turning the tickover up to about 2,500 rpm when the needles settled down and I could read them (about -20 psi across the board) - the bike now runs far better but it has me worried.

What would cause the fluctuating pressure at tickover? Does this show an underlying problem with the carbs or engine? Or have I bought a crappy set of chinese meters and should not be surprised?

Has anyone experienced similar? Any advice appreciated.

Do you mean the sync needles bounced around wildly?
So, you probably opened the knurled knob on each dial too much. Its a pretty common mistake when you first begin using them. The knob just needs to be barely cracked open.
Concerning the numbers on the dial, it doesn’t matter which number the needles hit, they just all need to be the same.
Finally, it’s a best practice to calibrate the dials. You do this by plugging each one in the same intake port, one at a time. Pick a dial to be the one that the others are measured against. Then you manually adjust the other needles to match the first one you chose.

Thank you, that would be it. I opened the valves fully.

Thanks also for the idea of calibrating the dials at pressure rather than setting them to zero as I did. Next time :-)

Glad it was easy.
The calibration thing is just so you can be more precise. Using the set out of the box will still give you great results.

Out of the box the dials were well out but instead of calibrating them against one cylinder I set them to zero as a static calibration.

Would it be worth redoing the job at tickover - it appears to be running well as is?

It can't hurt anything. Your idle should be at 1,200 I believe. It's a little bit of a dance. Make sync adjustment, readjust idle, repeat.
One thing I do when checking for vacuum leaks is I use a propane torch, like the kind used in soldering, unlit of course. I put the torch nozzle up near the isolation boots. Since it's a combustible gas, if there's a leak it will get sucked in and the RPMs will increase, letting you know you have a leak. The advantage I find is that it leaves no residue or mess to clean up.

Once again, many thanks for your advice.