Thanks for the replies.

I have bought a set of standard thundercat carbs (#38 pilot jets) and the fuelling is better and will accept WOT from 3K but still slightly lean off idle. It still needs 3.5 turns on the pilot screw to not hang on idle and blipping the throttle (i.e. pulling off from junctions before the slides open) still has a lean spot, so I have ordered some #42 pilot jets (£12 posted from New Zealand) to try.

plodder wrote: so I have ordered some #42 pilot jets (£12 posted from New Zealand) to try.

Bigger pilot jets have helped me in the past when I've had similar symptoms.

Just be aware that these will also alter your main jet fuelling as the pilots flow all the way through the rev range (not JUST low rpm as most peeps think!).

You'll probably need to go down a main jet size or two to suit the 42's.

The #42s were fitted last night and took it round the estate but can't open her up. No lean spot off idle and strong pull from 3K.

I understand the point about the pilots adding all through the range and will come back to that later once I have done some high load testing. But the change in the pilot fuelling is small compared to full main jet flow (~3-4%) so it will be pretty close. Maybe I can find some more air at the top end to compensate

Bike runs well with the #42 pilot jets.

Maybe a touch rich at idle, so I need to dial in the idle screw a bit.

Top end not affected. Fuel consumption not checked as not enough miles.

Maybe #40s would be ideal but I'll keep these as they are.

I still think the thundercat carbs gave better low end performance, maybe because they have 3 pilot holes for the butterfloy to uncover rather than 2 on the FZR carbs (but they were bad higher up the range, maybe due to the vent hose hanging in the wind). Once I get a new MOT on the bike I'll mess about with the thundercat carbs.

The seller of the pilot jets is on ebay in New Zealand (SME BIKE), they were £12 including post and took 8 days to arrive. Search for "N424-25" and his is the only listing to come up.