I agree totally! If Honda had thought it was better they'd have done it already! But I'm old and (hopefully) wise and the previous owner of this bike before my son clearly thought he knew better than Honda engineers. My son is unfortunately nearer to his thinking than mine and wants to keep it much as it is but working properly, of course.

It went onto three cylinders the last time he rode it because he ran the petrol right down, much against my advice. We naturally assumed it was blocked fuel jet(s) but actually I couldn't find any sign of blockage and the float bowls were all unbelievably clean for an old machine. The only 'dirt' I could find was very odd, to me. All the upper chambers had quite large amounts of fine sand underneath the diaphragms. I can't even imagine how it got there even though the K&N air filter was bone dry and obviously not doing its job. You could easily see through it so the sand could get through to the carbs but how did it get up under the diaphragms? The plastic slides are a little loose but I think they're probably about right. They have some marks but I don't see any severe wear on them. So my current guess is the bike went suddenly onto three because of an air blockage, rather than a petrol blockage. Does that sound possible to you?

It does sound to be an incredibly fast bike but I guess any sports 600 is like that. It's also even more incredibly thirsty. Personally, I'd have traded some speed for better accessibility and easier servicing! Thanks for your contributions.

 

As soon as I wrote it I realised how daft it was - the airway is huge and can't possibly get blocked. But you can get a jammed slide because of a grain of sand. I had this years ago with SU carb on an Austin 1100 and it completely immobilised the car. The fact that sand had previously got up past the slide into the space underneath the diaphragm supports this idea.

 

if its a 1998 the stock jets are 138/140 so a 142/144 with a full system wouldn't be to big 1996's are 135/138 theres a few little differences between the years

if its a dyno jet needle they have 6 slots, and factory pro i think 5? (when i had stock needles i ran 2 shims under them )
ive learned my dyno jet needles tend to work best or clip 2 or 3 from the top depending on what exhaust i have on it, my 1996 is currently running 142/144 jets 2nd clip from the top with 1 shim on the needle also ,1.5 turns on pilot, drilled slides and the decel circuit block off with a full vance & hines / k&n right now and it works well from 6-14k feet ,it doesn't work well with a paper filter at those altitudes so thats why i use the k&n

also this is random but make sure ur carb sync ports are blocked off, ive met a few people with f3s over the years that were missing a sync port cap or 2, and replacing ur carb boots can help if there old , there like 20$ on amazon

 

Thank you for that reply, Flyingdog. At first I hardly understood a word you said but you gave me clues about the world of re-jetting motorcycle carbs. I've read a few forums and discovered what an arcane sub-culture it seems to be - it has a language of its own which is the one you're using! I now know what Dynojet kits and some others are. I THINK these kits all come from America because I can't find a UK manufacturer claiming to do the same sort of Stage 1, Stage 3 etc tuning. I had never heard of decel pipes and unfortunately I've no idea if his bike has them. I'm very glad to say there's been no drilling of the slides. You're correct, my son's bike must have aftermarket needles and, because they have five grooves, they can't be Dynojet ones so I've no idea who made them. I still can't understand how anyone thinks that changing needles in CV carb from Honda ones can make a difference - am I just being thick? I'm interested that you have 142/144 main jets (like my son's) but you are at altitude higher than the whole of UK so your experience wouldn't necessarily match his.

Just to remind you, it's a 1998 cbr600F3 in UK. It has a (very, very dry) K&N airfilter in an otherwise standard filter housing with all that weird plumbing. It has a standard four into one exhaust with a baffle-less Derkovic PR 1833 'silencer' 200mm long. Have pity on this 79 year old English man! All I'm trying to do is get the so and so running properly for my son. Neither he nor I have any interest in trying to get yet more performance. We just need advice about initial settings for the fuel mixture screws, the circlip position on the needles and whether the present 142/144 main jets are too large (as I suspect). The pilot jets are standard.

In my trawling through aftermarket jet kit sites, I did notice that one of the jet kit firms offer an extended fuel mixture screw. I assume this is to make it easier to adjust the tickover air/fuel mixture since access to the underside of these carbs seems to be appalling. Would you recommend these, please?