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First off i used to have a problem of not being able to exceed 115mph, i found out it was because my airbox bungs were missing, i soon replaced them with some blanking grommets and managed to get 130mph out of the bike and it ran very smooth...

since then i had not touched anything on the bike but for some reason my problem has returned, i get to about 110mph and instead of brickwalling it just kinda bogs down, throttle becomes useless in other words unless i hold it a certain specific position, hard to explain really, if i release of the throttle a tiny bit whilst doing 110mph it will pick up slightly then if i wrap it back it will bog back down and have me jumping about lol... also if i pull the throttle all the way back say from 1st gear all the way through the rev range it feels very rough and slow, almost as if the bike is gurgling, the only way i can compare is that my 2stroke 125 accelerated A LOT faster than my 400 lol i also have a horrible flatspot under 4k where if i twist the throttle back far it will just bog and not move or slowly slowly gain revs till it passes 4k, once i pass 4k revs it picks up very well but iv heard all older 400s have this lower rev flatspot..?

just wanted a second opinion on whats wrong? i think it might just need the carbs balancing? the bike is FULLY derestricted, i stripped the airbox and silicone sealed ever joint possible and fitted new gaskets just to make sure there are no air leaks, over time i have also replaced the fuel lines and added a new fuel filter, silicone hoses, bunged airbox crank pipe hole and added small filter on engine crankcase and as far as i know it has been rejetted when the custom yoshimura exhaust was fit, also all spark plugs are a nice coffee brown color the way they should be..

not really sure what else it could be? iv checked everything :/. i think the last thing to try it to fiddle about with my carbs?

any opinions will be appreciated, thank you.

if the bike is fine in the rest of the throttle range (carb tuning is about throttle position, not necessarily RPM...) then you will need to see what main jets are fitted and probably go up 1 or 2 sizes and see if this clears it up.

when you are on full throttle see if opening the choke does anything, if the bike picks up then you are probably running slightly lean so larger main jets would help.

depending on what needles you have (either standard needles, or dynojet) consider going 1 clip position lower (for DJ needled) or adding 2 shims underneath the metal rim of the stock needle - this will richen the mixture in the middle of the throttle range - might help as well?

Yes that's what it feels like, almost as if the throttle is out of sync lol sounds stupid but its the only way I can describe lol thanks for the tips

Could also check fuel/air mixture screw, set it to standard 2 and a 1/4 turns out from fully in i think and see how that goes.

ZX4R-F3 wrote:Could also check fuel/air mixture screw, set it to standard 2 and a 1/4 turns out from fully in i think and see how that goes.

yes i will give this a go first i reckon just to start off with the basics, ill probably set each one to 2 1/2 out and keep down what they were originally at just incase i mess it up! which way is which? screw in rich (anticlockwise) and out lean (clockwise)? or other way round?

JoeZXR wrote:

ZX4R-F3 wrote:Could also check fuel/air mixture screw, set it to standard 2 and a 1/4 turns out from fully in i think and see how that goes.

yes i will give this a go first i reckon just to start off with the basics, ill probably set each one to 2 1/2 out and keep down what they were originally at just incase i mess it up! which way is which? screw in rich (anticlockwise) and out lean (clockwise)? or other way round?

screw in richens, screw out leans.