Hi,recently bought a 1981 xj650.

fitted new plugs,air filter,balanced carbs,bike runs good apart from 1 small problem.

After about 2 miles when changing gear revs are a bit sticky around 3000/4000 rpm they come down slowly but engine is revlng when changing gear,ok when engine cold but after short run sticky.

Removed diaphrams and cleaned needles, slides working ok,throttle linckage returns instantly when cold or hot even after 80ml run ,cold start working fine as well. engine not overheating either.but just this damed sticking all the time after bike warmed up.

Can any one sugest why?

Many thanks.

have a look at air mixture screws and carb balance.

easiest first---check for free play at throttle, you need a small amount so the idle is working of the idle screw not the cable.

vacuum leaks---could be vacuum line going to fuel tap, cracked carb boots or leaking gasket(boot to cylinder), also ontop of the boots the vacuum line blanking caps.

i had a slightly similar problem on mine---cold start good, let it warm up and take it off choke and the idle was sitting at a steady 1000rpm, then 2-3 miles into a run the idle would rise to around 1800rpm. Plugs looked fine but decided to lean the mixture out to see what happened, well it seems to have worked idle is rock solid at 1000rpm even after a run, but i need to run it on choke slightly longer before it will hold idle without it, also plugs still look the correct colour.