I am having problems with the valve guides bike is stalling and blowing blue smoke

OK, since you didn't mention a make model, year or number of cylinders this is just an old mechanics take on the situation. Blue smoke on deceleration (closing the throttle) is because of high vacuum in the cylinder pulling oil past either the piston rings or valve seals. Usually, blue smoke on a cold start is caused by the valve seals leaking into the cylinder while the bike is sitting. A quick check to see if the piston rings are at fault would be either to come down a long steep hill or accelerate hard then close the throttle, if it emits a sizable amount of blue smoke when you open the throttle again, the rings are most likly at fault.
The oil getting black is it doing it's job of keeping the engine clean, and the oil consumption you mention would not necessarily be cause for concern on an air cooled engine, especially in a warm climate. Is it possible this condition existed before all this work was done? If so, I'd suspect a carburetor problem, an over rich mixture, possibly because of a faulty float valve or the float itself contaminating the oil with raw fuel, especially if the bike is left on the side stand without the petcock being turned off.
To sum things up, a leak down test will show the condition of the rings and valves. Also might mention that if newer type (usually blue or green color) valve stem seals were used, there is a special procedure for installing them not all mechanics are aware of. The valve must be put in the guide first and the seal worked over the stem with the thumb then it can be tapped down onto the guide. Hope some of this helps, Frank

If it started to smoke after a service but didnt before, check for over-filled oil.
Otherwise it is a valve seal or piston ring gap issue.

If the engine idles for a while then blows blue smoke this is an indication of oil passing the valve stem seals and being burnt in the combustion process. ( When you are at lights and they change and you accelerate away leacing a cloud of blue smoke this is worn valve guides and failed valve stem seals) As for the idle it could be the oxygen sensor is coated with oil and soot and is malfunctioning henace the cpu cannot regulate the fuel setting correctly. Have the valve seals replaced and clean the Oxygen sensor and check for other fault codes

Blue smoke on decel is due to valve guide wear, you will need the head pulled and the valve guides replaced.

Continuous or just for the 5-10 seconds.
If continuous you may have oil seeping past your pistons oil rings or valve guides oil seals.
Check your spark plug see if they are fouled badly (covered heavily and oily) or just covered by oil and sooth lightly.
If 5-10 sec it is normal as the engine is heating up there are normally a little oil seeping past the seals.

That sounds like valve guides or valve guide seals.When they wear they the car blows smoke out the exhaust.Especially after leaving from a stop

valve seals are leaking oil or the valve guides are worn allowing oil to slip past the seals.

A lower octane gas woudn't cause it to blow blue smoke, more like it needs an engine job. There is oil getting into the combustion chamber, could be valve guides, valve seats, piston rings worn.

Most generally when an engine smokes on start up it is a sign of worn valve guide seals.If this engine has over 150,000 miles on it and/or it has ever been overheated it is very possible that the valve guides themselves may have escessive clearance and are the cause of the valve seal leak.These vehicles have a history of cylinder head problems and if this is determined to be the problem, the only true repair for it is to have the cylinder head rebuilt or replaced.This process can cost upwards of $900.00 to $1,200.00 to do so you might just put up with the problem until it become unbareable. Always be sure to keep the engine oil full and changed on regular interval of 3 mos/ 3,000 miles and use Lucas oil stabilizer to help minimize the oil useage .

This problem, ironically, might simply be the screen filter on the in tank petcock. The zrx is a gravity fed filter-less fuel system with only the screen to act as a filter. I had the same problem with my black 2001 zrx and did a full tune up before pulling the petcock and cleaning away 9 years of sediment from what appeared to be a very clean tank. I simply purchased a seal kit from Kawasaki before the tear down and waited till the tank was as empty as it would get by riding it. The screen was surprisingly clogged with junk...and once cleaned and reinstalled the bike no longer ran poorly when cold. Im not sure why the bike being warmed up made a difference but it did......now it starts and runs like a champ (colod or hot)!!!