No, given the proper (the normal) amount of oil in the crankcase, the scavenger section of the pump is going to evacuate this without getting overloaded and puking it out. The only time it should puke is when the bike has not been run for a while.

So, you may want to buy a new ball and a new spring and install those items in the front of the pump. It may be that the check ball is letting oil flow into bottom end, where normally it is a check and keeps oil from flowing past, until the engine is cranked up and the oil pump pressure lifts the ball off it's seat to allow oil to pass while the engine is running.

Vision, if you will, what is going on if the engine is stopped for 30 minutes and that ball has not re-seated properly.
Oil leaks past and the base becomes semi-loaded up. You go out and crank it up and there is a slight amount of excess oil down there and it gets puked out. Not much, but just the same.... some.
This is only my speculation, but it may be the cause of the tube burping oil. Worth the cost of a new ball and spring in my opinion. With luck it may take of the problem..... pg

 

iron mick and piniongear are correct on their diagnosis. your explaination of the tank leads me to think you have a xlh oil bag instead of the ch oil bag and you would see the oil return through the bag cap. one important thing to check on the vent line is that it is not restricted as this is the pressure equalization between the bag and engine c/c. for the scavenge to work most effectively, the pressures must be equal. i doubt any of the following is your problem but i will lay them out. there is an oil seal between the feed and scavenge sections on the stacked pump and if it goes bad, the scavenge side will have trouble keeping up. another spot is the main feed in to the engine which is through the pinion shaft. if you have leakage here, the cam case will have excess oil. the last point is you said that you did a top end. if the cylinders were done right, you do not have enough run time for the rings to fully seat in. this can cause excessive c/c pressure and to some extent will effect the scavenge, one telltale sign is pressure on the clutch side through the transfer valve but this will occur with really bad blowby. if you are not having issues, ride and watch it. but make sure your hose routing is correct. i too had the dreaded leaking check but i solved it by trashing the steel ball. you can try peening a new ball in the hole in hope to flaten out the seat but it did not work for me. i replaced the steel ball with a teflon ball and my problem slowly went away as the ball seated in. over time the softer material will conform to the pump body. i had the teflon ball in there no telling how many miles and no issues.

 

Bustert......
I can honestly say I agree with all you say regarding the puking from the breather pipe. Good points you make!
I still think the first thing to try is a new ball and spring. At least it is the cheapest thing to try. If that does not fix it, then a more serious approach will be in order......... pg

 

I'll go for the ball and spring. but i still don't understand why the oil suddenly decides to puke out at the end of a ride.

thanks and i'll keep you posted.