Problems on starting

Hi all,



I have old Briggs and Stratton engine lawn mower, model with aluminium tank. I had this lawn mower for free and fixed its carburetor. Also I changed oil (old oil was like mix of diesel and oil.. very thin)(I used this time 15W-40 Super HPD oil because I've heard this machine smoked like hell..) and tried to start it.. But that was with the plug in extremely hard to do. First time rope braked, then after I fixed this "rope", I could get it running, but it run rough, shaked.



Flywheel is on its right position. On copression, valves are closed tightly... But, I've noticed when there is going to be compression, exhaust valve opens for a little bit.. When the piston is in the middle, and closes right after. I thought it's compression release system, to make it easier to crank it. Can it be true? On this kind classic engine, I didn't saw this compression release system... I think so..



So, what could be problem? Too short valve? Problems with camshaft?



P.S.

Sorry for bad English, I'm writing from Finland.