one for the mechanically minded

one for the mechanically minded

i have fitted a zx9r b engine into my zxr750L after blowing up the 750 engine. it was smoking badly and blowing oil out its breather. with the breather pipe removed it was spraying oil to about the top of the airbox in height and smoking from the exhaust excessively. i stripped the top end and found what i thought was the problem, on all 4 pistons the ring gaps were in line allowing blow by. i set the rings properly and rebuilt the engine. the smoke has gone!!

i never touched the head in any way, oil is correct weight and level, no wear marks on bores or pistons, rings looked ok, everything was cleaned and oiled before reassembly

but the oil is still coming out the breather, its no longer coming out with force more frothing up and running out. it has changed in revs when this happens, before it was about 5000rpm now its if the engine is held at a steady 9000rpm. the oil is a dirty brown colour when it comes out as the froth settles it becomes black by the time it lands on the floor.

has anyone heard of this before? does anyone have any ideas on whats causing it and how to cure it??

please help before i have to go and buy another engine that i cant afford

pic for attention

7 comments
  • Is it carbs or injectors?

  • carbed

  • Sounds similar to a problem I had on GPz 500. The needle valves weren't sealing in the carbs, it allowed a trickle of petrol to run into the head. It flooded through to the sump and mixed with the oil. When I fired the engine it pushed the excess liquid into the airbox via the breather (and then poured it out of the overflow onto my rear wheel but lifes a bitch). Might not be the problem but could be worth checking.

  • Geoff Gillingwater stripped and cleaned the carbs last week, did the oil level rise with the petrol? mine isnt and doesnt smell of petrol. my oil lands on the rear tyre as well. 90mph rolling burnouts are fun in a scary kinda way

  • It did raise the oil level a bit and it did smell of petrol. I stripped and cleaned the carbs a couple of times without resolving the problem. Only 1 duff valve. Bought replacement valves one of which was also duff. All valves looked perfect. It might not be your problem just sounds similar. Hope you get it sorted whatever it is

  • cheers, i leak tested the valves with petrol when the head was off n they seemed ok. will lookinto the carbs

  • I'd run a compression test and cylinder leak test on all four cylinders as it sounds like you're still getting excessive crank case pressure.

    With the rings having been incorrectly inline allowing the engine to burn oil it's possible that the bores are " glazed".