explosive pressure in crank case

[dirtybike]

Has anyone ever had a problem with developing explosive pressure within the crank case and how did you fix it?  I have been building enough pressure, very rapidly, to blow out gaskets.  Both of the vents in the valve covers are clear and working.  I have checkedn the compression and it is good and even in both cyclinders.  The bike runs great but when it is warmed up it is like it periodically detonates fuel outside of the combusion chamber and find (or make) an opening to vent it.  The vents in the valve covers are not enough when it pops.  The bike does not skip a beat but it is a bit disconserting to have a cloud of oily comes back at you from some unidentified engine location.

[Humber]

First, check your valve clearances. Once the clearance is too small and the engine warms up the exhaust valves become lifted or i don't know how to put it -  "inserted". That creates a gap by which the fuel mixture reaches exhaust and it actually blows there insted in combustion chamber.

But anyway, I had the same problem and it was due to bad conditions of head and valves themselves (especially front one). It happened especially at high rpms. And while hoses where attached they blowed a lot of oil it the airbox.

While replacing the oil rings I decided to look at my valves and heads. It turned out that valve seats were covered with rust, the same was with valves and generally there were notches on them.

I had my valve heads reseated and valves grinded by a highly qualified grinder. I set clearances and it is OK now.

Think that your valve seats are worn as mine were!!! But I might be wrong

[Lucky]

I have had this happen once when i accedentially kinked one of the vent hoses while carelessly installing the airbox, so be sure the hosses are routed and connected properly. ?remember, if you take the airbox off to see what happened, you'll never find the problem

i'd be a little surprised to hear that it is a valve problem if the bike is running well and the vents are clear. ?be sure that you can feel or see the blowby coming out the vents, if not it might be that there is enough oil crud built up in the valve covers to be blocking the vent tubes.

one more thing comes to mind, and it is an easy to test: remove and inspect the air filter, then go for a short run and see if the problem is cleared up. ?if this works (or even if it doesn't, but the filter is clogged) then you know that the previous owner didn't care a crap about normal maintanence and that the crankcase is probably full of sludge. ?"GM crankcase cleaner" does an excellent job of cleaning engine internals, but i would only use half a can for an engine this size.
--Lucky

[dirtybike]

Hey guys,

You will not believe the source of the problem, and another forum string turned on the light bulb for me to look into this so keep your eyes open for the not so obvious.  It was the petcock.  The thing is vacume activated off of the crank case.  As with the other fellows, the diaphram has a problem and fuel is leaking by but in mine in stead of going the way of the carb, it is being sucked into the engine block.  Test to prove point.  I blocked off the vacume line and set the petcock on prime and ran like the wind.  No more problem.

Thanks for the forum.
Dirtybike