Dream nightmare
Ben Moor up in Inverness is building a Kawasaki/cb250n special. He freshened up the top end with new gaskets and changed the original CV carbs for Mikunis – but before fully plumbing them in, he dribbled a bit of petrol into the plughole to check the engine would fire up. It didn’t... but the rubber rocker box gasket blew out!
Says Ben: “I can’t work this out – I guess it’s igniting fuel in the crankcase and sending the pressure up the camchain tunnel – but why?”
Fuel and flames escaping the combustion chamber into the sump seemed the only possibility, so I suggested draining off some oil to see if it smelled of petrol and maybe trying a leak-down test in the bore. Leaky piston rings seemed a more likely situation than the (new) head gasket having blown across the cylinders when the engine hasn’t even run yet.
Ben replied that, having removed the head, the gasket looks fine. He also tried a leak-down test with petrol in the bore and the head – and found that both the rings and valves are sealing well.
The engine has a ‘wasted’ spark and I wondered whether, by manually adding petrol, this could ignite on the wrong stroke. A 180° motor could fire near bottom centre on the firing stroke, with the valves closed. With that piston already down, could the blast be directed elsewhere – maybe blowing past the rings? Not in this case – Ben reminded me it’s a 360° twin.
The odd thing is, there’s a rocker box breather Ben checked and it’s clear, so why burst the rocker gasket? Ben’s promised to keep me posted...