2nd valve starting to open before 1st valve is closed?

This is NORMAL. It's called valve overlap, and has been present on every poppet-valve 4-stroke engine since they started using cam-operated inlets (about 1910!). Without it, your 400CC engine would produce about 3HP, and use twice as much fuel. This, however, has nothing to do with the 'blow-back' from the inlet. This is part of the compression release mechanism, and is designed to reduce the effort needed to crank the engine fast enough to start it. At running speed, the inertia of the incoming charge from the carb is enough to prevent it being blown back out the air filter.

In short - whatever problem you may be having with the engine, this isn't part of it!

phil_saunders@bigfoot.com

Phil Saunders
January 2009

take the motor apart check all valves valve stems and camshaft check the camshaft is mark is meeting the timing mark on the crankshaft and if nt correct and it shud remove the problem

wilberforce
December 2008