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From: Philip Dingle
Remote Name: 22.214.171.124
Date: 18 Oct 2003
The manual recommends that the tappets should be removed and emptied of oil (this ensures that all settings are made from the fully collapsed position). If you have not already done so, taking the tappets apart and cleaning them could be worth while: Remember: Never mix tappet parts, as the pistons are individually matched to the bodies. Remember also that cleanliness is your friend, dirt is your enemy. Use of an ultra-sonic bath would be ideal.
My interpretation of your spit-back problem is that it was due to the tappet clearances being set too small. When cold, Jowett clearances are relatively small, but when the engine warms up, the aluminium crankcase expands more than the steel valve train so that hot, the clearances are correct. However, in the first few minutes of warming up, it is the valves (and particularly the exhaust valve) which expand most, thus taking up the small clearance that you had set. Eventually, once the mass of water has warmed up, the crankcase gets warm too, and had you run it long enough the clearances would have gone positive again. Go with the recommended 1.5 mm cold clearance, and let the hydraulic tappet take care of the temperature dependent clearance.