The running of unleded.

[Tony Fearn]

Hello Pat.

Does this go for the pre-war engines as well, or does it only apply to the higher compression engines in the Jav and Jup.? Regards, Tony.

[jondowning]

First increase the tappet running clearence, more gap longer the valve is closed in contact with head cooling.

pat, what clearance do you suggest?

[Forumadmin]

Why not keep them closed all the time? Saves lots of petrol! It is debatable as to whether increasing tappet clearance improves cooling as it means the exhaust gas is not purged due to small opening. I suggest the seats should be finished as per the tuning notes where a grey band is clearly visible on the exhaust to give good contact area. The ramp is fairly steep on the camshaft so the difference in time of contact is minimal. Much more important is to have good lift with a high-lift camshaft and longer opening angle. The wider opening should mean the gas exits slower through the gap and hence with less friction and hence colder. But that is theory! It is important valve guide is high on exhaust to keep the stem cool (unlike inlet which you can cut off). However, you should carefully open up the port around the guide and give a polish to get that hot gas out as quickly as possible. I am also not sure temperature is the cause of recession; at the back of my mind, I thought it was to do with lubrication caused by the lead. I have had a number of burnt valves on Jowetts caused by long motorway sections at competitative speeds. Now I throttle back every few k and let the valves cool down. Plus with the head temp sensors I can detect problems early. May have also been caused by some old two piece valves that had a tendency to separate on full chat. Most disconcerting. Driving a Jup with a spark plug removed and holed piston onto a ferry across the channel was headturning.