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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Very bad blow by - Help!
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<blockquote data-quote="timetraveller" data-source="post: 23184" data-attributes="member: 456"><p>If you can get a look at a spare head with the upper valve guide in it you will see that there is a very small hole at the side of the valve guide to let the air through. On the 'D's the cut out in the head was increased to improve the area of the through hole and hence let more gas escape. Sadly a lot of people are unaware of this and just fit a 'D' breather and then do not understand why it does not work as it should. The gas has to get up the push rod tubes, past the rocker and the rocker bearing, up past the valve stem and then into the valve spring chamber. It can do that but it has an easier life if the final small hole into the valve spring area is increased. I do not have the exact dimensions but a quick look will show you where the hole has to be increased in size. As I said,for a quick test, you can just block off the timed breather and fit the 'D' breather to any tappet adjuster hole (either of the middle two would be best) and then just see if you have temporarily solved the problem. You will certainly loose oil through the new breather but not so much as to damage the engine with a quick ten mile ride. If it cures the pressure problem then you can think about what to do next. If not then we all have to think where the excess pressure can be being generated. Good luck.</p></blockquote><p></p>
[QUOTE="timetraveller, post: 23184, member: 456"] If you can get a look at a spare head with the upper valve guide in it you will see that there is a very small hole at the side of the valve guide to let the air through. On the 'D's the cut out in the head was increased to improve the area of the through hole and hence let more gas escape. Sadly a lot of people are unaware of this and just fit a 'D' breather and then do not understand why it does not work as it should. The gas has to get up the push rod tubes, past the rocker and the rocker bearing, up past the valve stem and then into the valve spring chamber. It can do that but it has an easier life if the final small hole into the valve spring area is increased. I do not have the exact dimensions but a quick look will show you where the hole has to be increased in size. As I said,for a quick test, you can just block off the timed breather and fit the 'D' breather to any tappet adjuster hole (either of the middle two would be best) and then just see if you have temporarily solved the problem. You will certainly loose oil through the new breather but not so much as to damage the engine with a quick ten mile ride. If it cures the pressure problem then you can think about what to do next. If not then we all have to think where the excess pressure can be being generated. Good luck. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Very bad blow by - Help!
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