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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Poorish compression and piston leak by
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<blockquote data-quote="nkt267" data-source="post: 43102" data-attributes="member: 245"><p>I bought a new toy for myself and as I have thought that the compression on my Comet has not been what it should be I would use it to take a look through the plug hole. </p><p>I have 3 Vincent singles in my garage that I have rebuilt and mine has the worst compression.</p><p>I had the magneto fill with oil causing my last trip home on the breakdown truck and following various ideas put forward I have investigated further.</p><p>I don't have the equipment to do a leakdown test,but listening to the sound of air passing the piston from the timing case was good enough for me to realise that the problem is most likely excess crankcase pressure. So fix it properly or fit a big bore breather<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite11" alt=":rolleyes:" title="Roll Eyes :rolleyes:" loading="lazy" data-shortname=":rolleyes:" />.The bike runs well enough and I know that the engine will have to come apart,at least the head and barrell,but I bought this bore scope/microscope and decided to have a look.</p><p></p><p><a href="http://www.vincentownersclub.co.uk/photos/showphoto.php/photo/9811/title/comet-cylinder-bore/cat/906" target="_blank">http://www.vincentownersclub.co.uk/photos/showphoto.php/photo/9811/title/comet-cylinder-bore/cat/906</a></p><p></p><p>The bore looks quite scored to me,I hope it's not an optical illusion as I hate stripping and engine for no reason.</p><p>This was a new liner,muff,and piston about a 1000 miles ago. I really took it easy running it in,about 600 miles at a maximum of 40 ish, before I started to increase the loads and speeds. There is no sign of nipping up whilst riding and the bike still does about 80 ish flat out accelerating well.</p><p>I suspect that this is a ring problem but it will still cost me a new piston and rebore.</p></blockquote><p></p>
[QUOTE="nkt267, post: 43102, member: 245"] I bought a new toy for myself and as I have thought that the compression on my Comet has not been what it should be I would use it to take a look through the plug hole. I have 3 Vincent singles in my garage that I have rebuilt and mine has the worst compression. I had the magneto fill with oil causing my last trip home on the breakdown truck and following various ideas put forward I have investigated further. I don't have the equipment to do a leakdown test,but listening to the sound of air passing the piston from the timing case was good enough for me to realise that the problem is most likely excess crankcase pressure. So fix it properly or fit a big bore breather:rolleyes:.The bike runs well enough and I know that the engine will have to come apart,at least the head and barrell,but I bought this bore scope/microscope and decided to have a look. [url]http://www.vincentownersclub.co.uk/photos/showphoto.php/photo/9811/title/comet-cylinder-bore/cat/906[/url] The bore looks quite scored to me,I hope it's not an optical illusion as I hate stripping and engine for no reason. This was a new liner,muff,and piston about a 1000 miles ago. I really took it easy running it in,about 600 miles at a maximum of 40 ish, before I started to increase the loads and speeds. There is no sign of nipping up whilst riding and the bike still does about 80 ish flat out accelerating well. I suspect that this is a ring problem but it will still cost me a new piston and rebore. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Poorish compression and piston leak by
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