lost compression

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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lost compression

Postby dunsweepin » Sat Aug 11, 2018 6:42 am

hi all,got my csr running after being stood for 18 months.,with fresh oils and a carb clean i took her a short run.reset mixture on return,set tickover.20 seconds later she stopped.now have no compression.valves are moving up and down and no obvious noises.is it possible for valve timing to slip or is a a strip down job.i think it was 1962 last time i stripped a motorcycle engine.but willing to try ,all comments and advice welcome.
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Re: lost compression

Postby oldandsmelly » Sat Aug 11, 2018 7:33 am

It is most likely valve related and possibly that the bike has been stood for a while may be a clue. One valve may not be closing fully due to carbon build up and being dry for a while. Before doing anything major, try checking the tappet clearance as normal at top dead centre on the compression stroke. If you can now wiggle a pushrod up and down a significant amount then one valve is not fully closing.

If this test is OK then the piston rings may be gummed up and stuck in the grooves but this is possibly less likely as it was running OK albeit for a short while. Valve timing movement is not likely, but you can do a rough check to reassure yourself while looking at the tappets as either side of top centre on compression both valves should be closed and the exhaust valve shouldn't open until a fair way after top dead centre.

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Re: lost compression

Postby dunsweepin » Sun Aug 12, 2018 7:05 am

cheers for reply,all checks you mentioned seem ok,so its off with her head.oh dear the inlet valve seat insert has been moving up and down(with the valve i guess) and the head is burred around the edges.some piston slap also.not quite sure where to go from here.ps it is of course the exhaust valve seat,must get some glasses
Last edited by dunsweepin on Mon Aug 13, 2018 6:59 am, edited 1 time in total.
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Re: lost compression

Postby Janet » Sun Aug 12, 2018 7:37 am

From a previous thread* in the G3/M16 forum( viewtopic.php?f=17&t=22580&hilit=valve+seat ) if the seat is moving, it either needs replacing or pegging.

*So you know it's not me talking rubbish again.
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Re: lost compression

Postby dunsweepin » Sun Aug 12, 2018 7:45 am

thanks janet,i found the previous thread on the g9,sounds like alloy heads have this problem.
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