Where’s my compression gone ?

[Jay2903]

Hey guys

I’m trying to sort a few issues with my Yamaha xs500. I took it apart replaced a gasket and done a bit of valve grinding. I’ve reassembled it but I wouldn’t start, the compression seems very Low particularly on the right cylinder.
I thought I may have got the timing wrong and bent a valve. I checked the clearances which look fine.

Any ideas ??
Any Help greatly appreciated !

My best guess - the manual says if the timing is off by one tooth the timing will be off my 21 degrees which would cause the piston to hit the valves. Is it possible I got the timing way off and this just isn’t happening. I’m like 70% sure the timing is correct

[Pete.]

Plug, rings, gasket, valves. That's the only places you can lose compression.

Easiest thing to do is check the valve clearances. Get the engine on TDC with the valves not rocking and make sure you have clearances on both valves. If you don't, adjust the valves If you do, pour some petrol into the intake port. If it drain away quickly you've bent an inlet. If it doesn't pour some in the exhaust port and check again.

If you can borrow a tester you could do a leakdown test. You'll hear where the air is getting out.

Of course if you haven't run the engine there's no reason why you shouldn't pull the head off and re-use the gasket. Then you can check for bent valves by looking at the piston top and seeing if there's a shiny mark where it hit.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Jay2903]

[MCN]

When you say 'Valve Grinding' do you mean in a Valve Grinding Machine or 'Lapping' with grinding paste and a sucker?

The correct angles of the seat and face are very important and then lapping after the grind to provide an initial gas tight seal.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[Jay2903]