Need help: YS828 intake valve not fully closed on compression stroke

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Need help: YS828 intake valve not fully closed on compression stroke

dany
Hello, I have a weird issue with a YS828 engine... It is very difficult to start. When running I remarked that the carb was spitting out some fuel during acceleration. Did the following tests: 1- Took a compression reading, got 78 psi. 2- Put my hand over the carb inlet during cranking and felt succion and then some pressure (which doesn't look normal) Suspected bad valve timing or misadjusted intake valve, so I removed the cylinder head to look at valve opening sequence and everything is fine. I checked the valve gap at TDC and they are both at 0.006 (specs is 0.005 to 0.007) = OK Then I cranked slowly the engine and found that the intake valve doesn't fully close during most of the compression stroke (I can rotate easily the valve with a finger). The valve close fully only when the piston is at about 0.5 inches before TDC... This might explain the hard to start and spitting. My question is: Is it normal ? Is it a trick that the Yamaha engineers took to reduce the compression for an easier cranking ? This is not mentioned in the shop manual. Note: This engine camshaft has no compression release system, and usually those systems are partially opening the exhaust valve during the compression stroke, not the intake valve ... Does someone have a spare parts engine they could use to validate this ? I'm stuck, I don't want to grind the intake valve stem to allow the valve to close fully during compression stroke, I will end up with a valve gap of 0.016 at TDC and during all the explosion and exhaust stroke... Thanks for any help
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Re: Need help: YS828 intake valve not fully closed on compression stroke

English_Cat
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Re: Need help: YS828 intake valve not fully closed on compression stroke

dany
It would be wonderfull if you could test it.

To facilitate the process I uploaded a video here: YS828 piston & valves cycle

Can you do the same sequence and validate if the intake valve (the left one) can be turn with your finger at the same cycle time as mine ?

Thank you very much
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Re: Need help: YS828 intake valve not fully closed on compression stroke

dany
In reply to this post by English_Cat
Hello English Cat,
Did you look at the video? Do you need more details about what to check?

Best regards