Engine question

**powerliftmike** · 07-09-2015, 08:59 AM

On an overhead cam design can you adjust the valves on either the compression or power strokes, i.e. as long as the valves are closed? Why do they say "top dead center" only on compression stroke? I get the rise and fall of the piston, but why does that matter for the valves?

**lovbyts** · 07-09-2015, 09:29 AM

More info needed. Year and motor.

To understand it best YouTube it.

**bignath4607** · 07-09-2015, 09:42 AM

Originally Posted by powerliftmike

On an overhead cam design can you adjust the valves on either the compression or power strokes, i.e. as long as the valves are closed? Why do they say "top dead center" only on compression stroke? I get the rise and fall of the piston, but why does that matter for the valves?

Why do you need to know ? The reason being is the valves are linked to the timing of intake and exhaust gases top dead centre refers to the crank position on compression stroke im a mechanic but cant really answer as im not quite sure what your wanting to know/achieve

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**powerliftmike** · 07-09-2015, 10:03 AM

I wanted to know in general, but specifically Im trying to adjust a Ryobi 4-stroke that has OHV and OHC. The manual specs valve clearance at 0.006-0.008 in. I think Im having some compression issues so was trying to tune the valves as a possible solution. Isnt there typically a mark on the cam or timing chain to indicate TDC? The cam has a groove on one side in my case.

**lovbyts** · 07-09-2015, 10:37 AM

http://m.youtube.com/#/watch?v=YO1hvYNTUE8

**bdos** · 07-09-2015, 06:03 PM

Originally Posted by powerliftmike

I wanted to know in general, but specifically Im trying to adjust a Ryobi 4-stroke that has OHV and OHC. The manual specs valve clearance at 0.006-0.008 in. I think Im having some compression issues so was trying to tune the valves as a possible solution. Isnt there typically a mark on the cam or timing chain to indicate TDC? The cam has a groove on one side in my case.

That groove would be the timing mark for tdc, valves would have some form of adjustment but you need to figure out which valves open at your compression stroke then spin it over to find your exhaust valves hope that kinda helps just woke up

**DAAS** · 07-14-2015, 08:01 PM

Originally Posted by powerliftmike

On an overhead cam design can you adjust the valves on either the compression or power strokes, i.e. as long as the valves are closed? Why do they say "top dead center" only on compression stroke? I get the rise and fall of the piston, but why does that matter for the valves?

TDC means that a specific piston is at the very top on the compression stroke. its important because the cam has to be synced so that the valves for that cylinder are closed when that specific piston is fully up.

So TDC is only on the compression stroke because you need the valves to be in the right cycle. IE both closed and not the exhaust valve open.

Not sure if that answers your question

You should be able to tell by a groove on the crank being lined up to a groove on the crank case and a groove on the cam being lined up with a groove on the head. both in TDC position.

**DAAS** · 07-14-2015, 08:10 PM

Originally Posted by powerliftmike

On an overhead cam design can you adjust the valves on either the compression or power strokes, i.e. as long as the valves are closed? Why do they say "top dead center" only on compression stroke? I get the rise and fall of the piston, but why does that matter for the valves?

It needs to be on the compression stroke and not the exhaust stroke. Both involve the piston moving up but the valves are doing different things

**redz** · 07-14-2015, 08:12 PM

I changed cams on a DOHC engine before and it was an interferance engine meaning if the timing gears were not properly aligned at TDC then the valves could hit the pistons when cranked.