Compression check "" can it be done?

[kodibass]

I was given a 1999 30 HP Mercury 4 stroke (non running) , manual start only motor.. It pulls over has spark on 2 out of 3 cylinders, So I decided to do a compression check T-125, M-60, B-30 lbs. So I thought time to pull the head and take a look see, Well I got my new manual & was reading a bit it says that the pull start models have a compression release built onto the cam for the exhaust valve, So is it possible these low readings are the compression device working causing the low readings? Should I pull head or fix non firing spark & try to start? The manual is a mercury manual for this motor but I can find no mention of a compression test in regards to the decompression device. Pls weight in, thanks' kodi

 

[Grub54891]

Look further in the manual? There should be a way to disable the release mechanism. It usually is a mechanical device.

 

[Texasmark]

I read somewhere that lawn mower engines with compression relief have a shoulder on a cam lobe that partially opens one of the valves just before some part of the compression stroke causing the cylinder pressure to be reduced and provide for easier starting. Said that the process works for slow rpms like cranking....200 rpm sort of thing, but when the engine is running the 4 stroke cycle happens so quickly that there isn't time for the shoulder to effect the operational compression. If you have something like that I doubt you can check compression accurately.

 

[kodibass]

Texasmark, my manual says almost verbatim just what you said, So I feel you are correct, I think my next step will be to fix the non sparking cylinder & just see if it will start. This is only my second four stroke motor & the only Merc. I have had so its all kinda new to me, I'll keep at her. thanks' Kodi

 

[tommarvin]

Great start, fix the no spark plug or wire, coil whatever it is, change the water pump, its a rubber impeller on outboards, watch youtube for this, put the thermostat in a pan of cold water slowly heat it up, what temp does it open, when you put it back in put the very small hard to so hole up at 12 o'clock, drain the lower gear oil, ad 100% synthetic marine gear lube. Oh , change the motor oil, we have a two stroke, go have some fun !