Help !! Merc. 650 #1 cly. No compression!!!!!!!!!

[fab]

I have a 1975, mercury 650 outboard. The #1 cylinder has no compression, is it possible that it’s a stuck reed valve or is it the piston? if it’s the reed valve where can I purchase new ones?

 

[Clams Canino]

Re: Help !! Merc. 650 #1 cly. No compression!!!!!!!!!

It's not a reed. A piston is surely hosed.<br /><br />-W

 

[AMD Rules]

Re: Help !! Merc. 650 #1 cly. No compression!!!!!!!!!

Zero, as in less than none? That's not typically a great sign. Pull the plug and rotate the flywheel by hand. Is the piston actually moving? Shine a light in there and see if you can see anything peculiar.<br /><br />To get 'zero' compression the piston isn't moving, or there's a huge hole in there somewhere.<br /><br />I'm making the assumption your guage works of course....lol

 

[fab]

Re: Help !! Merc. 650 #1 cly. No compression!!!!!!!!!

Thank’s guy’s, the gage definitely works I got 125psi on cylinders 2 & 3. I could feel some air being pushed out of cylinder #1 but not enough to show up on the gage. So the piston is definitely moving. <br />Maybe it needs new piston rings? (I hope!)

 

[Chinewalker]

Re: Help !! Merc. 650 #1 cly. No compression!!!!!!!!!

With zero compression you need more than piston rings. Likely, that cylinder has a busted up piston and the bore is more than likely trashed, too. If you're lucky, going to the next size up in piston and boring the block to match will fix it. BUT, you need to fix the cause as well. Those 650s were notorious for cooking #1 cylinder, due to a steam pocket building up at the top of the cooling passage. There was a service bulletin put out about how to relocate the pisser outlet to the top of the block to alleviate the issue...<br />- Scott

 

[eli_lilly]

Re: Help !! Merc. 650 #1 cly. No compression!!!!!!!!!

Ack! I have a 72 650, where is the service bulletin!

 

[Clams Canino]

Re: Help !! Merc. 650 #1 cly. No compression!!!!!!!!!

The whole story is to block the pee hole install an elbow for a pee-hose over the top of the #1 cylinder. That's it.<br /><br />-W

 

[Chinewalker]

Re: Help !! Merc. 650 #1 cly. No compression!!!!!!!!!

A '72 is fine - it's a four cylinder. The service bulletin only pertained to the 3-banger 650s...<br />- Scott

 

[NIGHTFISH]

Re: Help !! Merc. 650 #1 cly. No compression!!!!!!!!!

I would check your cooling system b-4 you rebuilt the #1 cylinder that probably is the problem is that you just over heated it. I am not sure if you have a alarm on that model. Check to see if you do. If so and it didnt go off when the engines overheates you will need to get that fixed as well.