'72 Mercury 500 potential rebuild advice sought.

[vtdawson]

I earned a HS diploma in boats this summer. I'm only slightly less wet behind the ears as I write this. I ended up with an engine that needs help. I had it tuned, then the carbs rebuilt. It ran well at the repair shop but wouldn't start once I got it home, until my fishing buddy came over to help me trouble shoot it. He has rebuilt old motorcycle engines.

We were checking to make sure that the plugs were firing, and the engine started right up! We tested, and no matter which of the top three plugs we pulled, the same thing happened.

He would cover the plug hole with his thumb and it would stall. Except he couldn't get his thumb anywhere close to the third plug hole.

He thinks that the reed valves need replacing.

I'm really interested in what the folks on this forum thing.

Thank you!

Bennett

 

[racerone]

Well----- the reed valves have nothing to do with compression in the cylinder.-----Use a compression tester and report your numbers.----Reed valves are not likely the problem !!----Pull the intake bypass cover to have a look at the pistons and rings.

 

[vtdawson]

Thank you, I will do that. Any idea why it will only start with one of the plugs removed?

 

[Faztbullet]

Any idea why it will only start with one of the plugs removed?

Starter dragging, high resistance in battery cables or weak battery. Motor is not spinning fast enough to produce a HV spark under compression.