Mercury 500 slows down

[Bitmess]

1975 Mercury 500 50HP. I took it out for a test run today. It was slow to get on plane but gradually got up to speed. It took about 10 seconds. I ran out about a mile before it began to slow down. It gradually got slower and slower. The motor sounded ok but just wouldn't go. I tried compressing the bulb, then choking it. Nothing helped. I checked the throttle linkage. It looks ok, the gas tank vent is open. I rebuilt the carbs last week. They weren't really very dirty but the motor did this last time I went out - so I rebuilt them. I have replaced the fuel lines and tank as well as the bulb and fresh fuel. I checked the spark plugs and they are okay.
I did notice that the rectifier isn't bolted down very tightly. I don't know if that means anything?
I don't know what to look for next.


Thanks.

 

[Bitmess]

Re: Mercury 500 slows down

Update: I did a compression check. Top two cylendars are 120 third one is 70
I didn't do the fourth yet. Is this a lost cause?

 

[steelespike]

Re: Mercury 500 slows down

Speaking generically the rectifier usually needs to be bolted down to serve as a heat sink.Did the slowing seem like you were running through a heavy weed bed? Could be a gradual over heat motor begins to labor eventually stopping.CAREFULLY feel the head.It should be too hot to hold your hand on but not so hot it burns you.BE CAREFUL!A compression check is a good start.
A decarb using sea foam might improve the compression issues, follow directions on the container.

 

[Bitmess]

Re: Mercury 500 slows down

I tightened the rectifier back up. The slowing was as if I had very gradually throttled down.

I checked the compression in the bottom cylendar. It's almost 0. Just to be sure, I put my thumb over the sparkplug hole and turned the flywheel I didn't feel any pressure at all.

Any ideas?

 

[Bitmess]

Re: Mercury 500 slows down

If I were to pressurize that cylinder with compressed air and try to find the leak, would this let me know where the problem is? If so, what would I be looking for?

 

[Bitmess]

Re: Mercury 500 slows down

I tried the compressed air thing. That didn't work. With only 0-5 PSI on one cylendar what are my options?

 

[Bitmess]

Re: Mercury 500 slows down

I did the decarb with SeaFoam. It helped on cylinder 3. It's up to 90psi but 4 is still dead.
What's next?

 

[Chinewalker]

Re: Mercury 500 slows down

Sounds like #4 cylinder is toast. I'd start by tearing it down to see what happened.
Your options are to attempt a rebuild, find another powerhead, or replace the motor. If you choose to rebuild or swap a powerhead, make sure you have a manual to guide you. The forums are also helpful if you get stuck along the way...

 

[Moody Blue]

Re: Mercury 500 slows down

Yep that motor is done. No amount of Seafoam or anything short of a rebuilt will fix it.

You could pull the transfer port covers off and take a look inside to gauge just how much damage is done then decide what action to take.

 

[Bitmess]

Re: Mercury 500 slows down

I doubt that it's worth fixing since it's 34 years old. I bought it in this condition. When it's in a barrel of water it seems to run fine. Fires right up and sounds good, pumps water. It's a good looking motor, looks almost like new. So I thought it was a good motor. I won't buy another one without checking the compression. I tend to learn things the hard way.
I'm trying to decide if I should part it out and sell online or just sell it as is. Of course I would tell the buyer it has a bad power-head.
It does have some good parts and newly rebuilt carbs etc... Anyone had any luck selling parts online?

 

[grossm1]

Re: Mercury 500 slows down

I may be interested in the electronics off the motor - switch box, and distributor to be exact. Maybe. If the price is right. Let me know what you think.