1970 mercury 500 thunderbolt

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

I'm located in Harrisburg oregon n thx for stressing the importance of owning one of these tools I'm going to go get one asap

 

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

I'm located in Harrisburg oregon n thx for stressing the importance of owning one of these tools I'm going to go get one asap

Ok guys so I have a update on my motor I just had back surgery so my progress has slowed I did however bye a compression tester that I plan on using today or tomorrow if I'm up to it what range should each cylinder be in ?

 

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

So I've done a fee things today to the motor I did the compression test and the results are #1 110 psi #2 110psi #3 105 psi and #4 40psi so what's this mean? Does Number 4 have bad rings? Also I rebuilt the fuel pump and took the boat out for another test run and it seemed to be a lot better it stayed running it seemed to have better power and it got up and planed and made a wake. Whats everyone's opinion on what I should do about the compression in #4

 

[bgc]

Re: 1970 mercury 500 thunderbolt

Hope you recover soon!
There is a Transfer Port Cover on the starboard side of the block that gives you a good look into the cylinder. Remove the screws holding this down and get a look in there. Use a small screw driver and gently push on the rings. They should spring back freely. If you see any scoring or damage inside or if the rings have no spring let to them you're looking at a tear down. Let us know what you find.
This is an early 60's 44ci block but it gives you an idea of what to look for. Pic below it is a #4 scored due to water intrusion through the lower seal. Checking for a wet or really clean (steam cleaned) spark plug can give you an indication of a bad lower crank seal.

 

[Way Far North]

Re: 1970 mercury 500 thunderbolt

It appears that #4 has given it up. You could always dump a bit 2 cycle oil in the cylinder and do the compression check over again. If the numbers go up then its most likely just a set of weak rings. If not, you have a damaged piston/cylinder. Either way I think you will be redoing the whole engine. Hopefully you don't have to bore #4 oversize. I don't think Nikasil cylinders were around yet when these engines were built so thats a bit of a plus. Except for a few diesel engines I've worked on, if one jug needs work then I do them all, to keep the engine power balanced, especially since your compression numbers are getting on the low side for a 2 stroke. Try the oil trick for the other cylinders and see as well.

 

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

Can I use the motor under low loads like trolling, also I bought another motor it's a 69 mercury thunderbolt 500, the guy I bought it from said he knew nothing about it cuz it was like a package deal with his boat any way I know that it needs a internal wiring harness cuz they are all rotted but I was going to use the one one my 1970 but my question is what steps should I do to make sure it's a solid motor before I put it on the boat, how can I do a compression check on it when I have no pull start and no electric start as it's off the boat? And what other things should I do to it before hand I no very little about it. I can turn it by the flywheel and I hear the it puffing so I know that it's not seized, n there's some compression so anyone's input is greatly appreciated

 

[bgc]

Re: 1970 mercury 500 thunderbolt

Pull the top off and place an appropriate sized socket on the flywheel nut. Remove all the spark plugs and the lower unit. Use a variable speed electric drill to gently spin the motor.
If shes got water intrusion you could be doing more damage to the motor.

 

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

Thank u I'll give it a shot

 

[bgc]

Re: 1970 mercury 500 thunderbolt

Go slowly when starting the drill.....

 

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

Go slowly when starting the drill.....

Hey guys so I did the compression check on the 69 motor today and it seemed like it was really good all the cylinders were within 2-5 psi of each other I did try to see if it had spark by turning it over and grounding the plug on the block and I didn't have any luck but I figured it was cuz of the wires being rotten I feel reall good about this motor is there anything else I should make sure is ok before switching it out on the boat

 

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

Hey guys so I did the compression check on the 69 motor today and it seemed like it was really good all the cylinders were within 2-5 psi of each other I did try to see if it had spark by turning it over and grounding the plug on the block and I didn't have any luck but I figured it was cuz of the wires being rotten I feel reall good about this motor is there anything else I should make sure is ok before switching it out on the boat

Guess I'm on my own

 

[bgc]

Re: 1970 mercury 500 thunderbolt

Holidays.....

Get a gasket set and swap the blocks. Check the ground wires/straps before you put the new block in, easier to do now than later.

 

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

It appears that #4 has given it up. You could always dump a bit 2 cycle oil in the cylinder and do the compression check over again. If the numbers go up then its most likely just a set of weak rings. If not, you have a damaged piston/cylinder. Either way I think you will be redoing the whole engine. Hopefully you don't have to bore #4 oversize. I don't think Nikasil cylinders were around yet when these engines were built so thats a bit of a plus. Except for a few diesel engines I've worked on, if one jug needs work then I do them all, to keep the engine power balanced, especially since your compression numbers are getting on the low side for a 2 stroke. Try the oil trick for the other cylinders and see as well.

So I just though I would show some picks of what the #4 piston looked like on the 1970 mercury there really deep burn/ grooves. Right across the rings (the reasons there was very little compression) so what caused this? The other pistons are a little groves but now where near this one

 

[racerone]

Re: 1970 mercury 500 thunderbolt

This is typical damage of a motor that is not run properly .----Running with an old worn impeller.-Poor quality oil.---Lugging the motor with the wrong propeller for the boat etc.------No nicasil back then...Suggest boring it out and fitting new o/s pistons.-Might be cheaper to find another motor.

 

[bgc]

Re: 1970 mercury 500 thunderbolt

Looks like shes got a bad lower seal and possibly some pre-ignition.

 

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

Here's what's left of the lower bearing where did the rest of the ball bearings go? Any chance they could of got into the 4 cylinder and cause all the heavy scoring? And yes if your talking about the large O ring it was toast very brittle

 

[Bjohnson2345]

Re: 1970 mercury 500 thunderbolt

Does anyone need a 44 ci block and a crankshaft if the offer is good rnought I'll sell together the block looks to have made it without much damage might need a hone job I don't know any one interested pm me and we can go go
From there I might have other 500 parts to let me know