1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

[121cowboy]

Hi!, I have a problem with my 1970 65hp mercury and since spring is just around the corner i want to solve these issues asap.
This is the problem, this old girl will kill batteries as fast as my daughters goes threw cookies. When the battery is at full charge she will start as good as i expect it to. A little choke and a couple of turns and she takes off. After a while on the water i would bring her in or to fish... i cut the speed down to trolling speed, she would troll for a minute or two and dies. How it dies is...spudders for a bit and then nothing, kind of like a flooding problem or starving for fuel. I would try to get her back but she would give a heck of fight(start and die, sputter, ruff idle), also the battery sounds drained. Just like how any vehicle starts in -38c weather. Also, sometimes the battery is completely dead. After a couple times of this happening, i went out and bought a new marine battery with more cold cranks underneath its belt. Thats when i noticed the conditions changed a little...she started a little better. She would also run longer at trolling speed, but all it did was extend the inedible and thats the same outcome as before...runs ruff and then dies.
now what my plans are, are to to a rebuild the carb's, replace gas filter again, and a fuel line or two if needed.
I know that there is more, thats were i need help. What am i missing? should i be looking more to wards the stator and switch box. I was told once that the stator is one of the down falls of owning a older Mercury. down falls?? hmmm! this motor is older then i am and runs better at times. so if thats a down fall then I'm hooped...
seriously i need help so if you got any suggestions, have seen, or had this problem before ....pleeeeaaaasssseeeee help me out
Thanks

 

[Chinewalker]

Re: 1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

Last motor I had that ate batteries had a bad rectifier. It had burned out and was causing a short. Since it was outside of the key-start circuit, it was always connected directly to the battery.

 

[121cowboy]

Re: 1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

A BIG thanks!!!
I will definitely check that one out.
This is why i love forums, i would of never even thought to look for a rectifier as a problem...yup, I'm a newbie to the boating seen. only been on the water for two years. thats why its nice to be able to talk to the pro's.
Now for a really stupid questions wheres the rectifier located? Also, is there a proper procedure to check to see if this is the problem on a mercury?
Just in case this is not the problem, is there anything else i should be looking for to solve my issues?

 

[merc850]

Re: 1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

Here's a page from a manual on testing rectifier
I think yours is Fig. 3 model, this has been replaced with Fig. 4 model.
To test the stator put an AC voltmeter across the yellow terminals you should see a reading of 15v or so while engine is running.

 

[121cowboy]

Re: 1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

THATS GREAT, YOU GUYS ARE AWESOME!!!!
Now that i see what I'm looking for and what to do. should go pretty easy...knock on wood.

So if the rectifier is toast, did i do a big no no by running that motor a few times after the fact?
what I'm worried about is that i caused more damage then there was originally.

 

[merc850]

Re: 1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

If the engine is still operational then the Thunderbolt ignition components are ok. The only thing that might be ruined is the stator. Do the test suggested above. As a precaution you should make sure that all electrical connections are clean.

 

[121cowboy]

Re: 1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

Sounds good and i will.. thank you so much for your help.
I guess I got some work ahead of me. The way i see it is, if I'm going to learn about something. Then there is no better way then to do it myself. If i screw up... so be it, at least I tried.
I may have to call up on you for some of your excellent advice later on... if thats OK?
I got a leaky gasket in the lower unit and since i will have it all apart anyways. I want to change the pump. I guess there is no better time then now, since were i live there is still over foot of snow outside.

once again thank you and if you have anymore advice for me please throw it my way.

 

[merc850]

Re: 1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

Here's a general look at the waterpump/driveshaft assembly

 

[Mi duckdown]

Re: 1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

check the drive shaft at the pump seal. make sure it is not rusted and grooved notorius for that year, carbon steel shafts. .02

 

[Gomer50]

Re: 1970 65hp Mercury...problems with killing batteries and dieing at trolling speed

I would agree 100% on a bad rectifier.With a bad recitifier,it can damage the Stator.On that type of ignition system the motor needs at least I think if memory serves me right you need at least 9 volts to keep a half way good spark,if the recitifier is toast it doesnt charge the battery therefore once you have drained the battery down below what voltage that is required to run that motor (your dead in the water).I have hooked up a voltage meter in my dash and can observe if my battery in charging or not.If not bad recitifier time to check wiring etc. as well..