1982 Merc 50 hp. Loss of Power

Ramon · 11-02-2009, 07:53 PM

Could this be due to low voltage?

Bought brand new deep cell battery this spring. But last couple of times out boat was slow cranking. Was heading out today to do some perching under full steam. Suddenly she goes to about 1/3 power at full throttle. We make it back to ramp but battery is totally dead. Motor has power pack with 3 coils. S/N is 6148127. Is it possible that motor will not perform if battery voltage gets too low? It seemed like performance was slowly dying as we were approaching ramp. I could get it to lose it's lug by keeping some pressure on the primer bulb. Is the battery and the performance separate issues? Thanks for all the help.

Iowa Dave · 11-03-2009, 01:59 AM

Yes it sounds like it is due to low voltage but it is realated to not charging the battery. Sounds to me like a magneto is bad or could simply be a bad connection to the battery. when in doubt always check the battery connections first then verify that it is charging the battery and you can get your battery load tested as well.

Snakecharmer · 11-03-2009, 08:20 AM

Quote:

Originally Posted by Ramon

Could this be due to low voltage?

It seemed like performance was slowly dying as we were approaching ramp. I could get it to lose it's lug by keeping some pressure on the primer bulb. Is the battery and the performance separate issues? Thanks for all the help.

If pushing the primer helped, I would first check to make sure you haven't turned the vent screw closed on the gas tank. It needs to be open a little to prevent a vapor lock.

Ken G · 11-04-2009, 08:01 PM

The ignition system for that motor does require a full 12 volts from the battery for proper ignition. The electrical power is generated in the stator beneath the flywheel. Part of the stator output goes to the power pack which fires the ignition coils to generate the spark in each cylinder. The stator also sends AC voltage to the rectifier where it is converted to DC voltage which charges the battery. I'd guess your rectifier is fried. Luckily they are easy to diagnose, easy to replace, and relatively cheap around $40.00 several years ago. (Iboats has them for $26.57, check NAPA for the 18-5707.)

Charge your battery or use another known "good" battery. It should read 12.6 volts when fully charged. Start the motor and run it at a fast idle and battery voltage should read between 12.7 and 16 volts. If it is anything higher than 12.6 the battery will charge. Lower than 12.6 and the battery will drain and eventually the motor will quit.

Rectifiers can be easily damaged by reversing polarity of your battery cables or even disconnecting the battery while the engine is running.

Is your motor a 3 or 4 cylinder? My manual does not describe a 3 cylinder 50 hp until 1986 but the parts catalogs do show them. Inquiring minds want to know.

Ramon · 11-05-2009, 08:27 PM

Thanks for all the replies. I think we will investigate the electrical problem first. Perhaps the fuel problem is really not an issue.

The battery has been charged and it checked out ok. The boat will be restarted (in my dad's driveway on westside). If we can get a rev out of it, I will assume that it is not a carb problem. Do you think this will be a good test even though the motor is not under a "load"?

Having trouble finding an amp guage. I recall that in "the olden days" there was a guage that you could just lay on the positive cable leading to the battery and it would show by induction if it was being charged. Does anyone have an idea where to get one of these? Ebay has a clamp type but they want $140. Any ideas?

You are right Ken, it is a 4 cylinder.

Looking for direction in troubleshooting and diagnosing.

Thanks again for all the help.

Ken G · 11-05-2009, 09:09 PM

You may be able to get an inductive amp meter at Harbor Freight but it isn't really needed. A cheap voltmeter is all you need to check to see if your motor is charging the battery. You can test the charging system while running on muffs in the driveway. The system may show charging at regular idle speed but you may have to speed up the idle with the fast idle lever. If the battery shows any voltage over 12.6v while running then it should be charging.

Troubleshoot and correct any electrical problems before working on the fuel system. If you have a fuel pump issue it is probably coincidental and not related.

I'd recommend getting a Seloc manual for the motor also. It may not be as good as an OEM manual but they are easier to understand for a backyard mechanic. It will run about $35.00.

Ramon · 11-08-2009, 08:20 PM

Motor ran well today with the charged battery. It seemed to be willing to go to high revs, thought it was on the trailer and hence not under load.

The battery voltage while running varied from 12.4 to 12.56. Is this low enough to indicated that there is a charging problem? I know you said that 12.6 was the magic number.

If so, where do we go from here on the diagnosing?

Ken G · 11-08-2009, 09:28 PM

I found a test online to check the rectifier. Its a good bet that is what the problem is.

http://www.myoutboardengine.com/rectifier.html

sliprig · 11-10-2009, 12:55 PM

"The battery voltage while running varied from 12.4 to 12.56. Is this low enough to indicated that there is a charging problem? I know you said that 12.6 was the magic number. "

12.6V is the minimum for a healthy battery, assuming it is fully charged. Right off the charger high 12’s is not uncommon. I would be looking for more voltage when charging.
13-16V is good. A digital voltmeter is ideal for checking this. Ken is always working on his boat…..so he’s usually right!!

Sliprig

Ramon · 11-11-2009, 09:21 AM

Thanks for the help guys. I will be up in Cleve on Sat. and will do the diode testing. Thanks for the link Ken. I'll let you know what I find.

Ramon · 11-15-2009, 01:10 PM

Did the continuity test yesterday. Scored 50%! 2 of the tests passed, 2 failed.

Steps 2 & 3 both showed continuity. That is, when I measured from ground to A & C I got continuity no matter which way the leads were.

Steps 4 & 5 showed no continuity. That is when measuring from B to A or C, I got no continuity no matter which way I held the leads.

Not having knowledge of how the diodes are laid out, I'm not sure what's shorted or open, but I'm guessing I need a new rectifier? Makes sense?

Ken G · 11-15-2009, 01:33 PM

Sounds like its time to buy a rectifier.