50 Hp Merc power problems

[bigred69]

Hey all,

I have a 1975 or 76 50 hp Merc (Ser# 4359559). She is on a 1975 15 ft Imperial Tri . The problem is that she will not plane out. The motor starts and runs great in the water. I have rebuilt the carbs completly. New spark plugs. New batteries. New gas tank and 50:1 fuel. It has a good firm bulb.
When I get her past the wake zone and give her the go she will not get above 10 mph. Last fall she would do 35 mph but she was slow getting their.
I checked spark with a timing light and all four cylinders have good spark. All four cylinders have good compression. She does not miss at all.
Could their be something wrong with the stator and/or rectifier, and what would be the test procedure for that.

Any help would be appreciated.

 

[Chris1956]

Re: 50 Hp Merc power problems

Problems with the stator or rectifier would result in bad spark or no spark to one or more cylinders. is it possible the spark plugs are fouling out? If so, the fuel pump diaghram may be bad. if one or two of the plugs look brand new, you could be getting water infiltration.

 

[Chris1956]

Re: 50 Hp Merc power problems

One more thing. You might try a lower pitch prop. Maybe the motor was marginal with that prop last year, and the power has dropped off a few percent, and the motor is now overpropped.

 

[bigred69]

Re: 50 Hp Merc power problems

Thanks for the reply.

As far as the prop goes it is the original. The motor has less than 100 hours. I do not think that it has lost that much power from wear. She is not hitting the upper power band like she should. When I pump the bulb it will bog her down a little. That is why I think it is in the ignition. Could it be possible that the stator is not putting out enough voltage at higher rpm's?

 

[bigred69]

Re: 50 Hp Merc power problems

bump

 

[Chris1956]

Re: 50 Hp Merc power problems

I am not real familar with the ign system on that motor. It looks to be a dissy, but not the same as on the larger Mercs of the era. Normally on a dissy ignition, if you have spark at low RPM, you have spark at high RPM. You might check the plug wires for arcing.

Perhaps someone will come along with specific model experience.

 

[bigred69]

Re: 50 Hp Merc power problems

I have gone threw the fuel system. New lines, bulb, connections, and completely went threw carbs. But still have same problem. At wide open it is limited to 2500 to 3000 rpm. I have tested the trigger and at each trigger terminal connected to the switch box to ground I get less than 1 ohm. With all trigger wires disconnected from switch box I get the same results, less than 1 ohm. I am assuming that the trigger is bad?

 

[bigred69]

Re: 50 Hp Merc power problems

Anyone!

 

[andyusmc]

Re: 50 Hp Merc power problems

I am going through a similar problem with my 1987 Mercury Classic 50 (45 hp, 4 cylinder). Good compression, new prop. Cleaned and checked carbs, tried new fuel tank and line. Checked most of the obvious stuff. Then I bought a DVA adapter for my multimeter (any excuse for a new tool). What jumps out at me is the stator resistance on the high side is higher than spec (depending on spec), and the DVA voltage is lower than the blue side. According to the ignition troubleshooting guide linked below, under the "motor won't run above 3000-4000 rpm" section, the red to red/white wires are supposed to be higher than the blue wires. Mine increases with increasing rpm, but never gets as high as the blues. It makes sense to me that the lower voltage is not high enough to fire the cylinder under load. I ordered a stator today, and I figured if I am replacing that, I might as well do the trigger at the same time. I will let you know if it helps, but I expect it will be a week or so until I get everything and get it installed.
Andy
http://www.maxrules.com/cdi/TroubleshootingGuide.pdf