1975 Mercury 7.5 not getting full rpms

[Hunter44]

Hi, im currently working on a 1975 mercury 7.5 outboard for a customer of mine. I had put it on a 14 ft jon boat and took it out for spin. He didnt know what was wrong with it. When you give it full throttle it gives a little juice but then drops down to idle speed at full throttle, are the coils going bad? and how can you test the coils using a voltmeter, or is the switch box on it going out on it? and how can you test that. if you have any suggestions where to start that would be great!

 

[tpenfield]

Fuel pump diaphragm maybe would be something to check.

Also another thing would be if the insulation of the wires in the wiring harness look brittle and frayed.

what sort of business are you in?

 

[Hunter44]

Sorry haven't gotten back in awhile, I cleaned the carbs and the fuel pump and it runs fine in neutral. But still when you give it full throttle it goes down to a idle. I think it may be bad coils? or still possibly the switch box? let me know what you all think. would like to order the parts for the weekend to finish it up and get ready to winterize everything. Thank you !

 

[Chinewalker]

Do some basic troubleshooting. Start with a compression test, followed by a cylinder drop test, as it certainly sounds like it's only running on one cylinder. Inspect all wiring closely as original wires from that era are prone to corrosion and sheathing deterioration. Check the coils for cracks. If that all checks out, swap the wiring to the coils top for bottom and see of the no-spark issue follows the coil. If it doesn't, then it's likely either the trigger or power pack.