Alternator/Battery Issue? 1997 5.7 Mercruiser

[jplayland]

I have a 1986 Four Winns 21' open bow. It was repowered in 1997 with a 5.7L Mercruiser. I just bought it a month ago.

I had it out a few times with reasonable power (I need to do a tune-up, I'm guessing around 80% power.) The last two trips out I have been starving for power topping out at 3300 RPMs. On the last trip I was at 3100 RPM at the start and dropped as low as 2500 RPMs before being unable to restart it. It sounded like my old $500 car when I was in High School on a -20 degree morning. I stole a battery from my trolling motor and jumpered it to the starting battery. It started right up, but sounding like it was missing on a couple of cylinders. I'm assuming/hoping the cause is fouled plugs from unburnt fuel?

I charged my starting battery (from 2002 according to the date code) last night and had it tested at AutoZone this morning. They said it tested good. I'm quite surprised.

Should I assume alternator? Battery regardless of test results? Something else?

[dubs283]

have you checked alternator output??

should be approx 14.5 volts at 1500-2000 rpm

[jplayland]

Not yet, should I be safe to start it up with it missing to check on muffs? I want to confirm my theory is realistic in that the low electrical power could foul the plugs and cause the miss before I run it again and cause damage.

[dubs283]

why not fully charge the battery??

then the engine will definitely have enough voltage to run for testing purposes

[jplayland]

The battery is on the charger now to make sure I get a full 100% charge. I'm going to pull the plugs and clean them up while it charges and test the alternator voltage in a few hours.

[jplayland]

I decided to run the alt test with the trolling battery without cleaning my spark plugs yet. Ran like crap, but I expect that won't impact the test right?

At Battery Terminals
0 RPM 12.5v
1800 RPM 13.8v
2400 RPM 13.6v

Directly at the alternator is hard to reach due to the rear seat. Do I need to take that reading too, or is the terminal reading proof enough? I take at 13.6v-13.8v means I need an alternator?

[dubs283]

well, its putting out some, but to know for sure you need to check at the output stud on the alternator

the lower voltage readings at the battery could indicate corrosion or loose wire(s) somewhere

[jplayland]

I was able to get my brother in law to squat down in the engine bay and get a reading off the alternator. It shows 14.0v at 18-1900 RPM.

[dubs283]

in that case i would say the alternator is generating a good amount to keep the battery charged

the issue probably lies in the wiring between it and the battery, including grounds

[jplayland]

0.2v is the difference between good and bad?

[brn77]

i had this same peoblem replaced alt. and it was my fuse block check that it may be on the stud on ur starter where the main charging wire goes ....