Starter, Battery 1972 Mercury Thunderbolt 800 Issue

[krizm200]

Hi everyone,

I have a 1972 Mercury Thunderbolt 800 80 hp engine on my Mako 15. I just completed rebuilding the carbs because the engine has been harder and harder to start. The carbs are done and back on the boat. The last time I was out on the lake I tried to start the engine until the batteries died, they could no longer turn over the engine. I bought a battery charger from Walmart, a 10amp per hour, and charged the batteries 3 times over. I had removed the batteries to charge them. Tried to start the engine, but it barely turned over. Added my car battery to the mix with jumper cables, still barely turned over. So slow the engine will not start. I took the batteries to the auto shop and they tell me the tester says both batteries are good. I bought the boat used so I do not know how old the batteries are, but they are clean. There is no date on the batteries for age. All was fine with the batteries until I ran them down.

So my question, maybe I need new batteries? Or maybe I having a starter motor issue? Maybe something else? I have had the boat in the water and running 12 times this summer. Hope to get it back out soon. Any ideas?

Mark

 

[racerone]

Time to take the starter apart for inspection.-----Easy to do.---Take an ohm reading from commutator to stub shaft.

 

[krizm200]

I will try that. My Seloc manual says to test the voltage drop across the positive lead from the starter motor to a ground when turning over the engine. If it falls below 9.5 volts its battery. If not, I turn to the starter motor. DO you know what the ohm reading should be if I pull the starter motor apart and do the test recommended above, commutator to stub shaft?

 

[racerone]

It should read infinity on a digital meter.

 

[Chris1956]

On a motor that old, the battery cables corrode under the insulation, and the quick disconnect plug gets corroded. Run your fingers along the battery cables, and look (feel) for bulges. Often most of the copper conductor strands rot out, and the starter cannot pull enough current to spin well.

A test is to run new battery cables from the battery to the starter solenoid and ground. This bypasses the potentially bad quick disconnect plug.

Also check the condition of the ground cable on the starter. it needs to be clean, tight and good. id the engine wiring harness good? Most are bad by now.

 

[krizm200]

Hi everyone,
Thanks for the replies. I took the cowl off the engine last evening, and pulled the spark plugs out of the cylinders. Turned the engine over, and noticed the gear on the starter motor was sticking on the gear. Problem was the shaft was binding on the top bearing. Worked it loose, and then tightened the positive lead on the starter motor itself. The starter had no problem turning the engine with no plugs. Put the plugs back in, and the starter turned the engine over very quickly on one battery. Never did that since I purchased the boat in the spring. The engine is all back together now, waiting for the weekend!

I did have one concern, the teeth on the gear of the starter motor are rather pointed (not surprising for an engine built in 1972). I may replace it this winter. The crank shaft top gear looks great.

 

[merc850]

The starter drive clutch has beveled teeth to make it engage smoothly with the beveled teeth on the flywheel, this is normal.

 

[krizm200]

Follow up on the issue with the starter. I took the boat out Saturday to give the engine a try. It almost started when the starter motor bound up. Took the starter off the engine to find a copper sleeve pushing the starter gear up so it could not release from the crank shaft gear. I pushing the sleeve back down into the starter, and went back to the lake Sunday afternoon. This time the engine started up in less than 15 seconds (guess the carb rebuild really helped). I re-tuned the carbs, and the engine ran near perfect. I have a new starter coming Wednesday, and I am hoping once it is in, I will be set with the engine for a while, especially after replacing the prop, rebuilding the lower unit, carbs, fuel pump, all fuel hoses, new fuel tank, etc. I feel Cut Throat trout fishing coming on!

Mark

 

[krizm200]

One last follow up. The new starter motor came on Saturday. It was identical to the original starter except that the new one is 1/2 inch longer! The top section of the new starter fit into the top mounting bracket on the engine perfectly. Unfortunately, the lower mounting bracket was 1/2 inch lower, so the bolt holes did not line up. I had to drill out new holes in the lower aluminum bracket so the bolts would go into the head between the carbs. As of now, I have the new starter on the engine, and the engine turns over just fine. I'm hoping my engine rebuild is now complete! If the weather ever clears here in the Seattle WA area, I will go try it out!

 

[tommarvin]

We live in Seattle , back to seasonal temps at the end of the week.
Good for you..