1980 90HP will not crank

[mruocco]

I've been working on a 90HP inline 6 that I cannot get to crank unless the plugs are removed. Here is what I know.

-The battery is reading 12V output.
-It cranks with the plugs removed.
-The problem persists whether the lower unit engaged or not
-The starter spins when connected directly, and when going through the solenoid.
- It had been running last it was used, but has been sitting for the past 3-4 years.
- The starter spins when disengaged from the flywheel.
- In some cases, it will rotate the flywheel slightly, and slowly


I am able to manually turn the flywheel, but it is (obviously) much harder when the plugs are in. It seems like the starter motor does not have enough power to turn the flywheel. Any ideas?

 

[racerone]

Don't rush out and buy a new starter.------Take old one apart FIRST.----Inspect brushes and check continuity from commutator to the shaft.---Should read infinity.----Have battery load tested.----Inspect battery cables.----No money spent to do this easy work !

 

[mruocco]

Don't rush out and buy a new starter.------Take old one apart FIRST.----Inspect brushes and check continuity from commutator to the shaft.---Should read infinity.----Have battery load tested.----Inspect battery cables.----No money spent to do this easy work !

So, it sounds like the good news may be that its likely related to the starter / electrical system. I was worried about engine seizure, but I assume thats not the case since it turns with the plugs removed?

 

[sly_karma]

Sounds like voltage drop. Check all battery to starter connections for corrosion, clean with wire brush or file. Inspect cables and insulation for fraying, cracks or nicks.

Borrow a battery load tester and check yours for proper function, it might have a weak cell and be losing power quickly under load. Check all cells and top up with distilled water if low.

I'd look at those two areas before going inside the starter. And yes, the motor is not seized if you can turn it by hand with the flywheel.

 

[jbuote]

I too had an engine (Johnson 50hp in my case) that would crank slowly with plugs in...

While I'm no pro, I agree with all the above, but also check your ground connections. Mine happened to be a bad ground lug on the power head. Had to remove the stud, clean, and replace it.. Then it cranked just fine..

Clean ALL the electrical. Including grounds. If a component is grounded by bolting to the power head, remove it, clean the mating surfaces, and reinstall.. That's a ground too..

For what it's worth..

 

[Chris1956]

The battery cables on those older Mercs can corrode under the insulation. When that happens, the current cannot get to the starter. You can feel the battery cables along their length for bulges. Bulges are corrosion. In addiiton the connections at the quick connect can partially break or corrode.

A simple way to rule that out is to get some known good battery cables and wire them directly to the starter solenoid and block.

Also check starter ground cable and cable from solenoid to starter. You do have a starter ground cable, right?

 

[legalfee]

Coils fire with spark plugs out but not in: 1. Check for dragging starter or low battery causing slow cranking speed. DVA test stator and trigger. 2. Disconnect rectifier, regulator and retest. If the problem goes away, replace the rectifier and/or regulator

 

[mruocco]

Thank you all for the information. I plan to work through suggestions here tonight, starting with a battery test and connection checks/cleaning.

Can I use jumper cables from battery to the connections to rule out cable corrosion, or do I need to pick up some new cables?

 

[Chris1956]

You can try jumper cables. If she spins faster, that is the issue, however, jumper cables often do not make a good connection.

 

[jimmbo]

A 1980 model would still have that big electrical connector plug and receptacle on the starboard dude of the lower engine cowl. A great place for connection problems.