V8 Weak crank with spark plugs in, but only sometimes

The starter is from an older F350, and it cranks just the same when I hook up the jumper cables to the starter itself... The cables don't have any obvious pinches or damage, and I've cleaned all the contacts with a wire brush. The wire from the relay to the starter certainly does get warm, though..
The engine even gets physically difficult to crank by hand when this happens, but seems fine once I remove some spark plugs, crank it, then put them back in.

It even fired up and idled with 3 plugs removed (forgot to unplug the coil)

Regardless of the apparent condition of the battery cables I would replace them way too old.

Sounds like a possible starter issue from your video. Who cares how it spins when bench testing.......under load vs bench test is two different things

Also this is a point ignition????? Make sure the gap is correct and the base timing!!!!!!!

I am not sure you can reference turning the motor over by hand as a test..........

if it is in fact hard then remove the outdrive and retry to turn by hand. maybe an issue in back of the motor. (gimbal bearing/outdrive)



Oh and dump the screw driver for activating the solenoid!!!!!!!!!!!!!!!!!!!!!

Get a on off switch and 4-6 ft of ten gage wire and connect it to the switch and put alligator clips on the other ends of the wires and connect one side with 10 gage wire to the small terminal on the solenoid and the other straight to the battery plus and use the solenoid as designed.

Generating that kind of spark is not good in a boat!! Regardless of how you may feel about it!

That starter is not "ignition protected". Lose the starter and buy a marine starter before you blow yourself up.

When you had the plugs out, did you get any water out of the spark plug holes? Water doesn't compress so if your getting water in the cylinders this could prevent the motor from turning (hydrolocked).

Thanks for the responses!

capnlazy said:

When you had the plugs out, did you get any water out of the spark plug holes? Water doesn't compress so if your getting water in the cylinders this could prevent the motor from turning (hydrolocked).

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Nope. Directly before that video I removed all the spark plugs and cranked, there no sign of fluid coming out of the holes. It started and ran for a few minutes after I put them back in. I shut it off and tried again a few minutes later, that was when I filmed the video.

kghost said:

Regardless of the apparent condition of the battery cables I would replace them way too old.

Sounds like a possible starter issue from your video. Who cares how it spins when bench testing.......under load vs bench test is two different things

Also this is a point ignition????? Make sure the gap is correct and the base timing!!!!!!!

I am not sure you can reference turning the motor over by hand as a test..........

Click to expand...

Sorry my terminology isn't great, but my distributor is this style. Is this what you mean by point ignition?
The engine idles and runs quite smoothly when it actually starts.

I was also thinking of the cables and starter (since it is a junkyard starter)
Before I spend too much replacing parts and wires, here's another video:

That's from 2 months ago with the spark plugs in, but it's the speed the engine cranks immediately after removing the plugs, cranking, replacing them, and trying again (even using the boat's battery by itself). Would a bad starter vary in performance like that, or would it simply weaken overall?

Cranking by hand isn't exactly scientific... But since it went from 10-20lbs of force to turn the crankshaft with a 1ft socket wrench to over 100lbs, I thought it might be significant (Estimating, I don't have a torque wrench)
Also I know I keep going back to this, but it also goes back to the 10-20lbs to turn after removing the spark plugs and cranking by hand, then replacing them.

if it is in fact hard then remove the outdrive and retry to turn by hand. maybe an issue in back of the motor. (gimbal bearing/outdrive)

Click to expand...

That's probably next on the list. The drive was in neutral last time I ran the engine and I haven't changed it, but we're eventually pulling the motor anyway to take a better look at the boat. I'll try some more after we remove the drive.

Oh and dump the screw driver for activating the solenoid!!!!!!!!!!!!!!!!!!!!!

Click to expand...

I actually set up a remote switch after I got the engine running, but seem to have fried the solenoid. I'll grab a new one next time I'm in town.

Point ignition is under the cap. There is a plate with points and a condenser and the gap of the points must be close to specification. If you don't have points then someone may have converted to electronic.

If you do have points then any adjustment to them and you should make sure of this also is the timing MUST be set to specification.....maybe 8 degrees before top dead center at idle speed.

If the timing is way off it may run good but crank over badly



Any motor without spark plugs will crank over easily.

Once the spark plugs are in that allows compression to occur and thus the motor not spinning quickly.

And YES a faulty or weak starter will do what you describe.

Also are you sure all the valves are opening? you may have to remove the valve covers and with the spark plugs OUT crank the motor over and watch every rocker arm and make sure the valve is moving...........Sometimes after sitting for long periods of time valves will stick and NOT open