Mercruiser 3.0 only starts if i remove any one of the 4 spark plugs

[ihatemyboat123]

Long story, I'll try to make short. Bought a project boat last year, gutted and restored the deck and interior. Motor is a 1986 Mercury 140 3.0. Replaced the starter, carburetor, head gasket, plugs and wires. Started it up two days ago, ran great, took it to the lake for the first time. Ran fine, pulled the kids on a tube, everything seemed fine. Started noticing a hesitation when I accelerated and then it would run fine. Evening comes, head back to the dock. Kids want to swim so I stop a few hundred yards away. Go to leave, engines cranks and cranks but won't fire. Finally starts running just enough to limp within 30 feet of the dock and dies, have to paddle back in. Get it home and start to check everything out. It has fuel, streams shoot out the carb when it is throttled. It has compression, 150 or close on all 4 cylinders. It has spark but it looked a bit weak. Tested the coil and the ohms read correct per the manual. Replaced distributor cap, rotor and points. Still won't fire. Put starter fluid straight in the plug holes, still nothing. Pull a plug to test spark again and the engine fires and runs, runs rough but it runs. I can pull out any plug and leave it attached to the wire, or not, the engine will run. I tried adjusting the valves again in case it's an air flow issue, no change. I'm totally stumped, anyone have any suggestions?

 

[alldodge]

1986 Mercury 140
tried adjusting the valves again

How were the valve adjusted?

When it won't start is their 12V on the + side of the coil when cranking?
What voltage reading is there on the + side of the coil with key in the ON position?

 

[ihatemyboat123]

By adjusting valves I just meant the up and down lash. I did it by the manual, tighten until no play and then 1 full turn. I only checked the coil for ohms and not while cranking. I'll do that and let you know. Thanks for the reply.

 

[ihatemyboat123]

Set my voltmeter on 20 VDC put the leads on pos and neg and it read 0.7 while cranking.

 

[Bondo]

Set my voltmeter on 20 VDC put the leads on pos and neg and it read 0.7 while cranking.

Ayuh,..... Welcome Aboard,...... Go from the coil's (+) to Ground,.... not the (-) side, that goes to the points,...

 

[ihatemyboat123]

Got it thanks.
At + and ground while cranking, the voltmeter jumps all over from 0.2 to10.4, but never reached 12. Should it be jumping like that and should it be higher?

 

[fishrdan]

Changed points, did you change the condenser too. Did you use a dwell meter and timing light changing the points?

 

[ihatemyboat123]

I did change the condenser. I did not use a dwell meter or timing light, however the problem was happening before i changed the points and condenser, that was the last thing I did to try and fix it.

 

[achris]

Got it thanks.
At + and ground while cranking, the voltmeter jumps all over from 0.2 to10.4, but never reached 12. Should it be jumping like that and should it be higher?

That's doesn't sound quite right. While cranking, the voltage on the coil + should be 12 volts, dropping down to about 9v at the most (as the battery voltage drops due to load from the starter motor). I would expect to see that ^^^ sort of voltage swinging on the coil -, not the +. Check you have the coil wired the right way. + should have a purple and a purple/yellow on it, and the - a gray (tacho) and a black (to the points).


Chris.........

 

[Benny67]

Pull a plug to test spark again and the engine fires and runs, runs rough but it runs. I can pull out any plug and leave it attached to the wire, or not, the engine will run. I tried adjusting the valves again in case it's an air flow issue, no change. I'm totally stumped, anyone have any suggestions?

The removal of the plug (any plug) would just allow the engine to spin a bit easier due to the decrease in compression.

The valve lash is doubtful to come into play unless they are not opening at all...but the fact you said the motor ran for a bit would lead me to believe it wouldnt come into play in this case.

The fact of the motor would run due to the removal of one plug would make ME believe it's possibly a weak spark...so weak that the decrease in compression was allowing the remaining cylinders to fire and allowing the motor to run....run real bad, but none the less run.

I'd bet you have a bad connection in the ignition somewhere.

MAYBE a bad coil...

Comb over EVERY connection in the circuit.

Last time I jumped into one of these threads I got beaten up by a few guys who thought they were the cat's a$$ so I'll probably just watch from this point on.

Good luck.

 

[ihatemyboat123]

I'll give that a try, thanks. Your explanation of why it runs with the plug out seems to make sense to me. Thanks again.

 

[Benny67]

One more thing I will add to this is,you might want to think about loosing the points and adding the pertronix setup w/matching coil

I just feel it's way better than screwing around with points. You just set it and forget it...end of story.

Before everyone blows up and starts hammering me over the rocks...It's a suggestion... I am NOT PAINTING IT AS A ROUTE TO REMEDY THE PROBLEM

once again, good luck.

 

[harleyman1975]

How much voltage at the battery when cranking? I'm thinking it may be bad battery.

 

[Boating first timer]

I am having the same issue. Were you able to get to the bottom of it?

 

[achris]

Hello Boating first timer and :welcome:

This thread is 3 years old.. As we request in the forum rules, please do not post to threads more than 90 days old, start a thread of your own, detailing the engine, with engine serial number and the problem.

This thread is now closed.

Chris.......
iboats mod team