120 HP merc wont start

[In the dark]

I have had it on the water getting towed in after about a half an hour going fine. Now it still won't start.New cap rotor plugs condenser points all gone over several times it's getting a lot of gas and fouling plugs, pulling them out cleaning them up and putting back in.Replaced the batteries, but still wont fire. How would I know if it is a bad gas issue or do you feel that is a problem? I had some gas, over the winter in the tank with stabile in it and added more gas, on my first outing, but like I said it went fine for about a half an hour. My son in law is a mechanic and not just a back yard one, big trucks and cars, with a college degree in all this, so we are really stuck, Give us some more ideas please.
Thanks

 

[Bondo]

Re: 120 HP merc wont start

Well,........
It takes Fuel, Fire,+ Compression, At the right Time........For a motor to Run......

I suggest you run it til it Quits again,.......
Then figure out Which 1 is Missing.............

My 1st,+ only Guess is the possibility that maybe the Coil is getting Heat Soaked,+ Quiting................
But that's Only a Guess...............

 

[Don S]

Re: 120 HP merc wont start

We hired a 28 year old Ford auto mechanic.
He had no clue about carbs, how to start them, what a choke was for, how it worked or anything else. Then I was showing him the points distributor on a 470.............. He just stood there with his mouth hanging open. When I explained how it worked, he asked. "You're kidding" (or something to that effect)
So getting a college degree on modern trucks and autos doesn't help much with old 120's.

If you don't have spark, any fuel is going to keep going into the engine and keep flooding it.
When you put the new points in, did you set the dwell? If not, you have no idea if they will work or not.
Do you have power to the + terminal on the coil when the ign. switch is in the run position?

 

[In the dark]

Re: 120 HP merc wont start

This is all very true, thanks for opening the page up for me to see the bigger picture, It has to be there in the points and he is 27 years old and has kinda sluffed over the points and dwell. I will let you know. Thanks again, Don

Virgil Witt

 

[MojoRisin]

Re: 120 HP merc wont start

I'm having a heck of a bad year with my boat and am on this site seeking advice also. I know nothing about engines at all, but when we were diagnosing one issue on my boat, an issue just like yours started (lucky me).

My starboard engine started OK and ran and then out of nowhere died. We could not get it started again. My mechanic found we were getting a spark when the key was just turned but not when held trying to crank the engine. With quick short key turns, it started and then after time, would die. My mechanic basically hot wired the engine and it started and ran great. He then replaced the ignition switch and all is well - with this issue anyway...

 

[cheburashka]

Re: 120 HP merc wont start

I had a similar issue with my 140 HP Mercruiser. I had to be towed in. Turned out the points had slipped and weren't opening. At speed, the faster movement of the distributor shaft allowed them to open enough to keep it running. However once it was shut down, it wouldn't start at all. Have the points checked out. And at the risk of irritating some of the folks here who still like points ignition systems, I can't say enough good things about any of the Hall Effect ignition systems like Pertronix. I had one on my BMW R75 for many years and it never gave me any trouble at all. Just be sure to keep your stock points plate in the boat just in case.

 

[timobrien]

Re: 120 HP merc wont start

I have a 4CYL 120 OMC 1977, for many years it has persistently done something similar. It will run for appx. 30-40 minutes then the coil gets very hot, and starts breaking down and if the engine shuts off it will not start back. Numerous times I have changed the coil, points, condensor, and balist-resistor only to have the same problem. Last year i switched to pertronix electronic ignition, and a pertronix coil w/resistor it seemed to run a little longer but the same problem (coil overheating) occurs.

I bought a voltage regulator with an adjustment and adjusted to 13.5-14.5 output problem still persists, i even spaced the coil 3 inches from the engine to cool it, i even bypassed the ignition switch but it still occurs.

(i can cool the coil down with ice, and it starts right up and runs great once it cools)

Any suggestions would be helpful!