I need someone to outsmart my merc,cause I can't do it

[rolmops]

My 1984 75 horse 4 cylinder merc is giving me a serious headache.
For a few weeks now it is going back and forth between being a great machine and then refusing to start 2 hours later.
I drained the fuel tank and only use fresh seafoam laced fuel.The carbs are very thoroughly gone through and the fuel hose is brand new.The fuel pickup is clean.Last year I put in a new stator.
Today I ran about 8 miles and it started right up and ran like a champ.Half an hour later,it refused to start,but this time my nephew was with me and I asked him to start the engine while I checked for spark.There was no spark.So by now we eliminated fuel supply and discovered that there is no spark,although I suspect that tomorrow the engine will start up like nothing ever happened.
Is there a prime suspect for this sort of engine behavior?
Salmon season is starting to heat up and it is no fun limping back on a 9.5 little rudy kicker,even though it is currently more trustworthy than my 75 horse merc.
As always,thank you for your answers to this thread.

 

[daveswaves]

Re: I need someone to outsmart my merc,cause I can't do it

My 1984 75 horse 4 cylinder merc is giving me a serious headache.
For a few weeks now it is going back and forth between being a great machine and then refusing to start 2 hours later.
I drained the fuel tank and only use fresh seafoam laced fuel.The carbs are very thoroughly gone through and the fuel hose is brand new.The fuel pickup is clean.Last year I put in a new stator.
Today I ran about 8 miles and it started right up and ran like a champ.Half an hour later,it refused to start,but this time my nephew was with me and I asked him to start the engine while I checked for spark.There was no spark.So by now we eliminated fuel supply and discovered that there is no spark,although I suspect that tomorrow the engine will start up like nothing ever happened.
Is there a prime suspect for this sort of engine behavior?
Salmon season is starting to heat up and it is no fun limping back on a 9.5 little rudy kicker,even though it is currently more trustworthy than my 75 horse merc.
As always,thank you for your answers to this thread.

Sounds like the switchbox is failing when it gets hot.

 

[rolmops]

Re: I need someone to outsmart my merc,cause I can't do it

^^^^

 

[Laddies]

Re: I need someone to outsmart my merc,cause I can't do it

Have you disconnected the tilt (mercury) switch they can drive you nuts.

 

[rolmops]

Re: I need someone to outsmart my merc,cause I can't do it

Today she starts up like a champ.This makes it very hard to check for faulty parts,unless I replace everything, which I cannot afford to do.At least the coils are in very good working condition.
Laddies,congrats on your promotion.I saw just now that you past the 10000 mark.I thought that you had gone over to the "Motor Mouth" site.
I disconnected the tilt and trim switch.Maybe it will make things better.

 

[Joe_the_boatman]

Re: I need someone to outsmart my merc,cause I can't do it

I'll add that my engine would run fine for a few minutes, then wouldn't want to start again. We'd get it started after letting it sit, then limped back to the dock at idle speed.

Turned out we had a broken wire at the base of the trigger. Unfortunately, you need to pull the flywheel to inspect it.

You may want to start probing your ignition with a meter and you may find an obvious culprit.

 

[daveswaves]

Re: I need someone to outsmart my merc,cause I can't do it

Today she starts up like a champ.This makes it very hard to check for faulty parts,unless I replace everything, which I cannot afford to do.At least the coils are in very good working condition.
Laddies,congrats on your promotion.I saw just now that you past the 10000 mark.I thought that you had gone over to the "Motor Mouth" site.
I disconnected the tilt and trim switch.Maybe it will make things better.

Rolmops, I was serious about the intermittent switch box, since it runs in the morning take a heat gun or blow drier and heat up the switchbox while the engine is idling. If it is an open circuit caused by heat it will show up.

 

[thefairlaneman]

Re: I need someone to outsmart my merc,cause I can't do it

A bad condensor can cause this.

 

[Laddies]

Re: I need someone to outsmart my merc,cause I can't do it

rolmops, if you want to choke the engine out to stop it just disconnect the black/yellow wire from the switch box and run the motor a couple times that will eliminate the boat wiring, mercury switch and the kill switch, if that don't help then disconnect the 2 yellow wires from the rectifier and tape them off separately and run the engine a shorted rectifier will cause the stator to breakdown and kill the engine randomly. If none of this works then the only other thing that effects all 4 cyls. is the switch box. Thanks for the ataboy on the posts your the only one that noticed, I am a member of the MM forum but have just cut back with my posts here.

 

[OnTheWaterNow]

Re: I need someone to outsmart my merc,cause I can't do it

my 1994 mariner would do same thing my brother is an auto mechanic he said sounds like somthing in ignition was breaking down when getting hot he found bad switch box with heat gun and some kind of meter and was only running on two cylinders when finaly would start again when heated up bought new switch box for 150 solved problem

 

[rolmops]

Re: I need someone to outsmart my merc,cause I can't do it

I did check the switch box and it passed with flying colors and then I went through the tests that laddies advised and everything worked out dandy.
So I looked into other things and found out that although my battery was strong enough to crank the engine,at the end of the day it would only produce 9.5 volts which might not be enough to produce spark.Another battery was installed.Then I installed a 2 way valve, so although my kicker and main engine run off the same tank,there is no possibility for air to enter the system.
Today I went out on Lake Ontario and went about 14 miles out to get some king salmon.I ran the main engine,the kicker and the main engine again without any problems.Hopefully the battery replacement solved the problem.

 

[sschefer]

Re: I need someone to outsmart my merc,cause I can't do it

Rollmops, my 91 Mariner had a problem somewhat similar to yours. It would run fine for about the first 5 miles and early in the morning then it would start detonating. 5 miles was about my destination and if I throttled back to 3200 RPM it would stop detonating. I would then put on the trolling motor and fish for a couple of hours. When I tried to restart it would fire briefly then act like there wasnt any fuel. I'd crank and crank and finally after squeezing the bulb and pushing the choke in a few times it would start. When I took off I could only run about a mile before the detonation started back in again.

Does that sound anything like what your's is doing because mine turned out to be a bad NEW Mallory switch box. I replaced it with a CDI and the problem has since dissappeared.

The battery shouldn't affect anything in the starting circuit other than cranking speed. If you weren't hitting near enough the magic 300 rpm cranking speed then it may well have been your battery. The only thing the battery does is turn the starter but that turns the flywheel and that causes the stator to make starting voltage.