50hp Merc misfire on 1 cylinder?

[kerrybraswell]

I have a late 70's model 50hp Mercury that misfires on the #4 cylinder at all speeds. I have changed the spark plug and the coil and it is still misfiring on the same cylinder. I have hooked up a timing light to check for fire and it doesn't matter what RPM or whether the engine is hot or cold. That cylinder has an irregular misfire that i can't figure out. Is it possible that the switch box is the problem or is there something else to check first before i spend $300 for a new switch box? Is there any way to check a switch box with a multi-meter? I think the coils and switch box are the originals so it might be time to upgrade all of the electronics? TIA, Kerry PS, forgot to mention that this motor has the Thunderbolt ignition with the spark plugs that have the flat tips with no electrode sticking out.

 

[Laddies]

Re: 50hp Merc misfire on 1 cylinder?

It just about has to be the switch box if you have changed coils around

 

[roger3645]

Re: 50hp Merc misfire on 1 cylinder?

Have you checked the spark plug wire. Try switching one with #4.

 

[renoduckman]

Re: 50hp Merc misfire on 1 cylinder?

Make sure its not the plug, plug wire or coil.Check the ground wire and the coil wire to the switch box to make sure they are ok. I just put switch box on mine. Boatfix.com seems to have good prices on the switch box.

 

[kerrybraswell]

Re: 50hp Merc misfire on 1 cylinder?

I tried a complete new set of spark plugs, a new coil on the cylinder that was mis-firing, and switched plug wires with no improvement. All of the wires to the switch box are sound. I just don't know beyond the switch box, what other possible causes i might check before i change out a high dollar switch box. Nothing i have tried yet will make that #4 cylinder fire with any regularity. I will check Boatfix.com for prices, thanks! Kerry

 

[timmathis]

Re: 50hp Merc misfire on 1 cylinder?

Also check your distributor cap.<br />Tim

 

[kerrybraswell]

Re: 50hp Merc misfire on 1 cylinder?

This is the distributor-less CDI model. Is it possible that the stator or trigger could cause this sort of problem. They would certainly be cheaper to replace than the switch box if it was one of those two things. I am about ready to just replace the switch box and see what happens. Do the modern aftermarket switch boxes that cost less than the Mercury brand, last as long? TIA, Kerry

 

[renoduckman]

Re: 50hp Merc misfire on 1 cylinder?

Yes it can be the trigger or stator but its hard to test with out a peak reding meter. Im not the expert here but i guess you can test trigger by swithcing trigger wires and checking spark, without running motor of course! Let us know if you got it going or not

 

[kerrybraswell]

Re: 50hp Merc misfire on 1 cylinder?

I am thinking that i should just go ahead and buy a new switch box and if that doesn't do it then i can try a new trigger and stator. I can always sell the new switch box on ebay if it doesn't cure the problem and recoup some of my money to buy a stator and trigger. This motor runs pretty darned good even with the bottom #4 cylinder misfiring intermittently. I can hook up an inductive timing light to the #4 spark plug wire and watch it mis-fire every time i hear the engine sputter. It misses at all speeds but even running on only 3 cylinders, this motor will plane off the light boat i am running it on. I also have a 15hp kicker on this boat that pushes it pretty good, but i gotta get the 50 running right just for the principal of the thing. Thanks for the tips from everyone! I'll let you know when i get it figured out. Regards, Kerry