Not firing on all cylinders

[jwr_20]

Hello all. I am having a problem with my 1985 75Hp Merc. As most of you know, it is a 4 cylinder with 2 carbs. The problem I am having is that the bottom 2 cylinders aren't firing, or so I believe. I could get it running and idling at higher rpm with warmup lever just fine, but when I bring the rpm down and shift into gear it dies. I noticed that spark plugs 1 and 2 were hot, and 3 and 4 were cool, and fouled. The carbs are set correctly, and it is getting fuel. I did manage to get it into gear without stalling on 1 occasion, and it had no guts at all. The compression in all cylinders seems to be good as well. I think it has to do with an electrical component. The plugs are almost new, the wires and boots are in good shape. I had the motor running the day before, and it ran as well as it normally did, with lots of power, and speed. If anyone has any ideas, I would greatly appreciate the help. Please let me know if you need more info as well.<br /><br />Thanks a lot,<br />JR

 

[Clams Canino]

Re: Not firing on all cylinders

Could be a bad trigger or switchbox. Pull the 2 plugs in question and check visually for spark.<br /><br />Go from there.<br /><br />-W

 

[jwr_20]

Re: Not firing on all cylinders

Hello. Thanks a lot for the reply!<br /><br />I checked all the plugs, and wires. There was a strong spark on plugs 1 and 2, but nothing at all on 3 and 4. I checked all plugs with wire #1 and all plugs sparked normally. <br /><br />I have a Seloc manual for this engine and it mentions about some resistance checks to try on the trigger assembly.<br /><br />I think this may have been an ongoing problem, but I have always suspected the fuel as the source. One occasion, I had a hard time starting the motor, but did finally, and it wasn't running very well, with a lack of power, then it seemed to get a burst of energy, all of a sudden and the boat jumped out of the water, and planed out normally and the motor ran perfecly after that.<br /><br />Any more ideas are greatly appreciated.<br />Thanks again for the help so far,<br />JR