1973 Mercury 1150 Misfire on cylinder 3 and 6

[keithjones825]

New: trigger, plugs, wires, harness, key switch, rectifier (non-standard with voltage regulator). I used a timing light and tracked down poor performance issues to intermittent spark on cylinders 3 and 6 only, 1,2,4,5 are all firing away. swapped plug wires and problem stays on 3 and 6. cleaned distributor cap and lightly sanded contacts, no change. could this be an issue with the switchbox? it is dumb and doesn't differentiate between cylinders, trigger sends a signal and switch forwards that on to coil for a spark. bad distributor cap? I'm kind of lost.

 

[Chris1956]

I do not think you have an ign problem. At least not on the distributor side.
Make sure the part of the cylinder head that seats the #3 and #6 spark plugs is clean, so they have a good ground.

What did you see on the timing light? No flash on cylinders 3 and 6?

 

[keithjones825]

Plug photo. 1-6 Left to Right. 2 and 4 are a little clean, but 3 and 6 look normal.

I do not think you have an ign problem. At least not on the distributor side.
Make sure the part of the cylinder head that seats the #3 and #6 spark plugs is clean, so they have a good ground.

What did you see on the timing light? No flash on cylinders 3 and 6?

Random flashing on 3 and 6. Sometimes one or two flashes a second then none for a second or two, then a random flash here and there, vs constant strobing on all others. I'll pull those plugs and give them a good cleaning in the morning. I do recall a coating of fuel-oil mix around some of them when i pulled them and took the photo a week or so ago. I can put a ground wire straight on the base of the plugs as well and see if that gives me a good strobe on the timing light to confirm.

 

[Faztbullet]

Pull the boot of plug wire on plug end..Seen the spring rot off and cause no spark issues

 

[keithjones825]

I did a bunch of trouble shooting this morning. Started off by pulling the plugs on those cylinders and they were dirty cleaned them up and that had no effect. Pulled the distributor cap and all the wires and tested from the plug end all the way to the contact points on the inside of the cap and had nearly zero resistance on all 6, so that wasn't the issue. Finally I'm getting ready to put things back together and decided to check the little metal disc on the trigger to make sure it was clean and it was, but the thing I did notice as I rotated it around is that in the positions for plugs 3 and 6 which are next to each other that little metal disc was contacting the pickup or Magnet or whatever you call it that is suspended below the trigger housing because the disc was slightly warped. It probably got a little bent during installation when I changed the trigger. I flattened it out and made sure that there was about the same amount of gap during the full rotation of the disk put everything back together and now I have perfectly even strobing on all six plug wires with the timing light!

 

[Chris1956]

Good stuff, I hope that is the issue.

 

[keithjones825]

I'm going to take it out in the morning to give it a try on the lake. I had good idle and decent operation up to 2,500 RPMs the other day and then it would fall flat. This better be it because It's high in the low 60's after tomorrow and I'm ready to winterize, but not until it's running right again. Before all the shorts in the 40 year old wiring harness fried everything I had 5200 RPM at WOT and 26mph on a 20ft pontoon.

 

[keithjones825]

Ran like a dream. Came home and started winterizing. Lower unit had a bad shift-shaft seal, but I had one on hand. Waiting on impeller kit to put it back together. Impeller was still fine but the housing should have been replaced last rebuild (had melted plastic at the top) and gasket tore when I pulled it apart. I should have it all buttoned up later this week and ready for spring.

 

[Chris1956]

If the waterpump base is melted, replace it also. Also, if you suspect a driveshaft seal leak, replace the pump base. Make sure to clean the driveshaft and lube, before replacing pump base, or new seal will be damaged.