1987 Merc 45hp 4cyl ingintion problems

[jpellts]

I have what I believe is an ignition problem with a 1987 mercury 45hp "classic fifty". Here's some background. Last year I put the boat away and was having flooding issues. I had just rebuild the carbs the year prior so this spring I ran seafoam through it and cleared that issue. Engine ran great for a couple trips. Then out of the blue I lost spark to 3 of the 4 cylinders. Changed plugs and didn't resolve. I got my hands on CDI troubleshooting guide along with my manual and ran the gauntlet of checks and narrowed it down to 3 bad coils. After replacing all four coils, I still had no spark in one cylinder. After referring to CDI troubleshooting guide again I was inclined to replace the switch box, which I did and now seem to have good spark on all. However, since then, I started getting a "cough" at idle. It didn't always stall but would miss a few revs and come back up until the next cough. In order to stop the cough I had to go in on my fuel/air mixture screws to about 3/4 of a turn and adjust the idle stop screw slightly. Engine now idles great and seems smooth through the rpm range until just before WOT when engine starts to run "rough". It doesn't bog down, it doesn't lose rpm, it just hits a wall and doesn't sound right. Since then I have pulled and cleaned the carbs, tried a new gas can and new fuel line, started and ran engine with one plug removed at a time and all showed same noticeable difference from having all 4, did a compression check and all good (130 per I believe but no noticeable difference from one to the other), and verified synch and timing per manual (max advance checked while tied dockside). CDI guide has guidance for "doesn't accelerate beyond 3000-4000rpm" which I think is about where I'm at (doing 18-20mph now versus nearly 30mph before). That section of the guide suggested unwiring the stator from the rectifier which I did and didn't make a difference. Based on having new coils and new switchbox I'm leaning towards just replacing the stator and trigger assemblies. However, I still have in the back of my head the fact I had to go in so for on the idle screws on the carbs to stop that cough early on in this whole process and the fact it ran so well before any ignition components failed Could that be a sign of a faulty trigger throwing the firing sequence off? Maybe I have two separate issues going on? I did some reading on crank seals but sounds like typically one has to go out on the idle screws to compensate. Not sure I'm going to mess with it any further this year but any advice or thoughts would be greatly appreciated! Thanks!

 

[jimmbo]

Your drop in rpm sounds about right for the loss of a cylinder. Are the wires coming from the trigger losing conductivity or grounding out as the stator plate moves when the timing advances? When you say good spark. can it jump a 7/16" gap? How did you verify/set max timing at the dock?

 

[jpellts]

Thanks for the reply. I honestly haven't gapped the spark, which I should do. Based on running with one disconnected at a time and having a noticeable difference with each in my head I guess I had ruled that out, and the fact I don't have a gap tester. Max time was checked while tied to the dock, one person on the controls and me in the stern with a timing light. i did tweak slightly to get to 32btdc but made no notable difference. I verified base timing and sync prior to checking max advance as well. I have a Seloc manual that outlines the procedure pretty well. Im not sure about the trigger wires. I've done so many different checks but i know I haven't done any checks at the rpm range where things get rough. not sure I'd know how to go about doing that. Wires them selves look good. I'll get my hands on a spark tester this week and at least do that easy check. As long as I'm getting good voltage off the stator (which so far all my checks indicate I am), with a new switchbox and coils, if I do find weak spark would it likely be wires?

 

[jimmbo]

wires, poor connections, poor grounds, bad switchbox, etc
Your engines spark plugs are surface gaps I believe, not adjustable. check the plug insulator for cracks

 

[Faztbullet]

ran the gauntlet of checks and narrowed it down to 3 bad coils

This is hard for me to grasp as coils rarely fail.I can put all the coils I have replaced in a 1 gallon bucket from 30+ years of being in service industry. The only time I have replaced multiple coils was on a V-6 Merc that was hit by lighting on lift in boat dock. As to it hitting a wal take a timing light and see where timing stops when at WOT with load. If its over/under advancing check the wht wire on trigger and the bia's reading on switchbox.