Intermittent Miss

I have a 1991 Force 15 hp on a small tin boat that has given trouble since I bought it. I went thru the carburetor and thought I had it - put the boat in a local canal and it idled nicely and took off hard and strong when I opened the throttle - boat leaped into a full plane....then after a couple 100 yards, it quit. It would start, then die.

It pulled the threads on a carb mounting bolt and I had fits getting a heli-coill installed. While at that, I pulled the reeds and tho' not really familiar with them, I did inspect them and both seemed clean and similar. They appear to be in good condition and close completely.

Gas is fresh and mixed 50/1. I figured a piece of grit got into a jet, so went thru carb again and added a large inline fuel filter in hose from tank to motor. While I was at it, put a kit in the fuel pump and found the small inline fuel filter there, which seemed to be OK.

Bulb goes fairly soft when running on muff, but squeezing it back up makes no difference.

It seemed to run well on the muff, so put it back in canal and took off. It accelerated strongly, then "sort of" cut out. Seemed like one cylinder would quit - speed would drop to where it wouldn't quite hold full plane, then it'd catch again and boat would leap back into flat plane for a second or 3, then back to low power again. Seemed like it would go: mmmmm-p-p-p-p.mmmm.p-p-p-p-p-mmmmmm - etc.

Afterward, I took a short video of it on the muff. Toward the end you can hear it clearly:


The motor is VERY difficult to start when cold, then starts fine after the 1st fire up.

Wiring from flywheel isn't pristine and has obviously been worked on, but seems solid and secure. Connectors at primary side of coils are solid and tight but insulators are wrong size and are cracked and split. Spark plugs show fire with a cheap spark tester, spark leadss are tight and solid and plugs look new.

 

Cracked and split wires often arc. Run it at night and watch for arc/sparks. My guess is electrical, weak spark.

 

At the risk of sounding like an old cracked record, the first thing to do is spend a few dollars on a workshop manual that covers that engine, the manual will have a troubleshooting section you can consult. This beats guessing it "might" be x, y or z, that is the source of the problem. That is a laborious and often unproductive approach. Oh, and when I saw the thread title, "intermittent miss" it reminded me of the phenomenon of young women of inconstant affections ! Good luck in getting that motor sorted out.

 

Biglar,
You have your work cut-out for you.
I agree it's most likely electrical, but it could be fuel related too (or both).
Sadly, you just have to keep troubleshooting.
One variable at a time looking for any change whether better or worse.
Good luck!

 

I got a small tachometer in today that reads off wraps on a spark plug lead. It's for the other boat I have almost finished, but I think for the 12 footer it'll tell me if a cylinder is dropping out electrically. If the motor is running, but missing on 1 cylinder, if the tach is connected to that cylinder it should show a speed drop or no speed (rpms) at all.....it says here in fine print. We'll see.

 

Seriously, to even think of tinkering with an outboard, and that kind of simpler, older one is at least one that can be worked on at home, get the manual, and a simple tester that can be got cheaply at an auto accessories shop.

 

No. The crank spins. An electrical miss is easy to diagnose. Just look for a weak spark at night on either with a decent sp tester.

How do you figure weak spark translates to r's? Never heard that one b4.

 

Ya, when it's hesitating. You're going to need someone watching constantly for any sign when it hesitates.
That's cool you can use it for diagnostics like that. Clever.

 

My reasoning says that if the miss is electrical, then there will be intermittent spark at the affected cylinder. The tachometer reads rpms by counting spark impulses thru wraps around the spark plug wire. If that cylinder cuts out due to no spark, the tachometer would read zero, or at least fewer, rpms and the reading would jump around as the motor sputters. A good cylinder would show a steady rpm reading. If the miss is due to a fouled plug, I would expect it to stay fouled and cause a consistent problem. Been there, seen that.

'Course, I've been wrong a time or 3 in the past......??

I tend to think electrical because if it were fuel related it would affect both cylinders and the motor would die. When it had the problem in the water under load, it would come onto and back off full plane, but was making power the whole time.....so I think one cylinder is dropping out. Problem is figuring out why.....and it could be a bunch of things. Sooo.....I come to you guys for help. I did/do have the hope that someone would jump up and say, "hey, mine did just that and here's what the problem was."

 

Normally these small motors will run on one cylinder, so you pull off one plug wire and run it. If it doesn't cut out its not that cylinder, try the other one.

Most of the time it's the coil, your tach will pick up that failure, if it's the plug wire or boot, it typically won't.

 

...smart

 

You have to be careful disconnecting one plug lead, it doesn't take long for the coil to be damaged by that while the engine is running, it seems to be the case here of a home mechanic who has been tinkering quite a bit with this old engine in a largely haphazard way, and drawn a blank, when a more systematic approach with a tester and specs and procedures outlined in the workshop manual would likely have got to the bottom of it. If going to make changes to an engine without doing that, then about the only one would be to change the plugs, especially if the problem is intermittent.

 

Seems like every time I tried that in the past, I got knocked on my butt by the shock. I prefer less painful alternatives.

 

Yes, that shock is the punishment delivered by the God Of Tinkering. Then there is all the time wasted, air turned blue with curses etc. It always amazes me that when an old outboard plays up, the tinkerer heads straight to the carb, as if drawn by an invisible force.

 

You need to be smarter than the spark.

As I said, pull the plug and then run it.

Or, use some insulated pliers to pull it off.