Yamaha F4 motor, quits randomly

[bpsuls]

I thought I'd reach out here before I pull the trigger on a expensive part. In general I'm pretty good with these things, but this one has me wondering. Last year I was ripping around on the dinghy with a 05 Yamaha F4 and it randomly quit after years of working perfect. Wouldn't restart. Row back to the big boat. Pretty sure I remember trying it the next day and it ran. Ok, maybe the carb barfed. Anyways, take it on another trip, won't even start. Take it home, put a compression tester on it, 0 psi. What?? 0? Did I blow a hole in the piston how is this even possible? Take the head apart, stuck exhaust valve open. Ah ok, explains the 0psi. Why is it stuck? I read it could be because the valve leaks and allows oil on it that burns and makes it sticky. Ok, check the valve spec, it's good, replace the valve seal in the head, put it back together. Test it, runs awesome, great.
Out on another trip, quits randomly again. WTF? the valve again? Nope compression is good. Wait a while and fires right up like nothing is wrong and runs perfect. Again quits randomly, notice gas pissing out of carb this time. Ah ok, the float needle is done and is flooding the carb, explains random failure. Replace that. Test again, bring the carb to pressure with the bulb to make sure it hold tight, perfect. Out on the water, runs perfect WOT for 15 minutes, then as I slow down I notice it wants to stall so I bring it back to WOT slowly but after a minute, it then dies suddenly like you turned it off.
Row back to shore, try it at the dock, fires right up again, like nothing is wrong. Ok... Must be the coil pack overheating... ? It's not the carb and it's not vapour lock, it happens at idle too, I have thoroughly vetted it, cleaned it numerous times and the fuel pump pumps like a champ I verified this numerous times while it was running.

The TDI unit is $$$ and not returnable. Before I replace it I thought I'd check to see if anyone has run into this before. I've seen similar issues with overheating coil packs on other units before. Only other thing I could see is that dang valve still sticks sometimes but then pops back in and it starts again. Seems highly unlikely and the coil seems more likely. Kill switch is fine I even disconnected it to see if it made a difference, it didn't. I did do a spark check immediately after the failure on the water and it was hard to tell in broad dailight on the water but there was a spark I think, it looked weak though.

Long story I know but you need context to weigh in. Welcoming some feedback, this one has me scratching my head. Thanks.

 

[PacificElvis]

I thought I'd reach out here before I pull the trigger on a expensive part. In general I'm pretty good with these things, but this one has me wondering. Last year I was ripping around on the dinghy with a 05 Yamaha F4 and it randomly quit after years of working perfect. Wouldn't restart. Row back to the big boat. Pretty sure I remember trying it the next day and it ran. Ok, maybe the carb barfed. Anyways, take it on another trip, won't even start. Take it home, put a compression tester on it, 0 psi. What?? 0? Did I blow a hole in the piston how is this even possible? Take the head apart, stuck exhaust valve open. Ah ok, explains the 0psi. Why is it stuck? I read it could be because the valve leaks and allows oil on it that burns and makes it sticky. Ok, check the valve spec, it's good, replace the valve seal in the head, put it back together. Test it, runs awesome, great.
Out on another trip, quits randomly again. WTF? the valve again? Nope compression is good. Wait a while and fires right up like nothing is wrong and runs perfect. Again quits randomly, notice gas pissing out of carb this time. Ah ok, the float needle is done and is flooding the carb, explains random failure. Replace that. Test again, bring the carb to pressure with the bulb to make sure it hold tight, perfect. Out on the water, runs perfect WOT for 15 minutes, then as I slow down I notice it wants to stall so I bring it back to WOT slowly but after a minute, it then dies suddenly like you turned it off.
Row back to shore, try it at the dock, fires right up again, like nothing is wrong. Ok... Must be the coil pack overheating... ? It's not the carb and it's not vapour lock, it happens at idle too, I have thoroughly vetted it, cleaned it numerous times and the fuel pump pumps like a champ I verified this numerous times while it was running.

The TDI unit is $$$ and not returnable. Before I replace it I thought I'd check to see if anyone has run into this before. I've seen similar issues with overheating coil packs on other units before. Only other thing I could see is that dang valve still sticks sometimes but then pops back in and it starts again. Seems highly unlikely and the coil seems more likely. Kill switch is fine I even disconnected it to see if it made a difference, it didn't. I did do a spark check immediately after the failure on the water and it was hard to tell in broad dailight on the water but there was a spark I think, it looked weak though.

Long story I know but you need context to weigh in. Welcoming some feedback, this one has me scratching my head. Thanks.

I was tortured by a similar issue on a different motor. The problem: The kill switch had a short-circuit under its insulation (so not obvious to spot). When things got wet the short-circuit would trigger and the engine would simply stop working - apparently for no reason. When it dried she magically started working again. Wow I'm still traumatized by the work I put in trying to figure it out.

 

[Original]

I’ve had the kill switch wire short out at a plug connection before,
also seen the inner liner of fuel hose seperate and collapse,
another time I had a spark plug inner porcelain part come apart.
‘All of these were tricky to find. A new fuel supply was the easiest one to test.

 

[profisher]

Corrosion inside the spark plug boot could be the simple problem. There is a metal to metal connection internal to the plug which will turn black over time and start to loose continuity. Acts like a coil or plug acting up. Being a single cylinder if it looses continuity it going to quit. Check that before buying a coil.

 

[bpsuls]

The boot is an interesting one. I will check that. Also will put a new plug in it just to be sure, though i think that one is pretty new, could be defective. The imperative is that it works fine and then it doesnt, then it works fine again, but not right away. It screams a shorting coil to me, but I will definitely check the boot. Pretty sure its electrical, or some weird sticking valve thing but i'm just fabricating that i've never heard of that before.

 

[bpsuls]

So update on this. I replaced the plug and the plug cap. Ran it on the back of the boat in the driveway in a bucket, for 1.5 hrs. half an hour of that was at almost full throttle rev! even tried heating the coilpack with a heat gun, ran perfect. I though ok, maybe... but not convinced. took it to a lake yesterday, ran like a top for the first 15 minutes. i stopped to remove weeds, then wouldnt start again... got it going after cranking on it full throttle for a bit, sounded flooded. no problem, ripped around WOT for another 10 minutes, stopped for weeds again (Elk lake is rediculous for weeds) and it quit when i tilted it. wouldnt start again, dang, problem still there. got it started again after a 15 minute break. BUT: now i know its definitely fuel, and not the coilpack. It floods, especially in tilted positions. so... it seems like these motors do have issues with the floats and needles going wonky. not sure what to do here other than replace those but they look perfect. Yamaha parts are so expensive. Maybe a chinese knockoff carburetor...

As an FYI i ran it for another 20 minutes after that, stop and go, WOT for long periods. running it on tilt, stop on tilt, start on tilt, etc, ran perfect. PIA problem! couldnt reproduce it again for the rest of the day.

Thanks for all the responses though. Now i need to sort out how to get it to stop flooding the engine...