Yamaha outboard problem

[Caribboat]

Yamaha F4 4HP 4 stroke outboard dies ...
My outboard seemingly dies in a random fashion. It generally starts for the first time of use in any day. Then, after a ten minute run ashore it might die after I’ve slowed down for a minute or it might not. After leaving it for a couple of hours it usually, but not always will restart. It almost always restarts the following morning. It doesn’t matter whether I use the internal tank or the main tank. I’ve changed the spark plug, stripped down the carburettor and cleaned it, checked the fuel filter and put a dose of carburettor cleaner in the fuel tank. Sometimes the engine will work fine for a week, sometimes it won’t. I’m reluctant to take it to a service station because I won’t be able to get back on my boat which I live on and is at anchor. Also, because the problem is random and Sod’s law will mean that the engine runs perfectly when I hand it over to the engineer.
Any advice gratefully received.

 

[crewman]

I have the same outboard, but not the same problems. You seem to have checked the obvious - have you changed the HT lead? Breakdown in that could give this sort of intermittent problem.

 

[coveman]

I know I'm stating the obvious, but are you sure the air vent on the tank is clear? I imagine you have checked it! The other possibility is that the coil is breaking down when the engine gets hot. I had a four stroke lawnmower that would run for perhaps half an hour and then cut out. Leave it 10 minutes and it would start again, run for a while till it got hot and then cut out again. A new coil solved the problem.

 

[VicS]

I think you ave to try to determine if the problem is ignition related or fuel/ carburation related

Check the strength of the spark. It should jump 9mm or so in free air ( An adjustable gap spark tester is the best tool to use but it easy to improvise)

An inline spark tester will allow you to watch whats happening while actually running
Check the position of the coil/CDI unit. I guess it should be very close to the flywheel but the W/S manual might give more info

Run it after dark to look for sparks jumping places they shouldn't

Agree, check HT lead.

Fuel pump is common to both tank systems. Try pumping the priming ball with remote tank in use. If that will keep it going look for defects in fuel system or pump.

Maybe just needs a more thorough carb clean. Blow through all passages and poke a soft copper wire through jets.

You dont say what year .... maybe this manual will be useful. http://www.boatinfo.no/lib/yamaha/manuals/1997-2003yamaha.html#/0

 

[Milgrip]

Most outboard problems (Imo) are fuel related, foreign matter in the carburettor. The jets and passage ways are tiny.

If you can't get hold of an ultrasonic cleaner then clean the carb again and I mean a proper thorough dismantle and clean.

Sometimes a single carb clean is not enough. I have had carbs off three times before I got them running properly again.

 

[thinwater]

Ignition. Coil or lead. Otherwise you would have cold starting problems.

 

[john_q]

I have a 2 stroke 4HP Yamaha that started running erratically, stopping for no reason and sometimes very hard to start, this engine is used every day. It needed the fuel turned off every time I stopped or it would not start easily again. Went through the normal Carb cleaning and replacing every thing I could, took it to the Yamaha dealer 3 times, no joy, then found an independent who pressure tested the fuel line. Although a new needle valve had been fitted, the needle valve seat is not a replaceable part and had worn so was not sealing off when required, he cleaned the seat which sorted the problem for another 6 months before the problem returned. I bought another carb off ebay and the problem disappeared

 

[thinwater]

^^ This can happen, specifically with the new non-vented tanks. In the US, at least, a pressure/demand valve is available. A simple valve works.

However, since the OP is not having this problem with cold start, this is a different problem.

 

[Caribboat]

Thanks everyone for your advice. I’ll work through them all and let you know how I get on.

 

[[159032]]

I had the same problem on the same engine. It was a while ago so I am not sure that I can remember the solution. It was the thing about running slow before it dying that made me think of it.
I think the problem was the float valve needle in the float chamber. It had a very slight mark around the rubber tip. Not enough to cause alarm, but in fact it was jamming in the seat. A new needle plunger did the trick. But it wasn't cheap. About £20,I think.

 

[Caribboat]

Here’s my report back as promised:
In addition to cleaning the carburettor, etc as per my first post, I stripped the fuel pump down but found it clean. Another cruiser took the engine to a mechanic who diagnosed a faulty water pump. He stripped it down and cleaned it. But that didn’t cure the problem. My next step was to check the HT lead but before I did that I took a look in the main fuel tank. The petrol tank was clean but the petrol itself was pink. A nice rosé colour. It must have leached the colour out of the tank which is red. I bought a few litres of petrol and put it in the engine’s internal tank. It’s run now for about five hours over the past few days without a hitch, sometimes in quite choppy conditions. I hesitate to declare the engine fixed but I am regaining a little confidence in it. Fingers crossed!