Yamaha 50 not revving - suspected bad coil

alt · 08 May 2013, 00:48

Folks,
Have a 2stroke Yamaha outboard and on the weekend when out for a spin the revs died. It starts fine and revs fine in neutral but when under load will not do over 2700RPM (Normally gets up to about 5400RPM)

I changed the plugs and drained the carb bowls - no good.

When I took out the new plugs the bottom cylinder plug was very wet - I had no other tools at the time so didn't do anymore.

I suspect the coil could be shot. Thankfully there's another 50 here so I can swap a coil over tomorrow to test it.

My query is how would I check of the coil is shot?

Nos4r2 · 08 May 2013, 01:17

Check for a spark

Or us a multimeter if that's inconclusive.

Take one of the good coils as a benchmark and measure every different resistance you can.

Or just swap the coils over and see if the dead cylinder follows it.

Davie · 08 May 2013, 06:35

Fuel pump works on bottom cylinder in that model. Check the fuel pump diaphram first.

Jizm · 08 May 2013, 07:02

It's not limiting the revs in 'get you home' mode is it?

alt · 08 May 2013, 10:01

I can take a whole fuel pump with me from the donor engine - is it ust a case of un-screwing the old one? I know there's 4 screws on the face of the pump but not sure if that's to access the disphram or if it also mounts the pump to the block

It's definately not 'safe home' mode.... oil is full and registering as such, there were no alarms and I think the safe home mode allows less RPM than 2700

alt · 08 May 2013, 13:43

OK so I went to the boat this morning....
I have ruled out the bad coil - this is not an issue. I brought out a spare coil and swapped round - made no difference. I started the engine with only 1 HT lead connected at a time and each time it ran fine at idle with no load.

I cleaned out the carb bowls and the small filter again - looked clean.

I took off the air-box and noticed that when I revved the engine in neutral (it slowly made it's way up to 5300RPM) with no load that there was no petrol coming out of the bottom carb.

So I now suspect that it's either the carb needing a rebuild or the fuel pump. What was interesting is that when I tried to run under load and engine going only to about 2700RPM, squeezing the primer bulb made no difference - would this rule out the fuel pump issue I wonder?

A1an · 08 May 2013, 13:59

Quote:

Originally Posted by alt

- would this rule out the fuel pump issue I wonder?

No.

Whip the pump off and split it, check the diaphragm nearest the engine, if its split it fills the bottom cylinder with fuel and can cause the symptoms you are experiencing.

alt · 08 May 2013, 14:10

Quote:

Originally Posted by A1an

No.

Whip the pump off and split it, check the diaphragm nearest the engine, if its split it fills the bottom cylinder with fuel and can cause the symptoms you are experiencing.

Thanks for the advice. The engine actually belongs to a friend of mine and was just trying to rule out a few things before sending it to a mechanic. I won't be round to the boat again for 2 weeks so the owner will prob just send it into the mechanic.

It'll be interesting to see what it turns out to be! I was going to whip off the fuel pump on the donor engine but I saw a gasket behind it (although it looked plastic and re-usable) but didn't want to risk creating a problem on the donor engine so left it

two stroke mick · 08 May 2013, 15:13

which mechainic would you be considering for this work alt?

alt · 08 May 2013, 16:45

Quote:

Originally Posted by two stroke mick

which mechainic would you be considering for this work alt?

The guy who owns the boat has a mechanic (I do all my own, i'd be doing the work on this myself if it were my engine), think it's Moycullen Marine in Galway