Club 80-90 Forums

Any 'experts' here that maybe able to assist?

I've a Yamaha 4 stroke that all of a sudden has started playing up. Never once missed a beat until now.

Starts and runs beautifully, however, it now runs for approx a minute then dies, no coughing or spluttering - just dies. I've learned that it won't restart unless approx a minute has passed, it then starts straight away and runs for a minute, etc. etc.

The symptoms are so regular that I'm baffled, if it was the coil, there would probably have been a few indicators before it failed altogether.

I don't think the float is getting stuck, once, when it stopped, I quickly undid the float bowl and it appeared full.

Could just be the plug which is a cheap fix, just thought I'd ask around first.

PS. petrol is a couple of months old which in this day and age renders it stale but I'm not convinced it's a fuelling issue.

All help gratefully accepted, Mitch.

Is the choke staying on?

First thing to do is find out if it's sparks, fuel or compression causing it.

Sparks: When it's died, take the plug out, lay it on some clean unpainted metal on the engine case, pull the starter and see if it sparks. Take care you can get a nasty shock from a spark plug and or lead. If you have to hold it when pulling, hold it with a couple of layers of dry rag or insulated pliers.

Compression: When you first come to it, engine cold, pull it over and get a "feel" for the compression. Then, when it has died, do it again and see if it feels a lot easier to pull over. If it is easier then check the tappet clearance. Could be too close and when it gets warmed up, they will close more and may be holding a valve open. Not uncommon on older mowers. It's called valve seat recession and is caused by using unleaded petrol.

Fuel: When it has just died, carefully pour 1/2 an eggcup full of fuel into the mouth of the carb (air filter off) and try to start it (might take a few pulls as it might now be a bit flooded) and see if it fires at all.

My money is on sparks, likely a duff condenser. Is it points and coil or magneto ignition ?

Funny, how reading comments can trigger other thoughts.

I know it's not a petrol fuelling issue as there is sufficient fuel in the bowl when it stops.

I have checked the spark with the plug removed but I've not done so immediately after it stops which is when I'm expecting there to be a failure of some sort or another. This, I'll go and do once the kettle has boiled.

Balls, just realised that the other half has taken the plug to work so that someone can sand blast it. When racing my enduro bike, I'd often blast my plugs and some for other riders as it can bring back to life plugs that are apparently 'dead'. Anyway, that'll be the first test upon receipt of my shiny plug this evening.

I'll keep you posted. Determined to get the grass cut whilst it's dry.

Cheers, Mitch.

Revitalised plug inserted, fired up and.................................kept running for 5 minutes whilst I cut the grass and then.................the rain started!!!!!!


Thanks for your input chaps. Mitch.