4 HpYamaha backfiring

connel42

Hi
I have a 4hp Yamaha 4 stroke outboard.
I actually have a few of them and am well used to servicing them.
Usual problems are fuel related and I have the carbs off regularly.
I have one engine that is causing me problems.
Most of the time it starts ok but now and again it will backfire while pulling the start cord and the recoil will nearly pull your arm off.
The other day while running for about 5 minutes there was an almighty bang but the engine stayed running.
About an hour later after it was turned off for fishing 30 mins or so it would not star again.
I stripped it and cleaned the carb which seemed fine anyway.
The engine started first pull and ran in the tank for a good while.
The following day it kept cutting our and backfiring whilst running on the boat.
I stripped it again and cleaned everything again even though it is spotless - back into the tank and runs great for 30 mins or so.
Stopped the engine and left for an hour - went to restart - backfired and nearly lost a knuckle - Sorry if I am rambling but this is doing my head in and I am seriously thinking of writing this one off for spares.
Any ideas on backfire solution would be great
I dont see any adjustments on the carb and as far as I know the timing is fixed.

fergal.b

It could be a few things timing,fuel oil mix, bad float or missing spark are just a few but it sounds like an over lean/rich mix causing pre- ignition forcing the piston in the wrong direction. Do you feel the backfire is out the carb or exhaust.




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connel42

I have checked the spark and replaced the plug but I suppose the spark could be out of time with the stroke
It's a four stroke so the fuel /oil mix shouldn't be a problem
It's hard to know where the backfir is coming from
I think I will swap over the whole carb to see the problem is that.
If its the timing I think I'm stuffed as it may be fixed.

I will let you know how I get on thanks

fergal.b

When you take out the spark plugs have a look and smell to see if they are still wet with unburnt fuel.





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connel42

I was talking to Bobby in Killen Marine and he suggested checking to see if the woodruff key was broken
This would cause the flywheel to move which would knock off the timing
I'll let you know but I am hopeful

fergal.b wrote: »

When you take out the spark plugs have a look and smell to see if they are still wet with unburnt fuel.





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pedroeibar1

If anyone can help you on a Yammy engine it is that gentleman on Barnhill road!

connel42

Well
I pulled the flywheel and the woodruff key was perfect.
While the flywheel was off I decided to replace the coil and ht lead.
Back into the tank and running like a sale wing machine.
I'm not fully convinced I have solved the problem so will run it on a boat over the weekend
Can't think of anything left to check