yamaha 115 breaks up @ 4000

[FUNCAT-YAMAHA]

Help!
I have a 1986 Yamaha 115 outboard.
It runs fine and comes out of the hole perfect under load until the tach hits 4000 RPM, then it begins to break up.
If I back down the throttle below 4000 it runs fine all day long.
It seems to be ignition to me. I have good compression.
Any suggestions?

Thanks

 

[cousinabe]

Re: yamaha 115 breaks up @ 4000

A couple of thoughts.
Most problems seem to be fuel related.

What has been the service history of the motor?
fuel pumps, carbs cleaned, filters, plugs, maint, etc?

There is a known issue with the quick connect fuel fitting on the motor.
The motor side of that quick connect has a non servicable oring and your motor was not designed for ethanol fuels. It tends to swell up and cause a fuel restriction. You should either replace it or plumb directly to the motor fuel filter.

Have you performed any fuel vacuum tests?
Do you have a manual?
Have you performed peak output tests on the ignition?

 

[BaileysBoat]

Re: yamaha 115 breaks up @ 4000

Put in some fresh plugs.