Motor dies past 1/4ish throttle

[moffatt102]

Have a 91 48 hp Johnson. Idles great. Will go slow but dies when you give it more than half throttle. It’s just when the prop is in the water tho. The high idle lever makes it scream wide open. The prop come out of the water today and the rpms ran up. Seems like it only does it under a load. It has me stumped. Tried the toothpick in the bleeders and really didn’t change anything. Any help would be great.

 

[timguy]

Did the symptoms begin after a storage period? Has the motor ever been stored with ethanol fuel in it? Sounds like fuel deprivation. Ethanol, in due time, destroys anything with rubber in its composition as well as many plastics. It also provides an "open door" to fuel contamination as well as fuel deterioration. Yamaha techs have told us that running more than 10 percent ethanol in any boat, is not only unsafe, and destructive, but illegal. So my next question to them is "Why run ethanol at all, if we know it is harmful"?

 

[joereeves]

That engine will scream away on one cylinder with the prop out of the water or when in neutral as it loses all back-pressure.

Remove all spark plugs

Check the compression. What is the psi readings on both cylinders?

Plugs still removed... Check to see if spark will jump a 7/16" gap with a strong blue lightning like flame... a real SNAP!

Note that the 7/16" air gap is important. You can use a #2 philips screwdriver stuck into the plug boots, then hold the shank that distance away from the powerhead if you can't obtain a proper tester.