1979 75HP Evinrude 75943R - Throttle Issues

[lazyMlazyK]

Hey all, I've got a new issue with my old 75hp Evinrude outboard. Sometimes when I switch from idling to forward gear, then wide open throttle, I get some vibration and wide open throttle only gets me up to around 7mph. Usually gets up to around 35mph with my current boat. Usually, if I leave it in wide open throttle for a minute or so, it’s like something “catches” (nothing audible or anything), and it’ll ramp up and run fine like normal. Any thought on what might be going on? New plugs at the start of the year. Also, I've only noticed this happening when other people are on board with me - could the issue have something to do with load? I'm thinking not, but figured I'd throw that info. out there too. Could there be a spring or something somewhere that's catching on something, then jumping to the position it should be in? That's kind of what it feels like is happening. There's no clunking or anything when it switches from "limping around" to normal WOT.

Also noteworthy - during the time it only runs around 7mph in WOT, the motor is not revving up like normal either.

 

[racerone]

Does the timer base rotate easily ?-----Do this simple spark test.----On a test device does the spark jump a gap of 7/16", yes or no?-----Stop running this motor hoping it will fix itself.----Find the issue.----A plugged carburetor could lead to starving one cylinder of OIL.----That will be expensive.

 

[oldboat1]

Prop issue? Slipping?

 

[lazyMlazyK]

I have no tachometer, but the RPMs sound correct for 7mph when it wants to run that way, and the RPMs sound correct when it is running normally at 35mph. The speed and RPMs seem to make sense together, regardless of the speed. The vibration that I'm seeing is only when it's low RPM and low speed, but WOT. The vibration stops after something changes and the RPMs and speed pick up. I don't think the prop is slipping. I'll have to pull the plugs to see if I can tell how far the spark will jump. What test device is used for this that you speak of?

 

[oldboat1]

still could be a prop issue, though -- balance. A prop shop can repair most damage, and rebalance.