9.8 mercury problem
 

I have a 1970?s-ish Mercury 9.8 thunderbolt, motor starts fine and purs like a kitten. With just me in the boat it will start to bog when wide open but then eventually get up on plane to about 21 mph. When under load with 2 people in the boat, the motor will bog when wide open and stay sputtering only going maybe 5 mph. Any ideas what may be the issue? Thanks guys.

That sounds like the jets in the carburetor are "restricted" --- plugged up some. You could try spaying some carburetor cleaner [sold at Walmart or auto parts store] in it to see if the engine runs better. Spray in the carburetor and let it set for 5+ minutes and then try running the engine. If the engine is still "bogging" down..... give a short spray in the carburetor while it is running.




Or take the carburetor off and give it a good leaning to make sure all the small jet ports are not blocked. Also if you have not done it...? pull the spark plugs and clean them.

Thanks man ill try that this weekend!

I noticed today that when spinning my prop by hand it has a wobble to it. Could that be causing my issues?

In your first post?? you said that when you had 2 people in the boat that the engine "bogged" down. It also sound like the prop is fitting loose on the drive shaft. Maybe you should check to be sure the nut that holds the prop on the drive shaft is tight. This nut is supposed to be held "in place" with a cotter pin, so this pin must be removed before the nut can be turned.


Question.... Did the engine bogged down and actually slow down? Or did the engine keep running fast, but the boat slowed down?


If the engine kept running fast, but the boat slowed down I believe your problem is the bushing in the prop. Inside the prop there is a rubber bushing. This rubber bushing has a hole through it that fits over the engine drive shaft. This is a very tight fit, but over time this rubber bushing can ware out and start to slip and the prop does not turn as fast when under a load.


You can take the prop to a shop and they can replace the bushing. I don't know if there is a place near you, but the Prop Shop in Lafayette can to this.

Sounds like a bent prop shaft. Ever notice any vibration?

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Bent prop shaft
 

No, no vibration, im going to take the prop off today im assuming i should be able to see if the prop shaft is bent once i take it off

The engine both bogged down and slowed down. Thanks i will take the boat out today and try those things

Another simple thing that could cause this problem is a bad spark plug. I once had an outboard engine that ran ok, but would not run smooth at wide open throttle. When I finally changed the plugs.... it was like a new engine again.