9.8 Merc revs high then loses power at WOT

[likestofish]

I just bought a 9.8 Mercury (maybe 20 yrs old) and have it on a 14ft aluminum jon boat. I just had it serviced and it starts/runs good. Although, after a few minutes of going at WOT it will all of the sudden increase its RPMs while simultaneously losing power. Any suggestions? Thanks in advance.

 

[joblo33]

Re: 9.8 Merc revs high then loses power at WOT

Welcome to the forum likestofish! <br />It sounds like you might have a spun prop hub. The rubber hub in the prop will start to spin when there's load on it, causing the engine to rev up without spinning the prop. A way to test if this is the case is to put a mark on the prop going down past the rubber hub and go for a run, and see if the mark has separated. A prop shop can re-hub the prop if it's spun.<br />Eric

 

[Motor Boater Bill]

Re: 9.8 Merc revs high then loses power at WOT

Spun prop sounds likely--but if it isn't consider this other possibility:<br /><br />This could also be fuel starvation, since if it is not getting enough fuel to the carb, the level will drop in the float bowl, the mix will go lean, and it will speed up before it dies. You can see this if you just unhook the fuel line while the motor is running. Does the motor keep revving high and losing power, or does it die? Is the tank vent open? Pump the bulb when this happens and see if that affects the problem.<br /><br />I'd bet on the prop.

 

[dajohnson53]

Re: 9.8 Merc revs high then loses power at WOT

Originally posted by Eric B:<br /> ... A prop shop can re-hub the prop if it's spun.<br />Eric

It might also be better/easier to just buy a new prop - depending on how quickly and cheaply the shop can re-hub. In my town it was going to take a while and cost nearly as much as the new prop. Plus, if you get a new prop - there are two available (from what I've seen) for the older Merc 9.8's: essentially one for use as a kicker on a heavier boat (a lower gear, if you will), and another that is intended for use as the primary engine on a small boat - your application. If your old prop isn't the correct "type" for your boat, or if it is at all chewed up, I'd consider just buying a new prop if it's spun. Good luck and have fun.<br /><br />By the way - another poster talked about fuel starvation. That could be the case for sure. I found that for me (a beginner) it was pretty straight forward and easy to clean and re-build my '74 9.8's carb. An older engine like that should probably have a thorough carb disassembly/cleaning anyway so there's no harm in doing that and it may solve your problem.<br /><br />Also, it is easy to check the fuel line back to where it goes into the tank. There is a filter there, as well as a filter near the carb - either can get clogged up and possibly cause your symptom.

 

[likestofish]

Re: 9.8 Merc revs high then loses power at WOT

Thanks for the quick and multiple resonses. I took it out again today and tried marking it as Eric B said in order to check for a spun prop. Although, I'm not sure if I did it correctly...I didn't see any rubber part on the prop. I took the prop off and marked it on the inside and took it for a spin. I made it perform the symptom once, but when I got home and took the prop off nothing appeared to have moved.<br /><br />After hearing your inputs though, I'm thinking it must be a spun prop despite my failed efforts to confirm it. When it revs hig, it revs very high. In my novice opinion, much higher than the motor should. As soon as it does it, I always let off the gas and the motor never dies.<br /><br />Doug

 

[joblo33]

Re: 9.8 Merc revs high then loses power at WOT

Ya, sounds like a spun hub.<br />For future reference, this is where you'd mark the prop:<br />

<br />If the prop spins on the hub the mark on the nut won't match up to the one on the prop after.<br />Eric

 

[dajohnson53]

Re: 9.8 Merc revs high then loses power at WOT

Originally posted by Eric B:<br /> Ya, sounds like a spun hub.<br />For future reference, this is where you'd mark the prop:<br />

<br />If the prop spins on the hub the mark on the nut won't match up to the one on the prop after.<br />Eric

I see you have the same customized skeg that I have!

 

[tashasdaddy]

Re: 9.8 Merc revs high then loses power at WOT

i've got a skeg like that also.

 

[joblo33]

Re: 9.8 Merc revs high then loses power at WOT

Who needs a sounder when you have a skeg right <br />Eric

 

[likestofish]

Re: 9.8 Merc revs high then loses power at WOT

Thanks again... and the pics are very helpful. After looking at the prop more closely and with a light, it looks to me like it is spinning as you suggest. I'm going to call around tomorrow and see what the folks here in Montgomery Alabama charge to re-hub the prop. If it's too much, I'll look into buying a new prop, as D. suggests.<br /><br />I'll let you know how it turns out.

 

[Motor Boater Bill]

Re: 9.8 Merc revs high then loses power at WOT

If the spun one isn't too bad--like, it would work OK at half throttle--consider buying a new one and keeping the other in the boat as a spare--a "get me home" prop in case you hit something and bust or badly bend a blade and need to swap it out to get back to the dock.

 

[Motor Boater Bill]

Re: 9.8 Merc revs high then loses power at WOT

Rereading your original post, I am wondering again about the spun prop diagnosis. Normally a spun prop would let loose while you are accellerating, this is when the torque on the hub is highest, as the motor is pushing against the weight of the boat, rather than "after several minutes at WOT." Just a thought. Could this be cavitation if not fuel starvation?<br /><br />I have paid $35-$40 to have props re-hubbed for a motor this size.<br /><br />Check eBay for used or NOS props before you drop a bundl on a new one, but if you buy used, make sure they can guarantee it isn't spun. I have bought used props on eBay for around $20-$25. Of course they go up in high season!