Merc 9.8 won't go at full throttle.

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Bearcamp

Petty Officer 1st Class
Joined
May 21, 2003
Messages
341
Runs great at all rpm ranges, but at full throttle it just acts like it's in neutral. 1/2 and 3/4 throttle are fine and goes right along. When you hit full throttle it just revs up like something is slipping. Checked for prop spun and it's fine. Any ideas?
 
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marinegrh

Petty Officer 2nd Class
Joined
Sep 1, 2003
Messages
196
Re: Merc 9.8 won't go at full throttle.

have you run this engine with a different prop just for ssss and grins, you say it runs great except wot, is the engine rpm turning up? or what, try and be a little more descriptive, also what condition is your fuel and ignition systems in?
 
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dajohnson53

Lieutenant Commander
Joined
Apr 28, 2004
Messages
1,627
Re: Merc 9.8 won't go at full throttle.

To Bearcamp or other- how do you check for spun prop? I understand that with a spun prop there is a disconnect between the prop shaft and the prop caused by the rubber hub being loose. But how loose would it be to cause symptoms - so loose you could feel it by hand, or could you test it by hand, think it's tight, and it still be spun and cause problems like Bearcamp describes?
 
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marinegrh

Petty Officer 2nd Class
Joined
Sep 1, 2003
Messages
196
Re: Merc 9.8 won't go at full throttle.

as engine exhaust gases and cooling water the has been heated up from cooling you engine pass through and around you hub, the can make prop loose its grip on the hub, also the friction of the prop cutting the water will create a grab no brab situation... i would really check it with another prop first..theres a great article about props in boating life mag this month
 
AMD Rules

AMD Rules

Lieutenant Commander
Joined
Sep 23, 2004
Messages
1,707
Re: Merc 9.8 won't go at full throttle.

Remove your prop and mark a line on the back side on the innner hub and along the outer housing. Take the boat for a run and then look at your line. If the outer housing has moved relative to the hub the prop has "spun" and needs to be rebuilt or replaced.
 
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Bearcamp

Petty Officer 1st Class
Joined
May 21, 2003
Messages
341
Re: Merc 9.8 won't go at full throttle.

OK, the engine rpms do go up with full throttle. Sometimes if you let off the throttle, give it throttle, let off the throttle, give it throttle it will go into gear at times. But around 1/2 to 3/4 throttle it acts like it goes into neutral. What about the clutch dog?
 
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dajohnson53

Lieutenant Commander
Joined
Apr 28, 2004
Messages
1,627
Re: Merc 9.8 won't go at full throttle.

How did you test for spun prop Bearcamp? I'm just asking again because I'm still curious if a person could miss it if, for instance, you tried to make it let loose by hand as your test, that it would only let go with the pressure of the engine against the water, like a couple of guys above have said. I've never had a spun prop, but would think that this could easily be the case, thus my curiosity about your situation.
 
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mjbrueck

Petty Officer 2nd Class
Joined
Sep 7, 2004
Messages
108
Re: Merc 9.8 won't go at full throttle.

What are you using the motor for and is this a new problem? It also sounds like what happens when a prop is ventilating. If you're using it as a kicker and it's not deep enough or using a short shaft on a 20" transom, you'll get the same results.
 
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