Motor won't get to proper wot rpm

[Ats666]

Hi, I've got an old 18 foot pontoon with an early 80s 18hp Mercury (serial 7198298).
The motor will only rev up to 3260ish rpm in forward (verified with Tach/hour meter) .
Compression in the two cylinders is 155psi and 160psi.
Idles perfect and does not stumble to its maximum rpm.
It's got the factory prop on it (48-43586-13).
When I put it in reverse I can get the revs up for maybe 2 seconds but then it slows down again.
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No water in toons. Doesn't mater if it's 2 adults or 5 adults with fishing gear on board, rpm doesn't really change.
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This is my second season with the boat. I don't think the carb has been gone through in 10 years.

​​Linkages appear to be moving correctly.

Does it seem like the right prop?
Do I need to go through the motor?

Thanks
Allan

 

[HotTommy]

1. Google "Mercury prop selector" for a site that will tell you what prop is best for your boat.
2. There is a forum here on iboats dedicated to prop questions.
3. There is a forum here on iboats dedicated to Mercury outboard questions.
4. As your question are more about your motor than the type of boat it is on, either of those might be better at answering your questions than this one.

 

[NYBo]

Welcome to iBoats! :welcome:

The prop number seems to indicate it has 13" pitch, which is way too much. See if you can find one of around 6 or 7" pitch.

 

[Ats666]

Thanks guys
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I posted in the prop section

 

[5150abf]

Did the boat ever go faster than that or is it new to you?

If at some pint it went faster I would think clogged main jets, sounds like classic symptoms.

 

[Ats666]

This is the second season I've had the boat and it was like that the first season to.
(put alittle over 275 liters of fuel through it the first year and about 125 liters through it this year)

I add 1 can of seafoam to the gas tank at the start of each season with fresh 91 octane and 2 stroke oil.

I picked up a prop from a local rebuilder for cheap.
10 3/8 "
9.5 pitch
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​​​​Would it be foolish to try this prop out or should I spend the time and go through the carb first?

 

[ahicks]

Does it start easily?
Does it seem to run OK after starting and warming up for a few minutes?
If so, it's most likely fine.

Try the new prop....