9.9 Merc low RPM AT WOT

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retiredff

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I have a 92 9.9 Merc short shaft on a 1448 alum jon, no bulky decks or anything that added excessive weight. Just normal gear. With 2 180 lb adults I can only get 4300 RPM, I can't remember the RPM with just me but it is between 4.4 & 4.7K. The manual says normal WOT RPM is 5000-6000. I have the gas tank installed up front and I have a clear fuel filter installed after the bulb and at WOT there is very little fuel in it.

Do I need the 2nd fuel filter?
What size fuel line should I use for a 16' run?
Am I fuel starving the motor?

Overall, the motor seems to run fine, the RPM is just low.

Any suggestions on why the low RPM? At WOT my gps speed w/2 adults is 11-12, just me it's 17-18. The prop is standard for that model 3 blade 9x9, I found a 9 1/4 x 9 replacement, would it help changing the pitch?

Any Info appreciated.

If everything sounds normal, my only option is a 20 or 25 HP motor. I normally fish large lakes and @ 11 mph, it takes awhile to get anywhere. The boat is rated at 25 HP.

Tony
 
Not an expert opinion, but your speeds sound reasonable for the motor and boat setup you have. I wasn't getting my little 12 footer that fast with just me and a 9.5

RPM wise, how are you measuring? Is it reliable? Is the anti-vent plate flush with the bottom of the boat?
 
yes, the anti-cab plate is where it should be. I removed the prop and its a 12 pitch. Not sure how the pitch works, I'll research it. The manual says 5000-6000 RPM is the target zone for WOT. Your 9.5 is probably 80's or older and the HP for them was measured at the flywheel, after that the HP was taken from the PROP. From what I have read, that makes a big difference.

My tach is a mini accu-tach (not sure of the name). It come highly recommended on this site and others.

I still have to check for air leak in my connection at the fuel tank, the connector must be preyed off. It may be bad, don't know yet.

thanks for the reply.
 
[url=https://www.tinboats.net/forum/viewtopic.php?p=335695#p335695 said:
muskiemike12 » Mon Dec 02, 2013 9:15 am[/url]"]If your prop is a 12 pitch, that is your issue. Run a 8 or 9 and you should be in the proper range for that motor.


X2
 
Thanks. What will that do to my top end speed? If it's going to drop it, I'll just sell this motor and buy a 20 or 25 HP. I fish the bigger lakes and 11 mph is too slow for me. It takes too long to get anywhere.

Tony
 
I checked the prop again for numbers, I found:

A12
L 9P

I called a local merc dealer and he said the L 9P is a 9 pitch. So changing the prop would be a waist of $$$$$.

I'll find a used 20 or 25 and that should fix me up.


Thanks

Tony
 
Well good luck.

I went with a 1986 DT15 suzuki I found local for cheap and it'll get my boat and gear up to 21 mph WOT. Haven't tried it with two people, and I still need to jack it up and adjust the tilt.

Sounds like you are making the right choice going bigger. =D>
 

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