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Someone please help not getting enough rpms at wot

S

stugots

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"i just bought a 76 searay wee

"i just bought a 76 searay weekender. i am not getting full rpm at wot. i am only getting 3,000 rpms. my prop is a 15.25/17
the first thing i did was demo a 16/14 and i was able to get 3,500 rpms. my tach was checked and all is good. it has the correct gear ratio in the stern drive. it runnd like a top, both at idle and at wot. any ideas? the only other thing i can come up with is the timing may not be advancing enough. does anyone know what the timing should advance to at 3000rpms?"
 
don't want to sound like a

don't want to sound like a smart azz but I would go back to the prop that you demoed it with. Why are you looking for something deeper in the motor when you already know what the fix is????
 
"well the prop i demoed was se

"well the prop i demoed was severe difference from what was on there, and it would seem like there was more to it than prop size. the nex jump in prop size from what i demoed would be a 16/11 pitch. so my last thought is that maybe at wot with a load the timing may not be advancing"
 
What engine? If the distribut

What engine? If the distributor has mechanical weights in it make sure they are not rusted and move freely.
 
"the advance is corrected by a

"the advance is corrected by a visual inspection of weights and springs under the points plate - if they are all rusted and gummed solid, you clean and lubricate them. wouldn't worry too much about what the actual advance specs are.
Fuel filter is where I would start though.
Cleaning flame arrester, and checking clear hose from fuel pump - all the basics first"
 
"What HP engine? What size boa

"What HP engine? What size boat? For example, if you have a 165 HP engine and a 26 ft boat what you are seeing sounds about right."
 
"the weights are definately on

"the weights are definately one thing i was planing to look at when i got to the boat today. and a friend of mine did mention the fuel filter. with those both being a possibility, why doesnt it affect the motor when in idle? also if i rev it in neutral i dont have any problems getting rpms out of it."
 
"from what i can figure out ab

"from what i can figure out about the boat from the manual i have a ford 351windsor engine with a 233 hp mercruiser sterndrive. the gear ratio in the stern drive is a 1.33 to 1 ratio. it is a 24' searay weekender, and i gps my speed at 28mph at 3500 rpms with the 16/14 demo prop. i have not gps my speed with the origional prop but i could only pull 3000 rpm with that"
 
revving an engine to destructi

revving an engine to destruction in neutral takes a few tablespoons of fuel per hour.
Cruising down the lake takes several GALLONS per hour.

You have to change the fuel filter regularly. Once or twice a season usually advised.

The time for a visual inspection of typically neglected maintenance related items like fuel filters and flame arrester is long before analyzing gear ratios and advance curves.
 
"A 1976 boat; floatation foam

"A 1976 boat; floatation foam may be water logged. At 8 lbs./gal. of water; the foam can hold lots of water.

Peel the carpet back near the ski locker and poke the wood w/an awl or 16d nail and see if you get water. A water logged floor, transom and stringers will add lots of weight and make it very heavy."
 
"Joe...

Have you checked if

"Joe...

Have you checked if the throttle is opening all the way? Sounds stupid, but there may be something wrong with the linkage at the carb end preventing it from going to WOT. Or something holding it back in the control box? 2000 rpm is a lot for a prop to take care of. If you drop too many inches of pitch to gain rpm, you'll sacrifice all your top end speed.

Wrench"
 
Joe;
Are you sure about that

Joe;
Are you sure about that 1.33 drive ratio?
That is what is normally used with big block drives. Yours should be 1.50

Rod
 
"Hey Joe. I'm no expert, b

"Hey Joe. I'm no expert, but what about something simple like a fouled spark plug. I had one on my inline 4 cylinder and it only got about 2800 rpm's. It did not miss and at WOT it sounded kind of smooth just low rpm's and never could get up on plane. Finally my father inlaw was helping me and pulled all the plugs, scraped carbon off, re-installed, ran it and it ran great but missed until I replaced it. That fixed it."
 
they all sound like things i a

they all sound like things i am gonna look at. i am also leading toward timing chain. i am gonn look at the deflection tonight. the carb linkage distributor and weights all look good.
 
Timing chain would be the last

Timing chain would be the last thing on my list.
Ratio issue would be high on my list.
 
Joe;
What does the boat weigh

Joe;
What does the boat weigh?
Let me know and I can run some numbers for you that will predict top speed and the correct prop size.

Rod
 
"i have no idea, but it is not

"i have no idea, but it is not water logged and it has new stringers, new floor and everything"
 
"Joe;
I am guessing the weigh

"Joe;
I am guessing the weight at about 3750 lbs; who knows?
Based on that weight you should get 46-48 mph at WOT @ 4800 rpm, and the correct prop looks like a 16" pitch.

I would steer clear of a prop with more than about 14.5" diameter. More than that will be a lot of blade area to swing with that drive ratio.

I would start with a 15" pitch prop and see how that works. If it won't hit 4600-4800 rpm with that, then you have other issues; likely mechanical. A quick compression test will tell general engine health, then go from there.

Rod"
 
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