1977 Johnson 70 hp low top speed

[Ed27518]

I have a 1977 Johnson 70 hp with a 12 3/4 x 21 propeller on a 14 1/2 ft mini fiberglass bass boat. Top speed with just me and gear is only 33 mph. If there is weight in the boat it can take up to like 20-30 seconds to get up on a plane. Also, it is manual trim so it stays in the same position while in the water. I would imagine this boat would go at least 40 with this engine. Any help is appreciated. Thanks

 

[QBhoy]

What speed are you expecting out of this ?

 

[ahicks]

With no max rpm that you are getting now, my guess is you have too big a prop on it. A 17p or 19p would get you on top much quicker.

40mph may be wishful thinking

 

[racerone]

If motor does not have the rated power it won't spin that 21" prop.------So start with a compression test.----Followed by a spark test.------Run motor with a proper test prop to determine power output.----Or run motor on a dynamometer as a test.-----Dropping prop pitch on a motor to get the " happy RPM " is not always the best solution.

 

[Sea Rider]

Blind wild guessing without a wot rpm reading is Nada Bueno....RPM is all, speed is secondary.Install an induction tach and report back max achieved wot rpm to go from there...

Happy Boating

 

[racerone]

If condition of motor is unknown , then first thing to do is tune-up / test the motor.----See if it is making the power that the decal says it has.

 

[dazk14]

Racerone is giving you good guidance. Compression on those was ~150psi, or so.
That said and without knowing what your exact hull is like, 40mph is a bit much, especially without power trim. .
Also, those loopers had very low torque and didn't make power until they spun up, so a 21" pitch will always be a dog out of the hole with any load.
You didn't specify which specific model of prop is on the boat as that also is key to squeeze out performance.

 

[Faztbullet]

What is the hull brand/type???? Depending on set up/weight and trim high speeds maybe possible...just a note that

In 1974 Johnson set a speed record with stock 70hp on 14ft Allison of 70mph!!!​

 

[jimmbo]

Most 14 ft boats will run High 30s, may be 40 with a 70hp

As already Mentioned, if the engine isn't making its Rated power, then don't expect Stellar Performance

49,7 cubic inche
70hp @5000 rpm
WOT Range 4500 - 5500. This engine runs best when propped for 5400 - 5500
gear ratio 12 : 29

 

[JimS123]

Wow, lots of nostalgia in the threads this week. I had that very motor back in 1977.

She was on a 16' fiberglass closed bow runabout, dry weight about 950 pounds. It was propped with a 13x19 OMC SST prop. It ran pretty close to the 5500 rpm range and would run in the low 40's.

Now, my caveat is that this was before GPS and I was using an Airguide Tach and Speedometer installed by me.