76 Johnson 70 hp slow to plane out

[Coltrickle]

I bought an 84 Lowe 16' boat with a 76 Johnson 70 hp. The boat starts and runs great in a barrel and on muffs.. The spark will jump over 5/8 of an inch and it has 124 psi on two cylinders and 120 on the 3rd cylinder. When i went to the lake it starts and runs great but it does not jump out of the whole. It takes about 15-20 seconds to plane with me and my son in the boat.. Bout 350 pounds.. After plane it runs great through the rest of the throttle unless you allow the boat to settleback off plane.. Then we start the process over!! A friend suggested I check the prop. It has a 13 3/4x 19p on it. My 270 hp ski boat only has a 14.25 x 19p 4 blade and is 4 times the power.. I'm no expert but it seems a bit wierd for the power ratio.. Am I correct in my thinking or should I look elsewhere?? Thanks in advance for any advice!!

 

[Maclin]

That pitch seems a little steep. Need to know the RPM the engine can get to and at what speed.

 

[Coltrickle]

29 mph once it gets there just under 4800rpm. Thanks for the reply

 

[82rude]

12 1/4 x 15 would probably fit the bill perfectly.that motor has a wot range of 5 to 6000 rpm .

 

[steelespike]

Judgeing from your present prop size you have a large gear case.My info indicates wot 4500 to 5500 rpm
For the best performance you need to be near the top rpm. I think a 15" pitch will knock your socks off.
It will be 13 something by 15" You may have to watch the rpm.I also think a 17 " will do a good job Job.
Assuming 5500 with a 15" I get about 29 mph excellent hole shot, assuming 5200 rpm with a 17"I get about 31 mph
strong hole shot These aren't carved in stone.

 

[Coltrickle]

Thanks gentlemen!! I will try and find a loaner in these sizes before purchase if possible!! Will post feedback asap!! Thanks again

 

[Sea Rider]

If wot range is 4500-5500 for that 70 HP and you're achieving 4800 with a 19 pitch, there's only 700 rpm gap difference, a 17 pitch should put your revs arround middle to full wot rpm if that boat happens being light to medium light in weight while 2 are up.

Happy Boating

 

[82rude]

Opps didn't dawn on me that it was a large gearcase motor.For comparison I have a 16 foot 1976 starcraft bowrider with a large gearcase 2011 60 etec and for light to medium loads I use a solas amita 4 blade 17p.Thats up to 2 people plus I always carry 20 gallons of gas .Trimmed out at 5900 rpm im 30 plus but not positive how much as I tossed the useless speedo in favor of a cool looking airglide compass.B et you,d be pleased with a 4 blade solas in 17p as long as your not really heavy.

 

[steelespike]

I do think a 17 should work well though I believe the 70 has a 2.42 ratio and the 60 Etec has a 2.67 ratio. Just a little less rpm with the 70.

 

[steelespike]

I bought an 84 Lowe 16' boat with a 76 Johnson 70 hp. The boat starts and runs great in a barrel and on muffs.. The spark will jump over 5/8 of an inch and it has 124 psi on two cylinders and 120 on the 3rd cylinder. When i went to the lake it starts and runs great but it does not jump out of the whole. It takes about 15-20 seconds to plane with me and my son in the boat.. Bout 350 pounds.. After plane it runs great through the rest of the throttle unless you allow the boat to settleback off plane.. Then we start the process over!! A friend suggested I check the prop. It has a 13 3/4x 19p on it. My 270 hp ski boat only has a 14.25 x 19p 4 blade and is 4 times the power.. I'm no expert but it seems a bit wierd for the power ratio.. Am I correct in my thinking or should I look elsewhere?? Thanks in advance for any advice!!

Your 270 hp probably has a 1.45 gear ratio The 70 has a 2.42 ratio and is much lighter.What is the wot rpm and speed of the Ski boat.
It seems to me that a 270 hp would normally run a 21 or maybe even a 23.Depending on the size,weight and design of the ski boat.

 

[Coltrickle]

My ski boat trimmed all up will hit 52-54 with 5 people at just over 5k rpm the redline is 5600

 

[Coltrickle]

Changed the prop out and it's a little better but my grandads a 35hp evinrude on a fisher marine will smoke this boat out of the hole!!! I feel I may have a fuel or timing issue.. I don't know if this is important but when I use the engine to push the boat fully on the trailer it will not build Rpms and dies out.. It will fire right back up just doesn't have the push.. Thanks for your replys

 

[David Young]

My 77 70hp Evinrude on a 15 foot tri-hull will plane in 4 seconds and top out at a little over 35mph @ 5,500 rpm. I'm using a 13 1/4" prop with a 17" pitch. Picking out the right prop will drive you crazy

 

[Coltrickle]

I just run the boat in the barrel outside because I don't like them sitting to long. And now it does not want to idle and seems to be running shabby.. I have to keep the high idle on. If I back it off it dies.. It seems to have a miss.. Boats will drive you crazy!! The carbs have been fully rebuilt.. Now what???

 

[steelespike]

If that miss is like a sneeze it is a classic indication of a lean condition.Having to use the high idle tells me the idle circuit is not right,starving or blocked.
Is the tank vent open and working,are gas line connections/connectors tight.Fuel pump strong.
Did the motor sit for a long time requiring the carb rebuild? Did they go through the fuel system from tank to carb?
Does squeezing the primer bulb help?

 

[Coltrickle]

This boat did sit for a while(over a year maybe 2). And it does "sneeze" real hard between 5-15 seconds! Kind of erratic on the timing of the sneeze.. I did not rebuild the carbs myself the owners family had it done prior to me buying it.. The tank vents are open. And the fuel pump is new.. The tank and fuel line/primer bulb were new also but have sat in the sun a while.. All the lines on the fuel system under the cowling are new also..

 

[Sea Rider]

Before venturing playing with prop pitches you must have a healthy engine & prop that provides fast aceleration along a tach in good working order. I'll second what DY said about driving you cracy when attempting dialing best prop for your particular needs, if by second attemp you don't dial what you're looking for your experimentation asside from frustation could end in a costly one.

Happy Boating

 

[fishin98]

That boat should JUMP on to plane with that motor, to include trl loading.....You said the previous owners rebuilt the carbs???? I would rebuild/clean again to be sure. Then I woulkd check to make sure EACH cyl is firing BLUE spark. This should be at least a 40mph boat with 2 people on board.

 

[Coltrickle]

I am going to pull the carbs off and tear them down this weekend.. I do have a great spark.. Blue and will Jump over 5/8" in a spark gap tester... I thought the boat should be fast than it is once it does plane and wind out.. I will advise what I find after I disassemble the carbs.. Thanks for the advice!!

 

[Coltrickle]

Ok I rebuilt the carbs and soaked them in an ultrasonic tank at work while torn down.. All looked pretty good but I did find a partially clogged idle jet.. The boat now idles great and the hard miss(sneeze sound) is gone.. I have not yet put it in the lake but while running in the barrel the idle dropped way down at one point so I put my hand over the carbs until I found the one the didnt change the idle.. I left my hand there about 5 seconds until fuel built up in the throat of the carb and now it's purring like a kitten.. My question now is that as soon as I put my hand over the bottom carb it immediately chokes the engine out but the two upper carbs take about 10 seconds to change the idle.. Is this normal or is my paranoia getting the best of me??