Johnson 70 horsepower Model# J70ELCTD overreving?

[Big Bubba]

Hello again everyone,
I was out on the water yesterday andf I own a 1991 Lowes Pontoon boat with a 1983 year model 70 Horsepower outboard model# J70ELCTD. Right now it has a brand new Genuine Johnson/Evinrude 13.75 X 13 Aluminum 3 blade prop on her. The motor still seems like it is guttless to me and it overrevs to 6700 RPMs if I go wide open throttle with her but I don't want to run her like that so I keep it right around or under 6000 RPMs to keep from blowing the motor that I just put on my pontoon boat. I have decided to take the prop off and take it to a local prop rebuilder here in town and they saiud they can change the pitch to 15 for $35.00. I was going to take that new prop back to the lcal Johnny rude dealer but I kinda rubbed a little paint off of her so that is why I don't trhink they will take it back. Anyway, What do I need to do and am I doing the right thing about changing the pitch from a 13 to a 15 on that brand new prop. Do you think that my motor will take off a little better with a pitch change and also a little bit faster. At 6000 rpms it will run 15 miles per hour and at wide open throttle it will run 20 miles per hour at 6700 RPMs. Let me know if I am heading in the right direction and I also ordered 3 carb rebuild kits yesterday so I can rebuild the carbs becuase at the boat dock my motor was shutting off on me and I had to hit the key switch a few times to keep it going. Take care, Bob

 

[guy74]

Re: Johnson 70 horsepower Model# J70ELCTD overreving?

Are you sure you tach is right? I run a 1988 60hp johnson on my trihull with a 13" pitch prop I only got 6250 rpms @ 28mph. My 60hp is basically the same motor as your 70hp. Is the tach set on 12 pole, or 10? Those motors sound like a mad bee at 6000 rpm, if you are really getting the rpms that you say your prop slip would be over 50%. If you have a way to check the tach's accuracy, that is where I'd start.
Good luck,
Brian

 

[Big Bubba]

Re: Johnson 70 horsepower Model# J70ELCTD overreving?

Guy74,
You know you might be right about the tachometer, becuase when I bought the 70 horsepower motor it was on a 1975 Bonita tri-hull speed boat and it had a 13 1/4 X 17 prop on it and I used that prop and it took a little bit to push my 20 foot pontoon boat to 18 miles per hour at 5100 rpms wide open throtlle so I started calling around Marinas and they said I needed a pontoon boat prop and needed a 13 or 15 pitch prop so I elected for a 13 pitch prop and bought it a few weeks ago at a Johny rude Dealer in town. So when I put the 13.75 X 13 pitch prop on it I couldn't believe how much RPM's I had gained going from a 17 to a 13 pitch prop. I should have gained about maybe a 1000 RPMs but I gained 1600 RPMs so I have questioned the tach and at the 6700 rpms the engine sounds o.k. to me so maybe my tach is off somewhat but I am not to sure either and I am nervous if it is right to run that motor that way at that RPM. So if your boat is a tri-hull speed boat and you go 28 miles per hour then I can get up to 20 miles per hour in a pontoon boat maybe I am o.k.. Also I had a 25 horsepower 1991 Year model outbaord on the pontoon baot when I bought it but I figured it to be underpowered and sold just the engine and used my existing controls wiring cables used to power the 25 horsepower Johnson to operate the new to me 70 horsepower motor. I have never owned a pontoon boat before so I am not too sure how it should feel to me anyway and I need your guys help to answer my questions. Thanks again and let me know if I should go forward and change the pitch from 13 to 15 or leave it for now and rebuild the carbs and see what happens. I might pay someone to do a link and sink and verify my timing on that motor becuase when I got the motor it idled pretty high so I backed the timing off to lower the idle so I didn't mess the lower leg unit up on her. Take care, Bob

 

[guy74]

Re: Johnson 70 horsepower Model# J70ELCTD overreving?

If the tach is still set for the 25hp, it will be set on 10 poles instead of 12. That will give you high readings, when I installed the tach in my boat the instuctions with it had a misprint that led me to set it for 10 poles. Boy was I worried when I hit the throttle and pegged out my 7000rpm tach. But then I looked up the directions for a different brand of tach online and could understand them, it needed to be set for 12 poles on all of the 3 and 4 cylinder johnyrudes of the age. If your tach is still set for the 2 cylinder motor, your 6700rpm reading is actually 5583 rpms, which is probably pretty much where you need to be, if not just a tad low. If you were close by I'd lone you my 14X13 prop to try, it seems to bring the rpms up more than some other 13" pitch props I've run.

 

[Big Bubba]

Re: Johnson 70 horsepower Model# J70ELCTD overreving?

Guy 74,
Yes, I did not change the tach to reflect the 70 horsepower, did not know that I thought all engines used the same pole settings on the tach so I bet after reading your response that it needs to be set at 12 pole versus 10 pole and I will be right on target were I am suppose to be. Man, I love this site. I have learned a wealth of information on here and still keep learning something. I am going to rebuild all three of my carbs and put new floats and gaskets etc in all three. Were I keep my camper and pontoon baot at the guy that owns the storage place there Ricks Storage at Carlyle Lake Illinois let me use his shop to clean out my camper today and he gave me a guy that works on his boat there at Keysport Illinois to get him to do the Link and Sync after I rebuild the carbs to make sure it is right. Like I said I had a hard time around the docks keeping it running. It kept on shutting off on me and I had to raise the warm-up lever to keep her going so I think the carbs are due for a cleaning. Take care, Bob