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New to this forum, 1976 evinrude 70HP on a 1978 Crest 25 foot pontoon, RPM's only hit 3600.

M

meandmyharley

New member
Hello everyone! My name is Bill, and this is my first time on this forum. I purchased a 1978 25ft pontoon boat, with a 1976 Evinrude 70HP, back in June. I used it a couple of times, was not happy with it. It got up to a whopping 4MPH. I started to do a lot of research and talking to people. With that being said; Here is what I have accomplished so far! I started by stripping it down to just the pontoons and the crossmembers. It had two layers of 3/4 pressure treated plywood and 18 pressure treated 2X4's. The total weight in lumber was 860 pounds! I re-installed 3/4 inch Marine grade plywood. (one layer)! All new wiring, and new carpet. Total weight of all of the new decking was 300 pounds. needless to say, she its much higher in the water, now! I have done a lot of work on the outboard as well. I purchased a shop manual. Cleaned the carbs, compression check. All three cylinders are within 5 PSI. at around 120. replaced two of the coils, linked and synched, according to the manual. I also installed a Tach.


Now for my issue, the prop that came with this motor is a 14X19. The boat now reaches 20MPH. which is plenty fast enough. But the motor only reaches 3600 RPMs. I have found where WOT is 5500RPMs. supposedly to make the powerhead run as efficiently as possible.

Which way should I go with changing the prop to get the extra 1500 RPMs? The local marina here, Smith Boys/Jansen refuses to work on it and even has been reluctant when I have ordered parts. So I am kind of lost here. I cant just start buying props until I get the right one. Unless where ever I buy them has a very easy return policy. Any suggestions? Thank you, Bill
 
Lower pitch.
I just went through this with my 1200 lb. Starcraft.
I have a 15P I could let you have for the cost of shipping?
I went from the 15P to a 13P. Mine is now perfect.
If you can find a 17P try that first.
I went from 4600 to 5500 with that one change.
 
I have a 24' pontoon with a 1983 70hp Evinrude. I have a 14x11 3 blade prop (because that was as low as they had). I think I would be better off with a 14x9 if I could find one. I think the general rule of thumb is each inch of pitch equals 200 rpms. Going up 1 inch equals going down 200 rpms. Down 1 inch equals going up 200 rpms. At least that's what I've heard.
 
25 ' pontoon --70 HP ----------19" pitch ---------3600 RPM-----20 MPH--------------Either the tachometer is reading wrong , or the pontoon is very light and planes easily.----------If the tachometer is known to be correct then try a 15" inch pitch on this 70 HP motor and see what happens.
 
Thank you very much! I would be happy to take you up on your offer. I live in Dansville Ny. you could email me at; [email protected] to find out what i could do to get the ball rolling

jim18611865 said:
Lower pitch.
I just went through this with my 1200 lb. Starcraft.
I have a 15P I could let you have for the cost of shipping?
I went from the 15P to a 13P. Mine is now perfect.
If you can find a 17P try that first.
I went from 4600 to 5500 with that one change.
Click to expand...


i
 
racerone said:
25 ' pontoon --70 HP ----------19" pitch ---------3600 RPM-----20 MPH--------------Either the tachometer is reading wrong , or the pontoon is very light and planes easily.----------If the tachometer is known to be correct then try a 15" inch pitch on this 70 HP motor and see what happens.
Click to expand...


Thank you very much for your response. I do have say that the speed of the boat is only what it says on my hummibird gps/fishfinder. it does not have a speed sensor, but it get its speed reading from the sattielite, I think. How would I know if the tach is working correctly? I set it up according to the directions. which said to set it up on number 6. something about a pulse wave. is there something mechanicial to check the actual RPMS?
Thank you again very much for your help
bill
 
Bill, I emailed you, now edit your post and delete your email address.
If 15P gets you closer then you could guess at the best prop.
Cabelas had some composite ones that will fit these mid 70's OMCs and they are only $65. I think they are gone now though.
I found mine new on ebay from a poster here for $100.
 
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Update: I got a 15P prop sent to me from someone from this forum. BTW, Jim...check is in the mail! anyway, I installed the prop and the engine hits 4200RPM's. I am thinking that a 13 pitch should put me right where I want to be; I think.
Bill
 
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