1979 Johnson 70 hp over revving

[J.avant]

Hi...new to the forum.

I have a problem with my 79 Johnson 70 hp over revving.
The boat is a 1979 Skeeter SS-1 Fisherman (15.3)
The prop is an old SST 13 1/4 X 17.

The boat gets on plane OK I think. Now I'm on plane with the engine trimed with antivent plate level with the bottom of the boat. I guess you would say neutral? When I'm at 4000 rpm I am only at half or 5/8 throttle. When I go to WOT the engine will easly go to 6000 rpm which I don't leve it there long at all because I know the engine is only good to 5500 rpm. I would like to see 5200 or 5400

Could I go with a 13X19 SS? Also would anyone know if this engine has the small or large lower unit.

I hope I asked the questions in the right order?

Thanks,J.Avant

 

[steelespike]

Re: 1979 Johnson 70 hp over revving

What speed are you getting at 6,000?
while the skeeter may be fast I think you should check the tach.Varify against a known good tach. Be sure all connections are clean and tight. Note the position of the switch on the back operate it a few times to clean the contacts and return it too the setting.

 

[junior1113]

Re: 1979 Johnson 70 hp over revving

i wouldnt mind 5700-6000 on that setup with todays ethanol.

 

[J.avant]

Re: 1979 Johnson 70 hp over revving

HI steelespike

Sorry it took so long to reply been out of town.

Don't know what speed. I have a speedo but it was showing 22 MPH. 5500 is RPM max. Should I go past that?

There are 2 settings on the tach. The numbers go from 1-6 but the instructions say to use #3 for 12 pole and #4 for 20 pole. I used #3 for my Johnson.

I haven't tried another tach. though. I'll try and get one.

Operating the tach. switch sounds like a good idea. I will try that. Also I will check the connections.

 

[J.avant]

Re: 1979 Johnson 70 hp over revving

i wouldnt mind 5700-6000 on that setup with todays ethanol.

Do you think 6000 rpm would hurt the engine?

 

[steelespike]

Re: 1979 Johnson 70 hp over revving

The motor is rated for 6,000,max.
22 is very slow for a 17" prop with the motor at 5,500 either the tach is way off or the speedo is way off.

 

[J.avant]

Re: 1979 Johnson 70 hp over revving

Yeah I don't think the speedo is that accurate. I will take my car gps and find the correct speed.

I'm having the prop reworked at Mike Hood's prop shop. He does all the props in my area. He did say that it was bent,and did need to be redone with some cup put in it.

I've had the boat about 10 Months. I purchased it to restore because it had the original seats and they didn't look that bad.

I've gone far enough with the restore to be able to get it in the water. I got all manuals and paper work with it. The manual has 5500 for max RPM. Could you let me know if this is correct.

Sorry for rambling on but I don't want to screw it up after all the work I've put in it.

Thanks,John

 

[ozarkt165]

Re: 1979 Johnson 70 hp over revving

I have a 1976 ozark tri hull 16 ft w 70 horse Johnson. W oem prop i am turning 5k rpm @ 34 mph. Are you triming up after your holeshot? Assuming you have electric trim.... My boat won't come out of the water unless it is full down. Max speed 20mph in full down, trim up and get 34 mph. Be interested in conclusion. thought about trying a 13p to get my rpms up if it kills my top speed heck w it. also run ethanol free when possible. here in sw MO premium can be found wo ethanol.

 

[J.avant]

Re: 1979 Johnson 70 hp over revving

Its hard to find ethanol free gas here.

You running a 13 1/4 X 17? What year engine?

Yeah I'm all the way down on the trim and trim up after plane. I had 4 people in the Skeeter and still over revved. When I leve the motor down it gets about 5800 RPM.

I hope after the prop gets revamped I will see the RPM go down and speed go up. I'll report after next week about the changes if any.

 

[1kruzer1]

Re: 1979 Johnson 70 hp over revving

Yeah I don't think the speedo is that accurate. I will take my car gps and find the correct speed.

I'm having the prop reworked at Mike Hood's prop shop. He does all the props in my area. He did say that it was bent,and did need to be redone with some cup put in it.

I've had the boat about 10 Months. I purchased it to restore because it had the original seats and they didn't look that bad.

I've gone far enough with the restore to be able to get it in the water. I got all manuals and paper work with it. The manual has 5500 for max RPM. Could you let me know if this is correct.

Sorry for rambling on but I don't want to screw it up after all the work I've put in it.

Thanks,John

5500 RPM is the original top end spec in those years (My 70HP is from 1976), but many knowledgable members here have found that these motors are sensitive to being lugged, especially on todays fuels, and have found through experience that these motors actually last longer when revved more. 5800RPM seems to be the common goal, and indeed in later years on the 3 cylinder 70 OMC raised the max RPM to 5800 in their specs. The general recommendation now seems to be to use 5500 RPM as the MINIMUM speed for your normal load with 5700 or 5800 RPM being even better. You may find that your repaired prop will have less slippage and better efficiency, and may drop 100 or 200 RPM off your previous max. If that turns out to be true you should be good to go.

 

[ozarkt165]

Re: 1979 Johnson 70 hp over revving

My 70 is of 76 vintage. 17p is what i assume i am running. measured tonight at 12 inches. I am getting 34 mph (gps) @ 5000rpm with my 340 lb a#$ not a tub o 6'4", got a beer belly and plenty of muscle ...... LOL Want to hit 5800 rpm.... hey 1kruzer1 what hull you running????????

 

[ozarkt165]

Re: 1979 Johnson 70 hp over revving

My bad... 1kruzer1 and top speed?

 

[J.avant]

Re: 1979 Johnson 70 hp over revving

I got to run the recupped 13 1/4 X 17 Today. The RPM dropped about 200 RPM. The top speed was 36 MPH @ 5500 RPM. I had throttel left when I was at WOT the RPM was 59 to 6000 and speed was around 36 to 39 MPH. (I did use a GPS)

The 15 ft Skeeter is a tri hull at 1200 lbs with me and gear with 9 gal of gas.

I just read the post and will not be so worried about going to 6000 RPM with the engine. I will get better reading next time I was SCARED to open it up and keep it a 6000 RPM.

Thanks for the info. I will report any improvements.

John

 

[Fed]

Re: 1979 Johnson 70 hp over revving

The 'standard' tach setting for your motor is #6 but your tach may be different, you did read the directions right?
Would you know the difference between +6K screaming and 4K lugging, if not you should really confirm that tach.
I think tiny tachs are only around $40.

 

[J.avant]

Re: 1979 Johnson 70 hp over revving

The 'standard' tach setting for your motor is #6 but your tach may be different, you did read the directions right?
Would you know the difference between +6K screaming and 4K lugging, if not you should really confirm that tach.
I think tiny tachs are only around $40.

The tach. that I have has 6 settings but only 2 are used. #3 is for a 12 pole alt. and #4 is for a 20 pole alt.

The rpm doesn't sound right to you?

Thanks,John

 

[ozarkt165]

Re: 1979 Johnson 70 hp over revving

I ran a prop the other day that was only a 1/2 bigger than regular prop, dropped 100 rpm and 3 mph???????? Look to be same pitch... Not stamped anywhere on either prop.... Clearance to cav plate, 13 inch prob be real close. Love to be able to hit upper 30's Anyone running a 12 inch 13p on a similiar set up. 76 ozark trihull 16ft 76 -70 horse johnyrude? believe both current props to be 17p

 

[Fed]

Re: 1979 Johnson 70 hp over revving

Your tacho has convinced me John, go for it.

 

[J.avant]

Re: 1979 Johnson 70 hp over revving

Your tacho has convinced me John, go for it.

Sorry the pic was so big. This is the second tach i've tried and the reading is the same on both tachs.

Do you think the rpms are not right for the speed? I see where someone has a 70 hp and hitting 34 mph at 5000 rpm.

So if I'm at 5500 with a 13 1/4 X 17 what speed should I be?

Thanks,John

 

[ozarkt165]

Re: 1979 Johnson 70 hp over revving

Hey John, how much clearance between your cavitation plate and the tip of your prop?

 

[J.avant]

Re: 1979 Johnson 70 hp over revving

Hey John, how much clearance between your cavitation plate and the tip of your prop?

There is a 1/2" clearance.