Help with older Johnson Motor Please

[NEPATRIOTS]

I'm new to the forum but I have read all the great information people give here to other posters, I have a problem of my own. I have a 1974 Johnson V4 70 hp motor, the motor is on an 18 Ft Cobia sportster open bow, The motor starts great and idles good, the problem is at wot i'm only getting 3000 rpm,We took the boat out last evening and when my Buddy went to the front the rpm went to 3800. The prop on the motor is 13/25/17 should I try another pitch or is it something with the Motor itself. The gas in the tank is just a day old.

 

[JB]

Re: Help with older Johnson Motor Please

I am a bit confused. Please give us the model number of your engine. Does it have PT&T?

Is this performance new or has it always behaved that way since you have had it?

 

[jay_merrill]

Re: Help with older Johnson Motor Please

Try to find the data plate on the motor and post the model number here - Johnson did not make a V4 70hp motor. The 70hp model began in 1974, but they were three cylinder motors. Prior to that (1960 - 1966), there was a V4, 75hp model.

If the motor has a 17" pitch prop on it, I would say that is way too steep for an 18' boat, unless it is very light. You might be able to run a 15" pitch, but will probably not reach maximum WOT rpm. For most applications, a 13" to 14" pitch would be much better.



???

 

[NEPATRIOTS]

Re: Help with older Johnson Motor Please

See this is why I registered for this forum. You are right I went out and just looked at the Motor, Johnson did not make a 1974 V4 70 Hp I went by what I was told lesson learned. The numbers are as follows 85ESL74B ser # J4082983 4500 / 5000 RPM 85 HP 5000 RPM so it's Johnson 1974 V4 85 HP like I was saying im only getting 3000 RPM and when my buddy sat in front the RPM went to 3800.

 

[JB]

Re: Help with older Johnson Motor Please

You probably need to take this discussion to the Prop Forum, Pats, but first you need to determine if the engine is making full power on all cylinders.

Did the boat ever perform differently than now?

 

[NEPATRIOTS]

Re: Help with older Johnson Motor Please

Thanks JB I will ask over the prop forum, As to your question i'm not sure I got the boat in may. It was a project I took on, I fully restored the entire boat and just put it in the water this week.

 

[oldrudedude]

Re: Help with older Johnson Motor Please

If you gained 800rpm by having your buddy move to the front, you should trim the motor so the prop is closer to the tramsom. This will bring the bow down and allow you to plane easier. Don't bring it in too much though or the bow will plow into the waves.