1983 75hp Evinrude

[nmhellraiser]

The motor is mounted on a 17'6 bay boat. Here's my question, I don't think the tach is working correct. To me, it's reading higher than it should be. I have a 16P prop on it right now. If I go WOT, the RPM's go to 7k but at idle it's right at 850 RPM. I haven't kept it at the RPM range but when I'm at 5200RPM's I'm only going 25mph according to the GPS but it seems there's a lot more motor but the RPM issue keeps me from going much further. Is this a prop issue or possible tach issue?

 

[boobie]

Re: 1983 75hp Evinrude

Is the tach set on the right pole setting? It should be set on 6. If it is you may have a bad tach.

 

[mark1961]

Re: 1983 75hp Evinrude

Standard prop pitch for that motor is a 17 so that may partly explain the higher rpm's you are seeing with the 16, i suspect there's more to it than that though.

 

[eavega]

Re: 1983 75hp Evinrude

Has it always behaved like this? If this is new behavior, you may have spun your prop, thus higher RPMs are possible with no power. Otherwise, you are probably underpropped and may try to step up to a higher pitched prop. There are online prop requirement calculators, but you need to know your gear ratio and the weight of your boat as you run it.

To eliminate the tach as faulty, get a shop tach and compare the readings. I would think, though, that a faulty tach would be faulty at all RPMs, not just high RPMs. That having been said, your 850 RPM reading may not actually be accurate either.

I would tend to think that with a 16p prop, you are underpropped and need to step up to a 21p or so, but that is my uneducated opinion based on purely anecdotal evidence.

Good luck.

 

[jtexas]

Re: 1983 75hp Evinrude

Seems unlikely to me that it could reach 7k on a 17 1/2 ft boat even with just 16p prop. If it is, it'll be screaming like a banshee. Agree with the above answers, recheck the pole setting then borrow or otherwise aquire another tach to test it with.

The 75hp 15" shaft has a 28:15 gear ratio, 20" is 29:12.

Unless something is seriously wrong with the hull, or you're carrying a whole lot of weight, you should be way closer to 30mph at 5200, even faster with the short-shaft.

Paint or scribe a mark across the rubber inner and metal outer prop hub, then make a short run at WOT. If your marks still line up the prop is fine, otherwise the hub is spun and needs to be rehubbed at a prop shop.

 

[nmhellraiser]

Re: 1983 75hp Evinrude

I have a 19P prop and another tach but was wanting to get an idea before I go changing things up. I've had a prop that spun the hub but it acted different than this. I guess that could be a possibility. The motor is a long shaft. The hull is in great condition and nothing wrong with it. It is a tunnel hull. The whole thing is it doesn't sound like it's turning 7K RPM's. The tach is fairly new and I'm almost certain I set it on 6P when I installed it. This has been a problem since I've had the boat but I wanted to fish more than work on the boat so I've put it off until winter. I was afraid of losing my hole shot by putting the 19P on but I guess I'll give it a shot and see what happens.

 

[Sixmark]

Re: 1983 75hp Evinrude

I have a 19P prop and another tach but was wanting to get an idea before I go changing things up. I've had a prop that spun the hub but it acted different than this. I guess that could be a possibility. The motor is a long shaft. The hull is in great condition and nothing wrong with it. It is a tunnel hull. The whole thing is it doesn't sound like it's turning 7K RPM's. The tach is fairly new and I'm almost certain I set it on 6P when I installed it. This has been a problem since I've had the boat but I wanted to fish more than work on the boat so I've put it off until winter. I was afraid of losing my hole shot by putting the 19P on but I guess I'll give it a shot and see what happens.

Well that motor really shouldn't be turning 7,000 on a regular basis for any reason, so if it actually is then you want to remedy that anyhow. As stated the 17P prop is the standard on that motor, while you currently have a 16P installed.

Jumping to a 19P is probably going to be too large of an icrease in pitch.....jumping up 3 pitch sizes at a time may not exactly address the problem, it may just help mask what is really going on.

One thing that happened on mine was that the fuel solenoid went bad and was dumping fuel down the cylinders while at WOT. The odd thing is that is had some of the same symptoms.

Start a list and then check it off as you go.

Check the pole setting on the tach.

Check the condition of the stator and rectifier.

Test the tach against another known good one.

Check the prop and prop hub to see if it has spun, also check very closely for stress cracks in the prop, a prop that has fractures can "flatten out" at higher RPM which decreases it's efficiency and has mechanical and safety concerns as well.

It sucks to be off the water, but it's the price we have to pay.