Prop, or Engine Issue?

[wdirnbeck]

Hi all, I have never had a tach on my boat until recently and have found that rpm at WoT is low for my motor (is: 4800 should be: 5200). I wanted to consult here to see if anyone thinks my prop is causing this issue or if I need to be looking at the powerhead. I was wanting to get another prop as a spare so I’m in a good place to test a different one if needed.

Info:
Issue: Low RPM at WOT
Model: Vevor (amazon) part: 765183 Aluminum
Diameter: 13 1/4”
Pitch: 17”
Blades: 3
WOT RPM (one 160lb person): 4800
Speed at WOT: No speed data, maybe 25-30mph
Engine: 1975 Johnson 70hp Two Stroke (70esl75)
Boat: 1961 Glaspar Avalon, 16ft, ~800 dry?, fiberglass construction.

Thanks all, any help or advice is of course very much appreciated.

​

 

[Scott06]

yes prop is likely contributing but you are also correct to ask about condition of the engine. If you have good compression carbs and spark is in good shape then yes going down 2" in pitch should get you around 5200.

For your set up assuming your speed is relatively accurate I would say your engine is performing well.

going down in pitch you may see some loss in top speed but would expect much easier planning especially if loaded down more than one person

 

[airshot]

Those were great motors, owned two over many years of boating ! By all means check over your motor, should be done every year anyway, comp test, spark gap jump test and a can of engine tune or deadpan on occasion to keep thing working well. What do spark plugs look like...if clean then good but if fouled then why ? Chances are, like mentioned in prior post, it is the prop. You might want to check your tech to be sure it is accurate before replacing your prop. Not unusual for folks to have different props to switch back and forth, depending on what they are doing

 

[racerone]

Post the actual compression test values.----Should be around 140 PSI.

 

[Texasmark]

If you buy it put in a new impeller/water pump and a new thermostat. That engine doesn't have an over temp alarm. I had one that I bought from a friend and in short, the thermostat stuck shut at WOT and the engine locked up. Had to have the block bored and I installed oversized block components. Parts/machine work was $1k and that was back in mid 1980's.

 

[racerone]

Really-----Every big JOHNSON since about 1962 had a temperature alarm !!

 

[wdirnbeck]

If you buy it put in a new impeller/water pump and a new thermostat. That engine doesn't have an over temp alarm. I had one that I bought from a friend and in short, the thermostat stuck shut at WOT and the engine locked up. Had to have the block bored and I installed oversized block components. Parts/machine work was $1k and that was back in mid 1980's.

Thanks for the tip on the thermostat, first thing I did was put an impeller on when I got it. It did actually have an over temp buzzer on the dash harness when I got it but figured it was useless because of how loud the old thing is. I don’t know how but I was going to maybe just replace the buzzer with a led light.

 

[racerone]

The factory buzzer was in the control box and plenty loud.

 

[airshot]

The factory buzzer was in the control box and plenty loud.

My 1979 and the other that I don't recall the year had the alarm in the control box and was plenty loud when testing.

 

[jimmbo]

A 1975 70hp would have a WOT Range of 4500 - 5500 rpm, so you are within the recommend range, but being closer to 5500 rpm would certainly be better for Engine Longevity and Hole Shot.
However if the Engine is not in Good Mechanical Shape, or is not getting Fuel and or Spark, timed correctly, it can't make as much power as it can.

Horns were installed in the Control Boxes starting in 1973, prior to that, there was either a HOT Light on the Dash, 62-till about 68; or in the Control Box approx 68 - 72. The exact yr depended on the HP.

 

[wdirnbeck]

My 1979 and the other that I don't recall the year had the alarm in the control box and was plenty loud when testing.

I powered the one that came with it and it was not very loud but it did look crusty. I’d still prefer a light though so I might experiment, if it’s just grounding out a switch when it gets to temperature and sending 12v then I’ll probably just hook it up to a light.

 

[wdirnbeck]

yes prop is likely contributing but you are also correct to ask about condition of the engine. If you have good compression carbs and spark is in good shape then yes going down 2" in pitch should get you around 5200.

For your set up assuming your speed is relatively accurate I would say your engine is performing well.

going down in pitch you may see some loss in top speed but would expect much easier planning especially if loaded down more than one person

I can’t find find any 13 1/4” x 15” props anywhere. There are some stainless ones going for mid $300’s but that’s a little out of my budget at the moment. I’ll carry on double checking my motor isn’t having issues before I bite the bullet on that.

I don’t know much about props but I’m assuming the diameter is constricted by the lower unit design and has to maintain that 13 1/4” diameter. Am I wrong on that?

 

[Texasmark]

Thanks for the tip on the thermostat, first thing I did was put an impeller on when I got it. It did actually have an over temp buzzer on the dash harness when I got it but figured it was useless because of how loud the old thing is. I don’t know how but I was going to maybe just replace the buzzer with a led light.

Maybe mine was an earlier model.....It was a 70 horse loop charged Rude triple.....been a long time and I bought it used.

 

[racerone]

The horn was first used in 1972 model 50 HP and up.----Johnson / Evinrude had overtemperature in 1962 40 HP and on most electric start / bigger motors.-----And many people never realized this.-----Amazing as to how many folks said ----" Never knew it had that " ---After I showed them the simple way to test the overheat light / horn.

 

[racerone]

Most dealers would have a 15" pitch in stock.-----Very popular item used from about 1970 to 2005 models.

 

[Scott06]

I can’t find find any 13 1/4” x 15” props anywhere. There are some stainless ones going for mid $300’s but that’s a little out of my budget at the moment. I’ll carry on double checking my motor isn’t having issues before I bite the bullet on that.

I don’t know much about props but I’m assuming the diameter is constricted by the lower unit design and has to maintain that 13 1/4” diameter. Am I wrong on that?

You may be able to get your existing prop repitched by a prop shop.

No diameter is not tied to lower unit design except for max diameter a drive or outboard can use.

Normally when props go down in pitch diameter goes up some (not a 100% rule). So if you find a 15 " pitch prop that fits your hub and the diameter fits below your cavitation plate try it.

Would also do a compression test to and some basic tune up checks before buying a prop. If you have good compression, spark plugs look good (aka fuel is getting there) and have good spark strength on gap tester then engine is probably performing like it should

 

[racerone]

The 70 HP triple was first marketed in 1974.----And it sure came with a warning horn in the control box.-----Simply amazing that folks own these motors .---Know nothing about them or do not care to learn.-----Just turn the key and roar away from the dock.-----No wonder some of these fine motors end up on the scrap heap.

 

[Texasmark]

The 70 HP triple was first marketed in 1974.----And it sure came with a warning horn in the control box.-----Simply amazing that folks own these motors .---Know nothing about them or do not care to learn.-----Just turn the key and roar away from the dock.-----No wonder some of these fine motors end up on the scrap heap.

"Simply amazing that folks own these motors .---Know nothing about them or do not care to learn.-----Just turn the key and roar away from the dock." Yepper that was me!!!

Back in those days I didn't have access to "forms.iboats.com and was "admitmittedly" stupid. Don't know how I overhauled this engine.....but I did and it worked fine. Back in those days and earlier, I never changed an impeller some lasted about 7 years without a problem, selling the boat for something larger in about that time line, OMC, Scott Atwater, and Merc. engines, 5 1/2 to 125 HP. Just didn't know their life cycle. I did change the impeller in my current 115 Merc. last year with little deformation and no broken blades.

 

[JimS123]

"Simply amazing that folks own these motors .---Know nothing about them or do not care to learn.-----Just turn the key and roar away from the dock." Yepper that was me!!!

Back in those days I didn't have access to "forms.iboats.com and was "admitmittedly" stupid. Don't know how I overhauled this engine.....but I did and it worked fine. Back in those days and earlier, I never changed an impeller some lasted about 7 years without a problem, selling the boat for something larger in about that time line, OMC, Scott Atwater, and Merc. engines, 5 1/2 to 125 HP. Just didn't know their life cycle. I did change the impeller in my current 115 Merc. last year with little deformation and no broken blades.

Back in those days there was no XXX.com. BUT, the motors all came with an owner's manual which detailed everything you needed to know. PLUS, the dealers back them had been in bizness since the late 1940's, and all you had to do was ask.

 

[dingbat]

Simply amazing that folks own these motors .---Know nothing about them or do not care to learn.-----Just turn the key and roar away from the dock.-----No wonder some of these fine motors end up on the scrap heap.

The difference is that you see them as a “classic” motor from your past. Later generation(s) see them as “vintage” (nice way of saying old) motors, that are inexpensive to buy. That combination makes them disposable should something go wrong.

Bear in mind that vocational training in public schools was discontinued almost 50 years ago now. That’s two generations that don’t know the difference between a carb and a distributor and don’t care. Not that they need to know given that both components were replaced with new technology prior to their birth.