Evinrude 6HP Fisherman seems under powered

[petetomo]

Hi Everyone,
I am brand new to this forum and I am looking for some advice on my 1968 Evinrude 6HP Fisherman. I bought it this past May in non-functioning, but very clean condition. I cleaned and spaced the points and installed a new carb kit and that got it started, but it was still having some sputtering issues. After doing a leak test, I changed out the exhaust gasket which really improved the performance!

However, even though the engine sounds amazing, the performance seems to still not be quite what I expected. In idle, it reaches almost 6000RPM. In gear, in a garbage can of water, it will get up to about 4800RPM. However, when it's pushing my boat, it will only get up to about 3700RPM.

My boat is a pretty light inflatable and it will only get up to 5.25mph. Since the inflatable is flat bottomed and likely has a lot of drag force, I decided to try the engine on my friend's 12' aluminum V-hull which only weighs about 100 lbs. We still couldn't get in plane or get above 6.5mph. My friend normally runs a 3.5hp Mercury and he said that his smaller engine gets the same speed.

Any thoughts on things I might look into?

 

[Orville craig]

Looking for link and sync proceedure for 1977 25hp johnson seahorse

 

[racerone]

petetomo----Post the compression numbers.-------Did you install a new water pump impeller ?----Were the 2 coils new ?-----What is your elevation ?------Is the high speed jet in the carburetor bowl clean ?----Don't wind it up to full throttle in neutral.-----How did you measure 3700 RPM on the boat ?!

 

[Crosbyman]

transom height same as the outboard ?

 

[iggyw1]

petetomo, You stated that in idle, "it reaches almost 6000 RPM". Did you mean 600? Or are you meaning in "neutral" rather than "in idle?" If you are revving it up to 6000 RPMs in neutral, DO NOT DO THAT AGAIN. You will blow the engine!

 

[GA_Boater]

Looking for link and sync proceedure for 1977 25hp johnson seahorse

You are off by 19 horsepower. Start a new thread of your own or check the stickies on top of the Johnson-Evinrude section.

 

[tpenfield]

:welcome: to iBoats . . .

Not sure what bench marking info would be available for an engine of that age. I have a 1969 Mercury 7.5 and it gets my 8 ft. pram tender up to about 12 mph. Back in the day I had a Merc 7.5 on a 12ft Sears Game fisher, and used to get 15 ish mph with just one person (kid) in the boat.

However, what you are seeing is all that you might get out of that engine . . . Not sure that engine is all about speed.

When you first go up to WOT does the engine speed up then slowly fade? Or is it consistent? I would certainly test and post the compression numbers as racerone suggested. . .

Have you checked to see if the correct prop is on the engine? Maybe it has been changed by a previous owner :noidea:

 

[petetomo]

The compression was 90 in both cylinders. The water pump impeller is new. The carburetor was cleaned up with carb cleaner and an air compressor, but not soaked for any length of time. I will remember not to rev the engine that high again in neutral. I only did it for a few seconds to see how high it would rev without a load. I measured the RPM with the cover off with an audio tachometer (listens for the engine cycles).

On the inflatable, the transom was 6" lower than recommended in the manual. On the aluminum boat, the transom height was correct.

When I increase to WOT, the engine speed increases then stays consistent. It doesn't fade or fluctuate at all.

I had thought about the prop being incorrect for this engine. Any other ideas?

 

[petetomo]

BTW, my benchmarking that seems to indicate that I'm running slow are a number of posts on this site for this engine reporting speeds >10mph for even heavier boats. Also, youtube videos.

I know that neither is a perfect benchmark since every boat is different, but I did try two different boats with similar results.

 

[tpenfield]

Sell the Evinrude 6 and get a Mercury 9.8 hp and see how that goes. :thumb:

 

[petetomo]

Upgrading motor will not be realistic at this point. I just looked further into the prop and it is the standard 8 x 7.25. However, when I physically measured it, it looks like it is actually 7.5 inches in diameter. Could it actually have lost 0.5" in just wear and tear?

If it is actually worn down, shouldn't this lead to higher RPM at WOT, not lower?

 

[tpenfield]

Maybe it got dinged up in a previous ownership and the prop was ground down a bit. I'd see if you can get an 8" prop and try it.

 

[Rick.]

7.5 is larger than 7.25 so it makes sense that 7.5 would lower the rpm.

 

[petetomo]

The prop is supposed to be 8" diameter. 7.25 is the pitch.

 

[tpenfield]

1/2" diameter might not seem like much, but being at the outer extreme of the radius, it has a significant impact on the "area" of the prop, which relates to the amount of water it 'propels' . . .

 

[oldboat1]

You might check the h.s. orifice at the bottom of the float bowl, behind the plug screw. Clean it with a piece of soft thin wire, followed by carb spray with the plastic nozzle. Make sure the idle needle on top is set as lean as it should be (clockwise until the motor stalls or give you a lean sneeze, then c.clockwise about 1/4 turn. Make sure the motor goes into gear at idle without stalling.