1970 4HP Evinrude Lightwin questions

[z_dennis]

Hi,
I recently purchased a 1970 4HP Evinrude which appeared to be in excellent condition. The motor was professionaly maintained, had a recent tune-up. The seller started the motor in front of me, it started in 3-5 pulls, ran very well and smooth. It also idles VERY low.
I took it home and did a compression test. It tested 75 PSI for top and 77 PSI for bottom cylinders
I tried searching, and couldnt find the compression specs for this motor, but these readings seems low to me. Should i be worried?

I find it surprising that the compression is low, considering how beautiful the motor runs. I have only tried running it in a barrel full of water though, so i cant say how well it would run in a lake.

How much power can i expect the motor to output with these compression numbers? Also, how fast can i expect the motor to push a light 12ft aluminum V hull boat with 1 person in it?

Thanks for reading!
Dennis

 

[tashasdaddy]

Re: 1970 4HP Evinrude Lightwin questions

actually not that bad, all guages are different, and techniques in taking compression. they main thing is they are even. i would do a decarb on the motor, to clean out the innards. don't expect a lot out of a 4 hp, a 9.9 would be better, but that 4 will get you there. when properly maintained it is very reliable, easy to work on, and very sought after motor.

Decarb, take a can of seafoam put 3/4 of it in the gas tank, with only 1 gallon of premixed gas. put the rest in a spray bottle. start the engine, and let it come up to temperature. then remove plugs, and them some real good shot of seafoam into the cylinders, replace plugs, let sit 15 minutes. restart, and spray the rest of the seafoam into the carbs, so the the motor almost stalls, wait and repeat until the seafoam is gone.then take for a wide open spin. then put in new plugs, ad premixed gas to the tank, and take it for a wide open throttle spin. it is going to smoke like a house on fire, during this process.

afterwards compression.recheck

 

[z_dennis]

Re: 1970 4HP Evinrude Lightwin questions

actually not that bad, all guages are different, and techniques in taking compression. they main thing is they are even. i would do a decarb on the motor, to clean out the innards. don't expect a lot out of a 4 hp, a 9.9 would be better, but that 4 will get you there. when properly maintained it is very reliable, easy to work on, and very sought after motor.

Decarb, take a can of seafoam put 3/4 of it in the gas tank, with only 1 gallon of premixed gas. put the rest in a spray bottle. start the engine, and let it come up to temperature. then remove plugs, and them some real good shot of seafoam into the cylinders, replace plugs, let sit 15 minutes. restart, and spray the rest of the seafoam into the carbs, so the the motor almost stalls, wait and repeat until the seafoam is gone.then take for a wide open spin. then put in new plugs, ad premixed gas to the tank, and take it for a wide open throttle spin. it is going to smoke like a house on fire, during this process.

afterwards compression.recheck

Thanks for the reply. The motor is pretty clean inside, very little carbon build-up (as far as i can see through the sparkplug hole). I will likely do the seafoam treatment anyway, it cant hurt.
I ran it today for a while, sparyed some water mist into the carb, recheked the compression.
With about 6-7 pulls on the starter cord, the gauge goes to 105 psi in each cylinder. After 3 pulls, it shows 92 psi in each cylinder. How many times are you supposed to pull it to get an accurate reading? Doesnt seem to go higher than 105 no matter how many times i pull it.

The boat that i am going to be using this motor on is rated 3.5 HP max, so i cant use anything bigger.

Also, can someone give me an estimate how long this motor will run at max throttle on a gallon of gas? Im planning a camping trip where every pound i take with me will count, and i dont want to take more gas than i will need.

Thanks again!

 

[Rick.]

Re: 1970 4HP Evinrude Lightwin questions

Remove both plugs and put the gauge on the top hole and pull till the gauge stops climbing. Write down the number, move gauge to the bottom hole and pull the same number of pulls. Write down the number and that's your compression. I did a decarb on my 4HP 71 and didn't get hardly any smoke. a few days later I had the plugs out and decided to recheck the comp. and the compression had climbed over 10 lbs. per cylinder. The decarb must have loosened my rings even though I didn't have a great deal of carbon. I think you have great compression for what it's worth. I have never run my motor for an hour at full speed. Only trolling. I doubt you'll get more than 8mph with alight load, well distributed. Have fun. Rick.