1973 johnson 4 hp

[superd20]

i have a 1973 johnson 4 hp. it runs ok but i don't think it runs as fast as it should. at full throttle the engine skips. i think that it should push my 10 ft jonboat faster than what it does. does anyone have any advice?

 

[Daviet]

Re: 1973 johnson 4 hp

Well I guess that you need to put a tach on it to see exactly what RPM it is turning. A 2-cycle depends on RPM to develop horse power, if it is not turning proper RPM, it will not develop horse power.

 

[superd20]

Re: 1973 johnson 4 hp

when it skips the RPM increases and the motor seems to pick up speed for just a second then the RPM slows down and so does the motor. how do i get more RPM out of the motor?

 

[Daviet]

Re: 1973 johnson 4 hp

Is it running on both cylinders, use a timing light to check the spark on each cylinder to see if you are dropping a cylinder.

 

[superd20]

Re: 1973 johnson 4 hp

i don't have a timing light but it seems to be firing on both cylinders.

 

[Daviet]

Re: 1973 johnson 4 hp

All it takes for your engine to run properly is timed spark, compression and fuel. You are going to have to do some checks to find your problem.

 

[superd20]

Re: 1973 johnson 4 hp

should i check the compression first

 

[tx1961whaler]

Re: 1973 johnson 4 hp

Sounds like one cylinder is dropping out on you. The skip and pickup of RPM is likely the dead cylinder coming to life momentarily. Luckily that is a very simple magneto type ignition that is easy to fix.

 

[superd20]

Re: 1973 johnson 4 hp

@ tx1961whaler where and how should i start to fix it

 

[tx1961whaler]

Re: 1973 johnson 4 hp

Make sure that is the problem first. Yes, check the compression first. Then try running it and disconnecting the spark plug wires one at a time. If pulling one does not make a difference, then that is the dead cylinder. If you pull one and the RPM drops but it still keeps running OK, then the ignition is OK. If it dies completely when you pull the wire, then that is the good cylinder and the other one is the dead one. If you have a spark tester, each one should jump a 1/4" gap with a healthy blue-white spark.
If you find out that you have an ignition problem, then you will have to pull the flywheel off and replace some parts. I just replace all of them when dealing with a motor of unknown maintenance history and it's good for another 20 years.

 

[superd20]

Re: 1973 johnson 4 hp

what ignition parts will i need to replace?

 

[Daviet]

Re: 1973 johnson 4 hp

Did you check compression, what is it?

 

[superd20]

Re: 1973 johnson 4 hp

no not yet. have to get a compression guage.

 

[nwcove]

Re: 1973 johnson 4 hp

it does sound like you are loosing spark, but keep it simple, if the motor has reasonable compression, and its not starving for fuel at wot, the most likely problem is a coil .

 

[superd20]

Re: 1973 johnson 4 hp

ok just took the compression reading. while i was pulling the starter rope trying to start the motor both cylinders registered 20psi.

 

[Rick.]

Re: 1973 johnson 4 hp

If indeed it were twenty I doubt you could get it to fire so I have to think your test was poorly done. Remove both plugs before the compression test. Make sure the connection is good and leak proof. Put the gauge in one plug hole and pull briskly (counting pulls) until the gauge stops climbing. Write down the cylinder number and the test reading. Move gauge to the other hole and pull briskly the same number of pulls you gave on the first one. Write down the cylinder number and the gauge reading. Post the findings back with us. Best of luck. Rick.

 

[superd20]

Re: 1973 johnson 4 hp

thanks Rick. i will do in the morning and will post results.

 

[todd57]

Re: 1973 johnson 4 hp

You will also want to make sure that you have the throttle set to wide open. I've found I get much higher readings when the cylinders can get the highest volume of air intake.

 

[1946Zephyr]

Re: 1973 johnson 4 hp

You're motor is skipping? You might want to check out the prop hub and make sure the prop isn't slipping. Also, check and make sure the drive pin hasn't broke on the prop shaft.

 

[superd20]

Re: 1973 johnson 4 hp

ok i just checked the compression again and it was 25psi. i put the motor in a barrell and ran it. i pulled the plug wires off one at a time. when i pulled the top wire off the engine died and when i pulled the bottom wire off nothing happened. the engine just kept running as usual. so i think i have a dead cylinder. where do i go next? the prop is not slipping and it has a new pin in it.