compression test

[boatin12]

can someone tell me the correct way to perform a compression test on a 1973 65 hp johnson

 

[5150abf]

Re: compression test

Pull all the plugs and put them on a spark tester or at least something that will let the spark go somewhere, put the compression tester in a plug hole and crank the engine till the pressure doesn't rise anymore, repeat for each cylinder.

better to do it hot but if you can't get it running that is fine too.

Best to do it with all the plugs out as the engine spins faster and have a good fresly charged battery.

 

[F_R]

Re: compression test

Somebody will tell you to block the throttle open also. That is true on a 4-stroke, but not on a 2-stroke like your Johnson.
It's best to completely ground the plug wires to avoid the possibility of starting a fire as fuel blows out the plug holes. Don't ask me how I know.

 

[boatin12]

Re: compression test

thanks for your help its greatly appreciated, i did the test i got 110 on all three holes but how can i maintain it? is there something special i could do? thanks again.

 

[jtexas]

Re: compression test

here's a couple things that'll keep 'er happy:
Water pump impeller, every other year.
Seafoam. an ounce per gallon in the fuel.
Click over to the engine FAQ section of iboats, there's a thread about "DECARB".
Take 'er out & run her hard every 2 to 3 weeks, so gasoline doesn't turn to varnish in the carbs.
Take action at the first sign of performance degradation or any other symptom.
Get a prop that let's it run at the top of the RPM range at WOT.
Get the dings taken out of your prop.

Just a little bit of care, that motor'll still be running after we're long gone.

 

[boatin12]

Re: compression test

hey thanks for the tips the previous owner told me to run it out of gas after every use would you recomend doing this?

 

[iwombat]

Re: compression test

You can get away with that on single-carb motors, but not on multi-carbed motors. The carbs will not go dry at the same time, and you'll be starving some cylinders of lubrication until the rest of the carbs go dry. Put fuel stabilizer in your tank and leave 'em wet.

 

[5150abf]

Re: compression test

No, don't run it dry, there is no reason unless you are getting ready to put it up for the year and then there are drain screws on the float bowls for that.

Your lubrication is in the gas so when you run it out of gas you run it out of oil too.

The only time I drain my carbs is when I am storing it for the year.

 

[JJM.IV]

Re: compression test

From previous discussion, I pulled the following steps, to test compression:

>crack the plugs and re-snug(prevent thread damage)
>warm up engine
>remove all plugs
>ground plug leads to block
>crank engine over with wide open throttle
>crank equal number of hits per cylinder(5-7 hits minimum)
>don't use WD-40, as it will increase your readings and give you a false sense of security
>generic spec = 10% variation from highest to lowest cylinder

1. How do I ground the plug leads? 2. Does a spark tester work better? 3. Hints..?