Compression Test Noobie Question

[thewooddogg]

I've never done a compression test on any engine in my whole life. My boat didn't have many problems last year but I'm planning to do one this year.

That being said... I keep reading the instructions over and over and I still don't 100% understand. I need a picture book sometimes.

Does anyone have a picture of what it looks like to ground the high voltage lead out of the distributer cap? I have no idea what the high voltage lead is or the best way to ground it.

Also, when you pull all the spark plugs and ground that wire, you'd getting no spark, but isn't the fuel pump still putting fuel into the carb?

 

[Bt Doctur]

Re: Compression Test Noobie Question

you can also remove the wires on the + side of the coil.
you want no spark during the test because the cylinders will be pumping out a fuel /air mix.Guessing ,i`d say around 125 psi with a fully charged battery and carb wired wide open.

 

[achris]

Re: Compression Test Noobie Question

I have no idea what the high voltage lead is or the best way to ground it.

Also, when you pull all the spark plugs and ground that wire, you'd getting no spark, but isn't the fuel pump still putting fuel into the carb?

The high voltage lead looks the same as the spark plug leads and goes from the top of the coil to the centre of the distributor cap. Remove it from the dissy cap and make it touch the engine block (and not fall off as you're cranking the engine).

The fuel pump is still pumping and the carb is still letting fuel into the cylinders.... That's why you want to ground that high voltage lead.

1. remove all spark plugs.
2. ground high voltage lead (as described above)
3. open throttle fully and leave it open for the duration of the tests.
4. crank engine to clear excess fuel.
5. put compression tester into cylinder #1 plug hole
6. crank engine for about 4-5 compression strokes.
7. record compression reading, zero the gauge and put tester into the next plug hole.
8. repeat steps 6 and 7 until all cylinders done.
9. re-instate engine to previous glory.

You are looking for all cylinders to have not greater variation that 20 psi. (10 for an outboard)

Hope this helps you........

 

[John_S]

Re: Compression Test Noobie Question

"I have no idea what the high voltage lead is or the best way to ground it."

It is the big fat lead that goes to the center of the distributer cap. Remove the end from dist cap. Take a wire with aligator clips, connect one to the metal tab exposed on the high tension lead, and the other to a good engine ground,


"when you pull all the spark plugs and ground that wire, you'd getting no spark, but isn't the fuel pump still putting fuel into the carb?"

Remove all plugs and advance the throttle to wide open. Choke should be wide open. At slow engine cranking speed, and very little vacuum through carb bores, little fuel will be drawn into engine. Float needle has to drop to allow more fuel into carb.


PS: Looks like Achris beat me to the draw.

 

[achris]

Re: Compression Test Noobie Question

....
PS: Looks like Achris beat me to the draw.

 

[thewooddogg]

Re: Compression Test Noobie Question

Outstanding! Thanks guys for the explanations! Alligator clips with a wire sound perfect!

And I've never dealt much with carbs.. (rebuilding mine in the next few weeks, another first) so thanks for the explanation of the fuel delivery.

 

[thewooddogg]

Re: Compression Test Noobie Question

Couple more quick questions guys..

1.) Can I do this test with the drive off? I'm not planning on warming up the engine for it.

2.) How do I make sure the choke is all the way open? I think I have an electric choke.

 

[Don S]

Re: Compression Test Noobie Question

1.) Can I do this test with the drive off? I'm not planning on warming up the engine for it.

Yes, with the drive off it won't hurt anything. I would strart it for about 10 or 15 seconds if it's been setting for a few days. You won't burn up anything in that amount of time.

2.) How do I make sure the choke is all the way open? I think I have an electric choke.

Small screwdrive handle.