Compression check done correctly or not?

[LindaP]

Hello all,
I just finished checking the compression on my 787
challenger. I got 40 psi on both cylinders. I tried 3
gauges, 2 read 40 and 1 read around 100 for both,
but the needle on the gauge for 100 was very wiggly and
hard to read. These are regular psi gauges. I only
removed 1 spark plug would that change the readings?
Should I try again with a compression gauge? What
all needs to be sealed for this test? I have the carbs off
and all the fuel lines. Help!! I don't want to buy a new
engine. I have only used this boat twice, maybe ripped
off for some of the $4500 spent.
LindaP

 

[Dennis]

Compression Test

You need to use a gauge that is made for checking compression in engines.
I don"t see how trying to use a pressure gauge of any other kind would work.
It's OK to have the carbs off when doing this and have both spark plugs out.
Have the plug wires attached to their grounding posts before you begin. You don"t want any random spark that might cause a fire.
Install the compression gauge in the spark plug hole and crank the engine for about 4 seconds. Do this with each cylinder.
You want to see 150 PSI on both cylinders.
Dennis

 

[kustomkarl]

I agree with Dennis, the grounding post also help deflect the positive+ charge to keep the cdi units from too much discharged voltage. I usually count the revolutions. It will sound like a thub,thub,thub, I count 12 times or look to see when it stops rising on the gauge. Besure the compression gauge uses a oring to seal where it screws in the cylinder head. I recommend you don't use the gauge you have to hold tightly to the hole.The gauge should be able to hold the presssure untill you can read it and then you release the pressure.
If you need anymore help give us a shout.
Karl

 

[93'XP]

also when you are cranking it, hold the throttle wide open, this gives the engine no restriction for air, and you need to have both spark plugs out

 

[seadoosnipe]

check!

Good information here!........you should do a minimum of 3 checks on each cylinder and find the average. Each time you do a test, you'll see a few psi differences. It doesn't matter if both plugs are out or not. As long as you turn the engine over for several compression strokes, record the reading on the gage and do it again. The average needs to be no lower than 110 psi and shouldn't be above 140 psi. If you get 150, you got one tight engine.

 

[kustomkarl]

Uh, the idea of the plugs out is to keep the resistiance low so as not to drain the battery down when checking multi cylinders.