Am I checking compression correctly?

[v12mac]

I went down to Harbor Freight and bought a cheap compression gauge. Swapped out one plug at a time and cranked the starter. Battery was a bit weak but I think it was enough.<br /><br />Was this the correct method? Should I have pulled all the other plugs to let the engine spin a bit easier?<br /><br />The reason I ask is this was my first go with this gauge and one time it tried to fire and my readings went up almost 20lbs.<br /><br />Motor wont run but fires once and a while.<br /><br />Suggestions comments?<br /><br />Mac

 

[G DANE]

Re: Am I checking compression correctly?

All plugs should be out and grounded. You want to compare cylinders to eachother, which is only doable at a stable crancking speed. All should at best be above 100 at the speed where all plugs are out. If plugs are leaved ungrounded, power pack can be damaged.

 

[v12mac]

Re: Am I checking compression correctly?

Thanks for the quick reply. If I just jump the starter solenoid will I be ok?<br /><br />My numbers were horrible for a motor that ran great yesterday, 80psi and 75psi<br /><br />Mac

 

[Backlash]

Re: Am I checking compression correctly?

Yes, you can jump the solenoid (ignition off) without worrying about grounding the spark plug wires. Just make sure you have all plugs out and record the results for each cylinder. <br />Backlash

 

[JB]

Re: Am I checking compression correctly?

Cheap guages don't usually give accurate readings, but they are consistent enough to find out if your cylinders match pretty close.

 

[BoatBuoy]

Re: Am I checking compression correctly?

Also, make sure you have a fully charged battery so cranking speed for each cylinder is the same and open the throttle wide open.

 

[v12mac]

Re: Am I checking compression correctly?

Thanks for the info! <br />Mac

 

[v12mac]

Re: Am I checking compression correctly?

I pulled out all the plugs and hooked up the gauge and jumped the starter. I couldn’t keep the starter engaged. I would only get 1 or 2 bumps to the gauge before the starter would free spin. So I jumped the starter and put the metal tube with the rubber tip in the spark plug hole and the same thing would happen?? Is this sufficient?? The bad news is that I am still getting horrible readings. <br /><br />Just to check if the gauge was off I hooked it up to my jet-ski and got 118 lbs on both cyl.<br /><br />Any thoughts? How could this thing run great then drop to 85psi in one day?<br /><br />Mac

 

[v12mac]

Re: Am I checking compression correctly?

Bump^

 

[seahorse5]

Re: Am I checking compression correctly?

You really do not have to have the throttle open on a 2 stroke compression test as the exhaust port is open to the atmosphere until the piston ring passes the top edge of the port. Cranking speed is relatively slow, usually below 450 rpm. You will see about the same compression reading if you put your hand over the carb opening, blocking off ALL the air going thru it because of the air getting in thru the exhaust. Of course with a 4-stroke, the throttle HAS to be open.

 

[Paul Moir]

Re: Am I checking compression correctly?

Install spark plugs as needed to keep the starter pinion up while cranking. One or two compression strokes (ie, 1 or two revolutions) is not sufficient for an accurate reading.

 

[v12mac]

Re: Am I checking compression correctly?

Thanks, I'll give it another go.<br />Mac

 

[minocqa]

Re: Am I checking compression correctly?

Originally posted by G DANE:<br /> All plugs should be out and grounded. You want to compare cylinders to eachother, which is only doable at a stable crancking speed. All should at best be above 100 at the speed where all plugs are out. If plugs are leaved ungrounded, power pack can be damaged.

hi G Dane<br />i have a question also if you take out the spark plugs how do you ground them? or what do i ground them too. i need to check my compression also and i want to be sure i do it right.<br />thanks<br />Don

 

[marinemech]

Re: Am I checking compression correctly?

what do plugs look like? also modle make and year?<br />try putting plugs in rotate flywheel and feel compression (bump over moter) should have strong resistance on compression depending on modle make and year if its a reasonably new moter it may have exhaust reliefs drilled in cylinder walls if so comp. gauge will help i'm not sure if a leak down tester would help or not

 

[G DANE]

Re: Am I checking compression correctly?

minocqa<br /><br />I always lay the spark plugs in a way that I am sure their threads will touch a bolt at the coil or elsewhere where they will stay. If needed, I put another old plug in a hole to give resistance for the starter gear, usually not nessecary.

 

[G DANE]

Re: Am I checking compression correctly?

Although its no spark test, its a good opportunity to see a good blue strong spark at all four, too.

 

[minocqa]

Re: Am I checking compression correctly?

Originally posted by marinemech:<br /> what do plugs look like? also modle make and year?<br />try putting plugs in rotate flywheel and feel compression (bump over moter) should have strong resistance on compression depending on modle make and year if its a reasonably new moter it may have exhaust reliefs drilled in cylinder walls if so comp. gauge will help i'm not sure if a leak down tester would help or not

i have a 1976 70 hp johnson <br />my plugs are a light brown and a little oily.<br />thanks<br />Don

 

[marinemech]

Re: Am I checking compression correctly?

if compression is enough?? try spray some mixed fuel into carb throats and see if motor starts<br />if it starts and dies you have a fuel problem also while front is off airbox look to see if all choke plates are closing properly