Other How do you CORRECTLY check compression?

[seatsR4toilets]

I'm sure this is a stupid question, but I need to ask.
I sold a motor to a guy last year, who months later said (through somebody else and not to me directly) that I sold him a motor with 5 pounds or so lower compression than I said it had when he bought it.
I spoke to my motor builder who said that if you don't check it correctly that this can happen, even as high as 20 psi off.

Here is how I check it:

1) pull both plugs
2) thread in the tester and zero it out
3) open throttle fully
4) crank motor for about 4-5 seconds
5) read gauge

Can anybody give me some accurate insight on this?
Does it make a difference?
Am I doing it wrong?
Is there an accurate AND inaccurate way to do this?

 

[Johnson8537]

Thats How I have always done it!!! Some people don't do the throttle But thats right on as far as i know!

 

[Mark Gomez]

at my shop and for a few other shops in my area we do the same thing just finger thread a sparkplug into the opposite cylinder also

 

[crammit442]

Here is how I check it:

1) pull both plugs
2) thread in the tester and zero it out
3) open throttle fully
4) crank motor for about 4-5 seconds
5) read gauge

Can anybody give me some accurate insight on this?
Does it make a difference?
Am I doing it wrong?
Is there an accurate AND inaccurate way to do this?

-The motor will spin faster with one plug in and the tester in the other hole.
-Opening the throttle fully is correct.
-crank until gauge stops rising.
-manifold/pipe can make a couple of pounds difference.

Having a high torque starter/dead battery/old cables can affect cranking speed which will affect indicated compression. If you sold someone a motor with a claimed cranking compression and it ended up 15 or 20 lbs off, that might be a concern, but 5 psi is well within the range of difference from one gauge to another.

 

[Freakrider]

Groupk claims that contrary to popular belief the motor will spin faster with one plug left in...which would give a even higher reading...

if he did his test without the battery fully charged it could have dropped his reading lower....just like a high torque starter will read 5 pounds or so higher (from my experience).

 

[motoman96]

sounds good to me Ken, I leave one plug in hand tight too like mentioned above.

 

[Speedfreak]

Listen to Crammit... He posted the correct way. BUT 5 psi is nothing. He didn't use your gauge so there is no way unless both gauges are calibrated using the same equipment to make sure they are the same. Calibration can even be off it is according to where it is done.

I tested 5 gauges by clipping them to the air hose. The compressor gauge and regulator was set on 185. I got 4 different readings. 2 happen to match the compressor BUT I still don't know which one is exact.

 

[zeski25]

Wamilton always leaves a plug in. Test it twice, dont give it gas and clear the cylinder out before you test compression. He said that oil and gas being in the cylinder can give misreadings and ruin the gauge.

 

[norcal ex]

i do all of that but do not understand the throttle thing. i have tried it with or without full throttle multiple times and there is absolutely no difference.

 

[Matt_E]

With the throttle plates closed, the motor cannot draw in the full amount of air through the intakes fast enough.
That's the theory, anyways.

 

[nightrider05]

The throttle thing makes nearly no difference on 2 strokes(MAYBE a couple PSI max)

On 4 stokes it makes a huge difference in your readings.

 

[Mark44]

Hold your stop button in while cranking the ski over.

Mark44

 

[extreemthrottle]

The throttle thing makes nearly no difference on 2 strokes(MAYBE a couple PSI max)

On 4 stokes it makes a huge difference in your readings.

agreed, a two stroke has primary compression. and also a dry cylinder will have different readings from a wet cylinder.

 

[seatsR4toilets]

The "wet cylinder" thing does make perfect sense.
So, maybe change the process to cranking it over a few times first with plugs out and compression tester out, then put one plug in and the tester and then crank it.

I'm thinking maybe taking pics of the gauge when doing the test if it's a motor I'm gonna sell.
I've also got into the habit of taking a fine-tipped magic marker and wrting the compression numbers on the head (if it's painted) or on the cylinder itself just under the head. That way if I keep it, I remember what it was, and if I sell it, I have a written record right on the flippin' motor.

Thanks for all the input.

As far as a four stroke, what is considered a "HUGE" difference?

 

[Flash-FX]

:bigeyes:http://www.x-h2o.com/threads/12806&highlight=grounding+spark+plugs :bigeyes:

 

[seatsR4toilets]

:bigeyes:http://www.x-h2o.com/threads/12806&highlight=grounding+spark+plugs :bigeyes:

Someone posted to turn the fuel off.
Makes sense, if you're trying to get a "dry" cylinder..
What is the reason for getting the cylinder warm? Does that make a psi difference?

 

[Matt_E]

It gets the cylinder and pistons to operating temperature and dimensions.

 

[SuperJETT]

Air pressure differences from location to location (elevation plays in to that as well), battery voltage, battery cables condition, starter condition, top end condition, and even how freely the engine turns over will all affect the compression gauge reading.

5 psi is nothing when you factor in all the variables.