Compression Check

[MasterGuns]

Can someone give me a quick verbal (walkthru) of how to "properly" perform a compression check. Bike is an 86. I have a pretty good idea of how to check the compression, but since it has been literally years since I've done one on anything, I just need a quick review of the procedure.

 

Here's how I would do one.

 

Once the motor is exposed, I would

 

1) remove all the spark plugs

2) short out all the spark plug wires to not harm the CDI box

3) connect gauge

4) at full throttle, crank the motor over until max pressure is observed on the gauge for each cylinder

5) if low one tests low (what would be low??), squirt a little oil in that cylinder chamber then recheck

6) record all results

 

Good compression would be where no one cylinder is more than 10 off from any other.

 

Add anything you think I missed or should know before proceeding.

 

Thanks

[SilvrT]

Good compression would be where no one cylinder is more than 10 off from any other.

 

 

if you have 3 at 150 and one at 140 (10 diff) ... you don't have a problem.

 

From my old days of wrenching, I think we used a 25% factor so...

 

3 @ 150

1 @ .75 * 150 = 113 equates to a possible problem.

 

now, this was when I worked on cars (V8 or straight 6) ... a 4 cyl might be a different % factor.

[van avery]

You might put a battery charger on to make sure you don't run down the battery. Otherwise I believe you have it.

[Michel]

What I made when I check compression.

 

1 : I start the bike for have a hot engine.

2 : I remove the carburators ( for have a maximun air entrance)

3 : I remove the 4 spark plug.

4: " short out all the spark plug wires to not harm the CDI box "

 

In this condition I have this on my 1988 1200 venture

 

1 / 13,5 kg

2/ 14 kg

3/ 13 kg

4/ 14 kg

 

 

michel

[MasterGuns]

What I made when I check compression.

 

1 : I start the bike for have a hot engine.

2 : I remove the carburators ( for have a maximun air entrance)

3 : I remove the 4 spark plug.

4: " short out all the spark plug wires to not harm the CDI box "

 

In this condition I have this on my 1988 1200 venture

 

1 / 13,5 kg

2/ 14 kg

3/ 13 kg

4/ 14 kg

 

Compression should always be checked while the motor is cold. Any you don't need to remove the carburetors to do so. All else I agree with except an 88 Venture is a 1300 not a 1200/

[Michel]

Sorry but I have always check compression engine hot, before I works in mecanic , and we take pression like this. that right it s not indipensable to remove carburators......

In France 1200 venture: 1984 to 1988

1300 venture: 1989 to 1993

 

Here a picture of a 1200 venture 1988

 

http://img15.hostingpics.net/thumbs/mini_9166831988.jpg

 

michel

[MasterGuns]

Sorry but I have always check compression engine hot, before I works in mecanic , and we take pression like this. that right it s not indipensable to remove carburators......

In France 1200 venture: 1984 to 1988

1300 venture: 1989 to 1993

 

Here a picture of a 1200 venture 1988

 

http://img15.hostingpics.net/thumbs/mini_9166831988.jpg

 

michel

 

Wow, I didn't know that. Learn something new everyday. Thanks

[mmaleney]

Why test a motor cold rather than hot ?

[ACE50]

So you don't burn yourself!!??

[Black Owl]

Just my 2 cents here.

An engine should always be at normal operating temperature to obtain an accurate compression reading.

Or so my daddy told me.....

[MasterGuns]

Just my 2 cents here.

 

An engine should always be at normal operating temperature to obtain an accurate compression reading.

 

Or so my daddy told me.....

 

 

I was taught the motor can be cold, or warmed. I suppose you could get it to normal operating temp but by time you pulled all the spark plugs and screwed in for the first cylinder it would already be under normal temp. Anyway, I was taught to perform this test when cold and add 5% to the numbers.

[Squidley]

I too have always tested them cold, it was instructed to me that it would give a true idea if things were starting to wear. If you had a trouble cylinder, squirt a little oil in and that would suggest that the rings may be worn.

 

I have always gone by 10% difference between the cylinders, take the highest reading and the lowest and if the difference is less than 10% thats good. Throttle wide open and crank till the needle quits moving.....

[Flyinfool]

Why test a motor cold rather than hot ?

 

When hot the engine was just splashing oil onto the cylinder walls. This will always give a higher reading do to the extra seal provided by the oil.

While testing cold there will be less oil on the walls and you will get a better indication of the mechanical condition of the rings. This is why if you get a low reading you then squirt some oil into the cylinder to determine if the leak is rings or valves.

[playboy]

When hot the engine was just splashing oil onto the cylinder walls. This will always give a higher reading do to the extra seal provided by the oil.

While testing cold there will be less oil on the walls and you will get a better indication of the mechanical condition of the rings. This is why if you get a low reading you then squirt some oil into the cylinder to determine if the leak is rings or valves.

 

 

Sounds like a caveman could do it.