Compression Test

actions · 03 May 2011, 08:16

Hi,

Is this something that is fairly simple for me to do when buying a second hand engine.

Its a 4 stroke elec start Honda.

Can anyone show me an idiots guide on how to do it. I've read a number of posts but all have slight variations and are a bit more in depth for a mere beginner such as myself.

Thanks.

Nasher · 03 May 2011, 08:26

Hi.

It's quite easy really, but the first thing you're going to need is a compression meter, which is a gauge on the end of a bit of pipe with some adaptors.

If you were closer you'd be welcome to borrow mine.

Nasher.

jwalker · 03 May 2011, 10:37

As Nasher says, you need a compression tester. It's a gauge attached to a short length of metal tube which has a tapered rubber bung on the opposite end. The bung fits into the spark plug hole and the testers are usually hand held so firm pressure is required to hold one in place.
The engine needs to spin quite quickly so it's best to remove all the spark plugs to reduce the engine load and a well charged battery is necessary. Also, it's good to have a helper to crank the engine and work the throttle.

The tester is pushed firmly into the spark plug hole and held there, the engine is cranked over a number of times until the gauge ceases to increase its reading. It is important for the throttle to be held fully open while the engine is cranked. The gauge has a schrader valve in its end to hold the pressure within the gauge so that it can be read after the cranking has ceased, this valve must be depressed to release the pressure before testing another cylinder.

Good luck.

Steve_B · 03 May 2011, 10:47

Essential - ask Biff!

Nasher · 03 May 2011, 12:11

The one I use is a bit easier as it has a load of threaded adaptors to fit different sized spark plug holes so you don't have to hold it in place.

I'd suggest trying to borrow one, as although they can be found for @£30 it's a bit of an investment if you only use it once.

Mind you, as 250 says, ask Biff what can happen without one

Nasher.

Matt · 03 May 2011, 18:18

Yeh, I have a threaded one too. Much easier to use if you're singlehanded than the tapered bung ones, but both work fine.

actions · 03 May 2011, 18:46

Quote:

Originally Posted by Nasher

The one I use is a bit easier as it has a load of threaded adaptors to fit different sized spark plug holes so you don't have to hold it in place.

I'd suggest trying to borrow one, as although they can be found for @£30 it's a bit of an investment if you only use it once.

Mind you, as 250 says, ask Biff what can happen without one

Nasher.

Thanks Nasher, and thanks for the offer of yours, as you say if only i was closer. I'm going to take a trip down to Halfrauds to see what they have. I'm thinking £30 outlay as opposed to £3000 on a dead engine.

So, remove all the spark plugs, put in full throttle, place compression tester in one cylinder and crank over engine several times to get result. then repeat on next cylinder etc etc.

My question is, will the electric start engine crank over in full throttle?
Also he states that the battery as its stood for 3 years does turn the engine over but needs changing, should i buy one now and take it with me?

What sort of readings should i be looking for and do i need to earth the spark plugs somehow?

Sorry for so many questions.

actions · 03 May 2011, 19:35

This any good ?

http://www.halfords.com/webapp/wcs/s...Id_165557#dtab

States does 14mm and 18mm spark plug ports, any idea if this is the right size for an outboard.

Plus see more stupid questions in my previous post toooooo

Nasher · 03 May 2011, 19:59

That'un will do

It's hard to say exactly what reading to expect on each engine without data from the manufacturer, but what you should be looking for more than anything is very similar results between cylinders.

I usually work off @5% differance between cylinders being OK with 10% at an absolute maximum.

Nasher.

actions · 03 May 2011, 20:03

Quote:

Originally Posted by Nasher

That'un will do

It's hard to say exactly what reading to expect on each engine without data from the manufacturer, but what you should be looking for more than anything is very similar results between cylinders.

I usually work off @5% differance between cylinders being OK with 10% at an absolute maximum.

Nasher.

Ok will do, thanks very much. One last thing before you all pull your hair out with my Q's, the spark plugs, leave them connected to the leads or take them off altogether, if leave them on, do i need to earth them somehow?

Matt · 03 May 2011, 20:05

Unless it's a real sh!tter, in which case anything that's not 0 is a result!

Quote:

Originally Posted by Nasher

I usually work off @5% differance between cylinders being OK with 10% at an absolute maximum.

actions · 03 May 2011, 20:09

Quote:

Originally Posted by Matt

Unless it's a real sh!tter, in which case anything that's not 0 is a result!

Ha ha, Yea, 5% of bugger all and even i know to walk away. What should it be roughly, 50, 100, 150 etc ?

m chappelow · 03 May 2011, 20:09

As well as checking the compression on a cold engine its not a bad idea to see what they are like after its been running for a while or at least until its warmed up a bit.

Matt · 03 May 2011, 20:12

IIRC my xs200 in good nick was 150->155 across all 6.
My 1986 150 ended up at 95ish - it was pretty worn.

actions · 03 May 2011, 20:13

Quote:

Originally Posted by m chappelow

As well as checking the compression on a cold engine its not a bad idea to see what they are like after its been running for a while or at least until its warmed up a bit.

Ok, its been added to my ever increasing list, think i might change from A4 to A3 to fit all the bits on.

Seriously though, thankyou to everyone for all your advice, very much appreciated.

Any thoughts on what to do with the plugs whilst they are out, earth or just remove altogether from the leads?

actions · 03 May 2011, 20:15

Quote:

Originally Posted by Matt

IIRC my xs200 in good nick was 150->155 across all 6.
My 1986 150 ended up at 95ish - it was pretty worn.

Cheers Matt. Hopefully looking for around the 150 mark then. What should i walk away at?

Matt · 03 May 2011, 20:15

Actually, if yer gonna get very particular about it, you'd do a leak down test.

I've never felt the need myself though - the point at which I'm doing a compression test usually means something is already very wrong.

actions · 03 May 2011, 20:17

Quote:

Originally Posted by Matt

Actually, if yer gonna get very particular about it, you'd do a leak down test.

Now your just confusing me, leak down test. Is that making sure if "I" leak on it, it runs out?????

Searider · 03 May 2011, 21:46

Take the plugs out completely - from their leads.

You don't want any sparks!

neil.mccrirrick · 04 May 2011, 11:48

But do make a note of which lead belongs to which cylinder, they have to go back where they came from. Just in case you don't know, and sorry if I sound pedantic, trying to be helpful, the plugs can go back in any cylinder