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Old 03 May 2011, 04:16   #1
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Compression Test

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.
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Old 03 May 2011, 04:26   #2
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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.
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Old 03 May 2011, 06:37   #3
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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.
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Old 03 May 2011, 06:47   #4
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Essential - ask Biff!
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Old 03 May 2011, 08:11   #5
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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.
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Old 03 May 2011, 14:18   #6
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Yeh, I have a threaded one too. Much easier to use if you're singlehanded than the tapered bung ones, but both work fine.
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Old 03 May 2011, 14:46   #7
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Quote:
Originally Posted by Nasher View Post
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.
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Old 03 May 2011, 15:35   #8
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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
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Old 03 May 2011, 15:59   #9
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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.
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Old 03 May 2011, 16:03   #10
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Quote:
Originally Posted by Nasher View Post
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?
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