Compression Test

[jwalker]

Originally Posted by actions

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

On a standard marine 4 stroke engine I'd guess about 120ish.

If you're not sure about the engine re compression readings, squirt a wee drop of oil into the cylinder to seal the piston rings and do the test again. If this gives a big increase in reading it indicates bore/ring wear. If there is little increase in reading but you still feel the pressures are low it indicates valve wear.

[Boats&Outboards]

Your looking for 130psi at least on the Honda we discussed and I would not want to see anything more then 5% difference on such a new engine. Get it done when cold and after around 10 mins runtime to get the rings checked too. Sometimes they can show signs of failure when hotter although lets be honest if it's had the life your advised it should be spot on.

Sounds like it's going to be fine but appreciate the need to check as much as you can.
Peter

Peter ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

[actions]

Originally Posted by neil.mccrirrick

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

Trust me, you dont sound pedantic, i have never ever done this before, but for the sake of learning and buying a kit for £25, its worth it.

Also thanks Searider for the spark plug Q too.

I shall let you all know how i manage,
Going to practive first on my current engine before i go.

[actions]

Originally Posted by jwalker

On a standard marine 4 stroke engine I'd guess about 120ish.

If you're not sure about the engine re compression readings, squirt a wee drop of oil into the cylinder to seal the piston rings and do the test again. If this gives a big increase in reading it indicates bore/ring wear. If there is little increase in reading but you still feel the pressures are low it indicates valve wear.

Thanks for that, will do. I was wondering what would be wrong if i got low readings as the back of the testing kit just tells you there could be a whole host of issues, but doesnt tell you which is which.

Are both these issues expensive if in need of repair. I'm thinking yes as its boat related.

[Poly]

Originally Posted by actions

Are both these issues expensive if in need of repair. I'm thinking yes as its boat related.

All outboard repairs are expensive! I'd guess the valves are cheaper to fix than piston rings - but this engine is claiming to be in "as new" condition then if it has either of these issues it should be a walk away.

[actions]

Originally Posted by Polwart

All outboard repairs are expensive! I'd guess the valves are cheaper to fix than piston rings - but this engine is claiming to be in "as new" condition then if it has either of these issues it should be a walk away.

Cheers Polwart, Yes very true i've learnt from previous experience they are expensive.

Trust me, if i find any issues then i will be walking away. I would rather have a wasted day and two tanks of fuel burnt than an engine thats useless.

[Boats&Outboards]

Ring failure on such a motor is quite expensive, not difficult but anything that involves powerhead removal is going to hit the wallet.

Peter ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

[C-NUMB]

One question, was it not stated in on thread that one should not run/ turn the engine without having spark plug grounded(this might result in CD box failure)?

Would removing the plugs eliminate this potential issue?

[Nos4r2]

Originally Posted by C-NUMB

One question, was it not stated in on thread that one should not run/ turn the engine without having spark plug grounded(this might result in CD box failure)?

Would removing the plugs eliminate this potential issue?

No, but if you've got the killcord off when you do it then it won't spark-so won't damage the CDI box.

Always assuming you can turn it over with the killcord off that is.

[C-NUMB]

Originally Posted by Nos4r2

Always assuming you can turn it over with the killcord off that is.

Why did I don't figure out that my selves...Thanks, thats a usefull rule!

[actions]

Just had a quick practice on my current outboard before i go and look at the new one and i cant beleive how easy it was to do.

Results of 155 PSI on both cylinders.

[actions]

Woohoo, and Do'h.

Well did the trip to wales and after a great run up there finally found the boat and engine.

Looks clean as a whistle and i would decribe it as showroom condition

Started to do all my checks on the engine and all was perfect

Got my compression test kit out and bugger, it wouldnt fit the Honda Engine

So i was left with do i, dont i, do i, dont i going around in my head.

So got it running, pumps water perfectly and looks brand new so went for it.

Took it to my marine dealership today and they were gobsmacked how clean it all was and said engine is 100%. Phew

Anyway heres a pick of my new pride and joy. Whats better is my old boat sold this morning and has gone too.

Thanks everyone for all the advice, just a bugger the compression kit didnt fit.

[Commando C2]

screwfix do a tester for £16.99 and it has 12mm, 14mm, (the one for most outboard plugs) 16, and 18mm

just got one today but not used it yet.