4 hp low compression

[sharkman]

4 hp low compression

Following up on advice received earlier from this forum, and while awaiting my carb kit, I did a compression check on this motor which I recently purchased at a yard sale (Evinrude s/n 4206s)

With both plugs out, and throttle wide open, I checked the top cylinder first, with a reading of around 60 psi. Then I checked the lower cylinder and got a reading of 80 psi. Did this twice....same reading

Engine was cold of course as I can't get it going

Can anyone advise on what this means exactly..is this motor worse off than i first thought. Did I do the compression check right?

 

[CFronzek]

Re: 4 hp low compression

Re: 4 hp low compression

there are some differences of opinion as to how to check cylinder pressures. Your method is as good as any. You have one tired cylinder and one "good enough" cylinder.
This motor is on a downward slide. It will not develope it's original power any more.
In general when a motor loses cylinder pressure it gets to a point where it's difficult or impossible to start. Since you have an 80 lb cylinder that may help this motor to fire.
As long as this motor is reliable it is good enough for any duty that does not put too much strain on it. So, be kind.

 

[Xcusme]

Re: 4 hp low compression

Re: 4 hp low compression

Sharky,

Pull the plugs and lay motor with the holes pointing up. Rotate flywheel until you see the top of the top cylinder. Pour in a couple of ounces of Seafoam and let her soak for a few days. Drain any excess Seafoam from cylinder and re-do the compression test.
Frankly, if it was my motor,I'd pull the head and inspect the cylinder walls. If the bores are in reasonably good shape, I would split the powerhead and rebuild it. It's easier than you might expect, with a service manual (OEM). Most shops will tell you to forget it with an 'old motor'. If you didn't already know, it's a 1972.

 

[RPJS]

Re: 4 hp low compression

Re: 4 hp low compression

I agree with Xcusme on this.
Pull the head, if you've got any damage to the cylinder wall your flogging a dead horse trying to get this little motor running how it should, if the walls are in good shape you will probably find that the piston rings are stuck.
To split the motor, pull the pistons, hone the cylinders and free up the rings and put it all back together again will only take you a few hours.
If you do have to free the rings be very gentle, they snap very easily BTDT.

 

[sharkman]

Re: 4 hp low compression

Re: 4 hp low compression

Thanks gang. I think I'll try the sea foam, Xcusme can you advise on what that will do and maybe where I can get it?

Depending on what happens I will pull the head and look at the cylinder walls

Thanks again all

 

[sharkman]

Re: 4 hp low compression

Re: 4 hp low compression

okay curiosity is getting the better of me, I am considering removing the head now and taking a look inside. However I can see where the first 4 bolts of the head will come out easy enough, but what about the bottom 2? They seem to be blocked by the metal casing that surrounds the lower part of the powerhead. Do i have to remove the entire powerhead to get the head off?

Thanks

 

[Paul Moir]

Re: 4 hp low compression

Re: 4 hp low compression

The two bottom bolts can be loosened completely, but not withdrawn from the head until the head comes loose. That is to say, it's possible to completely remove the head without removing the engine, but it is not possible to completely remove the two lower bolts without removing the head.

When re-installing, you'll need a new head gasket (~$3), a box end wrench and a torque wrench. 60/80 doesn't sound like a head gasket but it could be a stuck ring if you're lucky. If there's signs of clogging in the water passages you should pull the exhaust cover plate too, which will also allow you to poke at the rings and see if they're stuck.

Click Here

There's a trick to torqueing the head right, so I'll try to get that bit done.

 

[sharkman]

Re: 4 hp low compression

Re: 4 hp low compression

Thanks Paul...I'll give it a shot and let you know

 

[sharkman]

Re: 4 hp low compression

Re: 4 hp low compression

quick question.....when you say the rings may be stuck......what am I looking for when the exhaust cover comes off? will they not move around the piston or will they prevent the piston from moving upwards/downwards?

thanks again

 

[Paul Moir]

Re: 4 hp low compression

Re: 4 hp low compression

If you take the exhaust cover off, you'll be able to see the side of the piston and it's rings through the exhaust port. By probing the rings with a plastic or some other safe implement, you'll be able to determine if they're all nice and springy sometimes.

It should be fairly obvious what's going on once you pull the head though. If the problem is cylinder wear or rust, it'll show.

 

[sharkman]

Re: 4 hp low compression

Re: 4 hp low compression

Thanks again Paul

update - got the head off

With the untrained eye I see a fairly shiny cylinder wall with a few rust spots on both #1 & 2. There doesn't appear to be much corrosion on the inside, but on the outside, in the space between the outside cylinder wall and extreme outer wall (don't know what that gap is called) there is alot of white stuff that looks like hardended foam, but its not on the inside of the cylinder. A bit of oil present
What do you think? next step?

Always grateful

 

[Paul Moir]

Re: 4 hp low compression

Re: 4 hp low compression

The white stuff is calcium carbonate - the stuff that seashells are made of. It will come out of suspension if the temperature of the cooling water much exceeds about 140F which is why they use low temperature thermostats on all marine engines with raw water cooling. You've got to dig that stuff out. Check out the cooling circuit map on my 4hp shareaproject site and make sure both cooling passages are clear.

If the white stuff seems packed in around the top cooling passages go ahead and take the exhaust cover off so you can clean out the passage that runs over the top of the engine block. You'll need to use a drill bit or something similar.

This is without question the result of neglecting the cooling system and the water pump in particular, which is unfortunately common on smaller outboards.

My 4hp when I got it had that condition. If you look close at the top piston you'll see it's got shiny sides in places. That's from the piston heating and expanding until it was larger than the bore, at which point it dragged the cylinder walls (and likely stalled).

The gap is called the "water jacket".

No sign of a blown head gasket I take it? There is usually a noticeable chunk missing.

 

[sharkman]

Re: 4 hp low compression

Re: 4 hp low compression

Great Paul.....where can I find your site?

No sign of damage to the head gasket, it appears in good shape, a bit old though

 

[Paul Moir]

Re: 4 hp low compression

Re: 4 hp low compression

Oh, sorry, that "click here" I gave you a few posts up.

 

[sharkman]

Re: 4 hp low compression

Re: 4 hp low compression

got it thx.....didn't notice there was more then the one page on the link....great drawing, clearly shows the waters path

and got the exhaust cover off....top rings did seem to stick but gently pushed on them a few times and they move freely now......