Johnson 9.9 1980 compression number

[svey]

Good day,

I have 100# on the top cylinder, 80# on the bottom one. With those number, should I take the powerhead off and re-ring? Or will I be able to use motor for the season? It is used on a sailboat to get in and out of the arbour and sometime to motor in bad condition (no wind or wind in wrong direction)?

Tx

Yves

 

[iwombat]

Re: Johnson 9.9 1980 compression number

You've got a 20% difference in the compression across cylinders. That's out of spec. Time to pop the head and see what's going on.

 

[svey]

Re: Johnson 9.9 1980 compression number

Tx for your response,

I've opened the bypass covers (head and left hand side). I didn't see any scoring on cylinder.

I've never removed the powerhead before. I've only done water pump, thermostat and lower unit seal replacement. I have the Johnson service 1971-89 outboard 1 to 60hp manual. Is that a very hard procedure to do? I've had the engine 4 years and I know that the PO was not incline in maintenance (leaving the motor on the boat over the winter attached to the sailboat, I live in Montreal, Canada).

Tx

Yves

 

[iwombat]

Re: Johnson 9.9 1980 compression number

You don't need to remove the powerhead to take off the head. Could just be a blown head gasket.

 

[svey]

Re: Johnson 9.9 1980 compression number

Iwombat,

Sorry, I guess I don't understand the lingo.

In the back of the motor, I have the water blanket plate, where the thermostat is located. Under that, I have the cylinder head attached to the powerhead. Is the head gasket between the cylinder head and the powerhead?

How can I test to see if it is leaking? I could try tightening the bottom bolts if I can fit a socket wrench there.

 

[JB]

Re: Johnson 9.9 1980 compression number

The bottom cylinder has a problem, Yves. It may be a blown head gasket, which can be changed without removing the powerhead. It may be a buildup of carbon preventing the rings from functioning properly, which can be corrected with a decarbonizing procedure (see Engine FAQs), or it may be mechanical failure of rings or piston. That would require replacement of the broken stuff and maybe a rebore of that cylinder.

I would start with the decarbonizing. If that doesn't bring the compression up to match within 10% I would then remove and inspect the head gasket for leakage.

Too many guys tear the engine apart only to discover that the rings on one piston are stuck with carbon. Could have fixed that in half an hour without even getting out a wrench.

 

[svey]

Re: Johnson 9.9 1980 compression number

I've followed the instruction for decarbing my motor with Seafoam. After the 3 session (15 minutes run, 15 minutes wait) I get 100# top cylinder and 85# lower cylinder. That still over the 10% mention before.

Is there a test to check for head gasket leak? Or I just remove, scrape and put a new one? Do I need a special tool to reach the lower right bolt?

Tx

Yves

 

[iwombat]

Re: Johnson 9.9 1980 compression number

Take a look at your spark plugs. Does the bottom cylinder appear steam-cleaned? If so, that's a dead giveaway that you've got water intrusion.

I think you'll probably want to remove the head anyway, just to see what's going on. But, you're on the right track - remove the head & scrape off any leftover gasket. It's a good idea to resurface the head too. You can do that with a piece of sandpaper and a flat surface like a piece of glass. Just rub the head over the sandpaper in a figure-8 motion. After all that put in the new head gasket. Most head gaskets come pre-coated these days so you won't need any gasket sealers.

 

[svey]

Re: Johnson 9.9 1980 compression number

Should I also remove water blanket cover and replace check and replace thermostat?

 

[iwombat]

Re: Johnson 9.9 1980 compression number

I don't see any particular need unless you just want to do it as part of preventative maintenance. T-stats and gaskets are pretty cheap.