Low compression on Johnson 45hp pull start.

[Mystic Mariner]

Hello, new to the forum; I have a '91 Johnson 45hp commercial pull start motor. I pulled the plugs and opened the throttle to full. I hooked up a compression tester and get 30lbs on the top cylinder and about 90lbs on the bottom. I pulled it about 5 times and read the gauge. Should I be getting higher readings like an electric start? Motor runs, but now I'm freaking out with these readings. Any help would be appreciated.

 

[racerone]

Remove cylinder head.----Inspect cylinder.----Lucky you if it is just a blown head gasket.-----Model # of your motor?

 

[Mystic Mariner]

Remove cylinder head.----Inspect cylinder.----Lucky you if it is just a blown head gasket.-----Model # of your motor?

J45RCLEIA

 

[Mystic Mariner]

Remove cylinder head.----Inspect cylinder.----Lucky you if it is just a blown head gasket.-----Model # of your motor?

Do you know if the head can be removed w/out removing the powerhead; or do I need to pull it all apart?

 

[maineiac5586]

I would try a different tester. The 40 and 55 commercial motors I've recently worked on have 150 psi in both cylinders

 

[Mystic Mariner]

Another symptom, running on earmuffs the telltale is streaming like crazy, but as the motor warms up steam starts to come from the telltale and it switches from water stream to steam. I'm going to order a water pump kit from MarineEngine.com along with a head water-cover gasket and thermostat. Figured I'd hold off on the head gasket until I do a borescope.

 

[racerone]

Pull the head off now.-----The head can be removed with powerhead left in place.----The head is warped and needs to be resurfaced before new gasket is installed.---

 

[Mystic Mariner]

It looks like the head is part of the powerhead mounting.

 

[racerone]

As clearly stated the head can be removed with powerhead in place !

 

[Mystic Mariner]

How would I know what level of "wear" or "scoring" that would be acceptable once the head is off? Is there a way to determine if honing/boring and new pistons are needed before I reassemble? Sorry, but new to outboard repair, I'm used to working on car engines and they are evidently more forgiving.

 

[racerone]

It should be easy to determine once head is off.----Post picture of top cylinder.

 

[Mystic Mariner]

It should be easy to determine once head is off.----Post picture of top cylinder.

My Seloc manual should arrive any day now and I'll pull the head.

 

[Mystic Mariner]

OK, decided to try another compression tester that I know works (from my friend's automotive shop). This is what I got; 65 psi top cylinder, 110 bottom cylinder. I guess now that I have accurate readings I'll pull the head. Any comments?

 

[Mystic Mariner]

For some reason the two videos won't play, but the pictures show my compression on top and bottom cylinders, respectively.

 

[racerone]

You now have information that you did not want.----Pull the head off now.----Easy to do as indicated earlier.

 

[Mystic Mariner]

Head’s off; only one head bolt broke off in the block. Looks like a gasket fail, is this common? Cylinders have no scoring.

 

[racerone]

Fairly common.----The head is warped so it needs to be resurfaced before you install a new head gasket.-----Go buy a lottery ticket.

 

[Mystic Mariner]

Fairly common.----The head is warped so it needs to be resurfaced before you install a new head gasket.-----Go buy a lottery ticket.

Would the head warp due to overheating? Anything else I should look for/do going forward?

 

[racerone]

Every head I have ever taken off a motor has been warped to some degree.----Just the nature of an aluminum casting !!

 

[Mystic Mariner]

Waiting on parts, made good progress and was able to remove the broken head bolt.