Motor won’t start. Low compression

[CeeJ1791]

I have a ‘73 50HP Johnson that I purchased last summer. I always have a really hard time getting it to start. Once it warm it starts fine, but if it sits idle for a week or so same issues. I just recently de-winterized it, and the only way I got it to fire was by spraying a bunch of starter fluid into the carb intakes. I was thinking it was maybe a fuel issue so I just completely cleaned and rebuilt the carbs. After putting it all back together, I still can’t get it to fire. I tested the spark and that’s good. I tested the compression and the top cylinder showed 65 PSI while the bottom showed 120 PSI. What can be done about one cylinder being so much lower than the other? Where should I look next?

 

[boobie]

Pull the cylinder head and take a look.

 

[F_R]

Something is seriously wrong with that 65 one. You can hope that it is just a blown head gasket. Then face the reality that it is something like a blown piston.

 

[CeeJ1791]

Pull the cylinder head and take a look.

What should I be looking for?

 

[boobie]

F-R said it all.

 

[CeeJ1791]

I did a little more investigation today and got it going but discovered that it’s only running on one cylinder (I pulled the cap off the top spark plug and the motor kept running). Is this more cause for concern or is it possible I messed something up in the carb rebuild or wasn’t careful enough in cleaning them?

 

[Bosunsmate]

Pull the head gasket, top cylinder is in trouble

 

[CeeJ1791]

Pull the head gasket, top cylinder is in trouble

Im pretty new to this all. I have the manual but not sure exactly what I’d be looking for.

 

[Bosunsmate]

Do it gently and then remove the gasket and analysis it for any worn bits where compression gases may be getting lost through
Sometimes its obvious. You can also look at the piston dome with the head off.
Take pixs if not sure.
Back out those head bolts inner to outer alternating between up and down, same when doing it up but you do it up in two stages

 

[CeeJ1791]

Got the bolts out but there seems to be some sort of sensor wire (that I can’t figure out how to remove) preventing me from removing the head completely. I was able to slip the gasket out and have a look. New one is ordered and will arrive in 3 days.

 

[jbuote]

That wire (if I'm not mistaken) is your temp sensor.
There is a head cover that is bolted on top of the cylinder head.
To remove that, if you're going to, you have to take the head cover off.. It has a gasket too..

No pro, but that does look like a blown head gasket...
You might be lucky here!!

I'd still see about taking the head off completely and getting some pics of the pistons and cylinder walls too though.
You're already in there...

The guru's will know best however...
(EDIT: :facepalm: Forgot about the rubber sleeve and the knife connector for the temp sensor as pointed out by interalian and flyingscott below... Don't pull the head cover... Left that info however, so you know the head cover is there..
That's why they are the gurus.... )

My best to you!

 

[interalian]

That's a blown head gasket for sure.

The tan wire can be disconnected by following it back a few inches. You will see a rubber sleeve covering the wire (edit: it's right there in plain sight, clamped just to the right). Slide the sleeve back then disconnect by folding the connector. This is called a "knife connector".

Once you have the head off, send pix of the inside of the head and the piston tops. You may just get lucky and need to replace the head gasket.

Before re-fitting the head, you need to re-surface it by laying a sheet of medium sandpaper on a FLAT surface, then work the head face in a 'figure eight' pattern. If there are shiny and dull spots, keep working until it's shiny everywhere.

 

[flyingscott]

You can disconnect the temp sensor by the black rubber tube in your pic. Peel it back and there is a blade connector under there. Make sure the piston is okay and the metal on the block is good. It is also a good idea to resurface the head.

 

[CeeJ1791]

Ok not the best photos because it got dark. Top cylinder looks really dry compared to the bottom. Also the top one is pushed out while the bottom is recessed (does that make sense?). Is this normal?

 

[jbuote]

Top piston pushed out, while bottom is recessed is absolutely normal...
I do see a distinct difference between the two cylinders there, but I don't feel comfortable commenting on what it might be..
It may be nothing... Not enough experience here to say one way or the other..

I would offer though, if you can rotate the flywheel by hand slowly and get pics of both cylinders when they are mid-stroke (even with each other) and then top is recessed and bottom is pushed out..

That'll give these gurus a good look at the pistons and the cylinder walls too..
Top is the one with low compression, and could just be the head gasket, but rotating flywheel till top is "recessed", bottom is pushed out, and getting pictures of that, then these gurus will be able to see the cylinder walls too...

I believe that would give them more insight to offer good advice..
Hope that made sense...

 

[interalian]

The pistons look different as you were not getting proper combustion on the one that has low compression. Per above, turn flywheel (clockwise as viewed from above) and watch the pistons move.

With one piston all the way out, shine a flashlight into the gap around the piston and move the piston to make a gap between it and the cylinder wall. Look down the gap to check the condition of the top piston ring. It should have a gap in only one place, likely at the top. Repeat for the other piston. Report back. We're looking for broken rings here.

 

[Bosunsmate]

Looks like your top carb may of being blocked so it was running lean so that will be on your job list too.
Do as they both suggest. And also remove that water cover on the very back of the head as you will want to clean in there too and check the thermostat and poppet valve in there.
I agree that your head gasket is stuffed. I cant see any signs of ring bits hitting the piston or head so you may be lucky there as well but its still quite possible they are broken.
Id also oil up that cylinder wall with a oily rag ASAP and clean it back while you can so that no corrosion makes permanent damage to it

 

[CeeJ1791]

Here are some better pix of that top piston (and one of the bottom for comparison). What do you think gurus can it be salvaged?

 

[CeeJ1791]

Looks like your top carb may of being blocked so it was running lean so that will be on your job list too.

This all started with a carb re-build. When I took them apart I found one of the floats (can’t recall which but now I’m assuming the top) was missing the little piece that attaches the needle to the float, so the needle was lodged in the seat.

With one piston all the way out, shine a flashlight into the gap around the piston and move the piston to make a gap between it and the cylinder wall. Look down the gap to check the condition of the top piston

I don’t think I see any signs of broken rings, though it’s hard to tell bc the piston is pretty flush against the wall (little to no gap). Because of how dry it is, it doesn’t seem to move as freely as it should. How do I remedy this?

 

[F_R]

OK, I'll be the bearer of possible bad news. That upper cylinder has been ingesting a lot of water. Likely from the blown head gasket, but possible from the exhaust gaskets as well (has it been overheated?). Whatever the case, in my opinion it is junk unless/until it is torn down and the bearings inspected for rust and pitting. Good luck.