No Compression

I have a 60hp three cylinder Johnson, have had it since new it would have been purchased around the mid eighties.

It has only ever sunk once, but that was years ago, we got most of our money back from the insurance company.

It is now suffering from disasterously low compression, all cylinders around the 60LB mark.

It has never been cooked, we have not done that many miles, it has been left unused for up to say 6months at a time
( unusual in Queensland we don't have winters here). The paint on it is like new, original grease on the linkages.

It is fuel injected, I have however always run it with oil in the fuel as well as the injection.

I suspect the rings might be carboned in, then again perhaps the bores are Glazed.

I am soaking one cylinder with fuel injector cleaner to try and get the carbon out, but if anything that is reducing
the compression even further.

Any suggestions?

If I pull the thing apart I am going to end up with a box full of needle rollers!

Re: No Compression

Gotta love that statement "it has only been sunk once". Anyhoo, I'd guess you are correct on the stuck rings theory. Extra oil is not always a good thing. If soaking doesn't help, and it probably won't, then you will have to start disassembly to see what you have. Start with pulling the cylinder head. If that doesn't pan out, then remove the exhaust cover so you can see the rings. Yep, it you have to go beyond that, you will have a bunch of bearings to keep track of.

Re: No Compression

put a little oil in the cylinder, retest, if the compression gets better, its likely the rings

Re: No Compression

Does it run? If so, I would do a serious decarb and retest compression before tearing it apart.

Be sure that your battery is 100%, load tested, and all cables and connectors from it are free of corrosion.
Follow the procedure in the Top Secret File ^^ exactly.

All cylinders testing near each other is a sign that there is probably a fault in the testing equipment or procedure. The likelihood of all three going bad by the same amount at the same time is very low.

Watch out for surgeon's disease. A headache does not require neurosurgery.

Re: No Compression

readpigg said:

It has only ever sunk once, but that was years ago. it has been left unused for up to say 6 months at a time.

Click to expand...

When was the last time that this engine ran as it should... without problems?

Re: No Compression

Thanks you for the replies.

If you add oil the compression comes back up to the 100Lb mark.

Definitely Rings.

Obviously it is oil injection Not fuel injection. OOps

Re: No Compression

Took the head off.

B%%% of a job, had to drill out the four bolts round the center cylinder.

When retaping the holes always terrified the tap will pick up on a bit of busted stud and shear, leaving an insolvable problem.

Head gasket in good condition , no corrosion in the aluminium.

Absolutly no sideways movement on any of the pistons.

Neighbourhood expert reckons excellent condition.

It used to take a day for the cleaner to soak past the piston.

That leaves the reed valves.

Took the carbies off and had a look inside.

Looks good, no gunk, no rust, can flex them with one's finger.

Suggestions please.

Re: No Compression

Suggestions? I gave you suggestions to do the simple stuff, but you preferred major surgery. Reed valves do not affect compression, head gaskets do not go bad on three cylinders at once, Carbs do not affect compression.

There was something wrong with your first compression test. Now you have major surgery.

Good luck.

Re: No Compression

I am doing my best to resist major surgery.

As the piston travels up the cylinder to compress the gas it sucks air into the crankcase, the reed valve has to open to allow
this ingress of air.

As the piston travels down on the power stroke it compresses the air fuel mixture in the crankcase , against the reed valve.
if the reed valve is leaking the mixture simply gets shoved back out the carbie.

If the reed valve is either stuck open or cemented closed you get no air into the cylinder to compress.

There are a set of reed valves per cylinder, for low compression on three cylinders three sets of reed valves have to fail at the same time.

My Kindom for an Expert.

Re: No Compression

At compression testing speeds scavenging is not an issue. The cylinder may be filled through the exhaust ports on the down stroke.

Reed valves do not affect compression test readings.

A poor gauge or flawed technique affect compression readings. Low but equal readings on three or more cylinders is strong evidence that this is the problem.

Re: No Compression

im with the guy above, youve taken the engine part that didnt need taking apart.
you dont know how to do a compression test and like said above.
you can take the carbs and reeds off if you like it still wont affect compression.
compression is once the piston goves ABOVE the exhaust ports and the RINGS keep a good seal.
Now i would of put a nice drop of decarnoning fluid down there and left well alone for a day.
Then started it up to clear the crap out,;et it run for about 10 minutes then done a decarbong process again.
Once i had done this and then got a proper compression tester and seen it in the regieon of around 130 to 145 psi
i would then of been happy.
Plus too much 2 stroke in an engine that dont get used will cause sticky rings.
the engine needs a bloody good blast to clear it out, and someone who knows what they are doing when they take it apart.

I dont suppose you warmed the engine up before undoing the head bolts !!!!!!!!
and when was the last time this engine had a decarb ?
and a new impellor ?
and a new thermostat ?
timming checked ?
fuel pipes changed for the later fuels ?

If you dont trust vro and i dont blame you if you dont then do away with it and run at 50-1 all in the one tank.
to have just 60 psi you would be able to pour oil in the bore and watch it go away almost instantly.

go buy a new compression tester, or check your reading psi and not kpi !!!!

if its kpi then that would be 2.2 times 60 = 132 psi !!!!

before anyone says no such thing as a kpi tester, ive got one in the garage,its pretty old must be from the 1970's
and its rubbish. not accurate at all and one day up the next day down.

i would get a second comp test before doing open heart surgery on a outboard.

i hope you get your engine sorted out and back together all ok,but do just run 50-1 and play safe


phill

Re: No Compression

There was very little carbon in the motor.

The motor won't start, I used the oxy to get the bolts hot I mean HOT.

Leaky reed valves will have the same effect as leaky poppet valves

I will put the head back on with the old gasket and lots of silicone and try again.( I have ordered a new one, from the States you are Cheap)

The motor has been standing around a lot, perhaps the four rusted bolts cracked the head interface and the cylinders are blowing into
the water jacket.

There are no burn marks on the gasket ss rings though a slight touch of colour/rust on the block side.The Aluminium is perfect.

Re: No Compression

CORRECTION

Before I get abused, the poppet valves holds the compression pressure, the reed valve does not, They are NOT the same

Whew!!

Re: No Compression

Put it all together.

Squirted some engine oil diluted with diesel into the cylinders.

Spun her over, then checked the pressures

120Lb, 90Lb,110Lb

Squirted diesel into the cylinders, spun her over to wash the oil out, then checked, similar pressures.


Recap

Symptoms

Was difficult to start and getting worse.
Would fire on two, with the third firing when the engine warmed.
Compression pressures similar and low.
Compression pressure would increase when you squirted oil in.

With head off

Little carbon
No corrosion/ blow through marks on aluminium
Gasket in excellent condition
Four center bolts rusted out
No sideways movement in pistons
No wear on the cylinders, though probably glazed, coffee colour.

I wonder how many ring jobs have been caused by a blown gasket?

Re: No Compression

If it ain't broke, fix it until it is!

Re: No Compression

If you are missing center bolts on the head and center cylinder has lower compression than others, I'd say that these two are related.