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Acceptable Compression Variance on a 1982 75 hp Johnson

M

mike77cj

Regular Contributor
"I am trying to decide if it i

"I am trying to decide if it is worth the time and money to troubleshoot and repair an engine of mine. Scenario as follows:

Second owner since 1992. Engine well maintained, ran excellent. Pickled and sat for four years. Started right up, idles like a gem, falls on face when throttle is applied under load.

Compression as follows: #1 - 145, #2 - 140, #3 - 120

Pulled head. #1 and #2 cylinder walls within spec. #3 slightly out (0.0029", limit is 0.0020"). Decent amount of piston slop. A rebuild is obviously in order, but I do not have the time at the moment.

I reinstalled head with a new gasket, then dropped the carbs, tore them down, soaked them and put them back on. Now the engine will only idle with 3/4" choke.

Am I wasting time here with such a large variance in compression? Like I said, before pickling the engine ran excellent; smooth idle, adequate power.

One last item: a black oily, soupy mixture is draining from the prop exhaust. I have ruled out fogging oil because I have run the engine long enough, have cleaned the inside of the cylinder walls and cleaned the carbs. This NEVER occured before, when the engine was running well. Thanks."
 
"I'm not quite sure what Y

"I'm not quite sure what You mean by 'out'?
Out of round? Outside dia (Bore)? Out for lunch?

What might be more of a worry is that #3 is low on compression.
I would not worry about the black oil coming out, probably only dissolved carbon/soot from exhaust, unless the gearcase is empty of oil!

As for idling problems, I guess You must have missed something during Your cleaning and install. Most likely an idle channel/jet.
But a total answer of the engines condition You will not get without a complete tear down, or at least removal of exhaust covers. That again requires that You lift off the powerhead."
 
"Hi Morten,

By 'out&#39

"Hi Morten,

By 'out' I am referencing the cylinder bore. I would attribute the low compression to blow-by between the piston rings and cylinder wall.

Since my original post, I located a receipt from the prior owner's service. It reads "Number three cylinder reed valve shows signs of excessive wear. Engine will run as is, but replacement recommended." Perhaps this lead to less fuel entering cylinder 3, thereby reducing lubrication and increasing wear over the years?

This weekend I will pull the carbs, break them down again, pull the intake and inspect the reed valves."
 
Well if somebody could " s

Well if somebody could " see " that the reeds had excessive wear then they had them out and should have replaced them.If those reeds on #3 were in fact worn the others would be worn as well. Now in many years I have niot seen reed valves that had " excessive wear and they are very reliable.Reed valves have nothing to do with " cylinder compression "
 
"Ikia,

I am not stating th

"Ikia,

I am not stating that worn reed valves directly relate to compression. My thought is, if the reed valves are worn and less fuel is entering the cylinder, in turn less oil will be lubricating that cylinder during high rpms. Perhaps sixteen plus years of this condition led to the wear of the cylinder wall thereby leading to lower compression."
 
"I am speaking specifically ab

"I am speaking specifically about cylinder 3 in the post above, assuming conditions are normal for 1 and 2."
 
"I have quite honestly never s

"I have quite honestly never seen worn cylinders on any outboard except for a very few that has been used for heavy commercial use with some 2000+ hours!
However I have seen quite some brand new ones on the limit and even slightly out of the factory specs.
Reeds does nor 'wear', but they may crack and break, normally if engine is run with crank-case combustion or reved above max specified for a period of time. I guess the previous owner means: do not close 100%, and even that is normal.
The low compression is most likely caused by sticking rings and/or damaged piston. If problem creating a proper vacuum in the crank case for cyl 3, that might be the same reason, or a damaged lower crank shaft seal."
 
"The factory reeds are made of

"The factory reeds are made of a good grade of stainless steel... they do not wear. However the aluminum reed "blocks" do corrode away. I have seen that a few times.
I don't know how much is "too" much, of free flow between the reed and the blocks to really affect the engine.
I know you guys have seen this too. Could this be what was ment on the service slip?"
 
"Gene
I would lean more to a

"Gene
I would lean more to a tech that wanted more work than a fogging as for the # 3 cyl i would say it more from use than any thing else,I have a 60hp sitting on the stand now did the compression test on it's 100 #1 108#2 95# 3 it don't run as strong as it did new but the owner said when it will not run that's when he'll let me rebuild it it's in for lower end right now he hit a stump he thinks prop and bearings are all gone more likely from running after it was full of water"
 
The small identical looking je

The small identical looking jets may have inadvertently been switched.

Top front idle jet = #67

Side idle jet = #59

And of course the high speed jet that can only be installed in one place = 56D
 
"I found a stuck float which w

"I found a stuck float which was keeping the engine from idling. Repaired float, engine starts right up and idles, but with a repetative backfire every six seconds.

Spark is good, solid blue jump over 7/16". I put new plugs in today and after running for five minutes the #3 plug is heavily fouled.

I suspected a faulty compression gauge so I bought a new one today. Revised compression results are:

#1 - 130
#2 - 125
#3 - 122

I pulled the intake and inspected the reed valves. All appear to be in order. Fair amount of rust on the upper portion of the reed block.

I replaced all the fuel lines inside the engine. Running the engine from a jerry jug with a new primer ball. Disassembled the fuel pump and inspected it, all looked good.

I pulled the flywheel, key was in fine shape. There is a fair amount of rust along the inside perimeter of the flywheel. Could this affect the timing?

Timer base is advancing properly when throttle is pushed.

The backfiring, through the exhaust, leads me to believe the problem is timing. Any ideas? Thanks."
 
"rust,wont affect timing------

"rust,wont affect timing--------if u r backfiring thru exhaust,...u have wires in wrong location/flywheel key bad/mislocated.....
backfiring thru exhaust,..is a definate,..ign problem....u could have internal probs,if all else is ok."
 
"If You are running on muffs,

"If You are running on muffs, exhaust 'backfiring' is natural as You do not have any exhaust back-pressure from water level. Put it in a drum and check again."
 
"I agree,what "we" may

"I agree,what "we" may call "normal"..may be mis-construed" as a backfire,by the normal engine trblshtr."
 
"Engine is running in a test t

"Engine is running in a test tank, still backfires with the back pressure of the water.

Flywheel key was in proper location. No wires have been removed / altered. Keep in mind this engine ran excellent for eleven years. Then I pickled it and she sat for four years. Problem began when I fired her off.

What part of the ignition system should I troubleshoot next? Keep in mind I am getting a solid blue spark at the plugs.

Plug 3 continually fouls but I am getting good spark. The carbs do not have needle adjustments. How would I lean the mixture? All jets are clean.

What internal problems might one suspect?

Thanks for the help guys!"
 
"A bit confusing.
'Backfi

"A bit confusing.
'Backfires' is normally caused by too early ignition.
Then we have the situation where combustion travels down into the crank-case, which is normally caused by a lean-burning condition or defective pistons/rings.
'Backfiring' through exhaust is a bit 'off, as that is normally where it should go. I expect that what was meant was that combustion continued after exhaust port had opened.
What might be is that the cylinder with fouling plug runs far too rich and gives a slow burning creating what might be taken as a back fire symptom."
 
"Backfiring is also caused by

"Backfiring is also caused by excessive fuel. Since it started when you repaired the stuck float, I would go there first. Replace the float, seat and valve. Make sure the float clearance is set at the proper level."
 
"i'm not sure,i follow thi

"i'm not sure,i follow this,.....i have had engines gushing fuel from every site,and excess fuel,has NEVER caused a backfire...for me...maybe a fire or 2....but thats a diff story!"
 
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