81 75hp Johnson problems

[MD Hook]

Hey All,
I got a motor a friend gave me and I'm having problems with it.It ran great with the muffs on and was pumping water.Its had a new powerhead put on it but I don't know when or how many hours it has on it.You can see in the pic below because it was never painted.


When I took it out it ran fine for a just a few minutes then started acting like it was running outta fuel. Brought it home and cleaned the tank/filters,replaced the fuel lines and connectors and fuel pump.
Took it out again and it ran fine for a while but lost power sorta like it was running outta gas.
Brought it home and did a compression check. Top cyl was 140 the other two where 150.
Thought maybe it was a blown headgasket and it was running too hot and losing compression.
Figured the best thing to do would be pull the head and inspect since a head gasket is pretty cheap.
Here's a pic of the top of the pistons and cylinders.


Here's a pic of the head.


I noticed the top piston looked like the left side of it was burnt off or something.Sorta looks like it got dropped on it edge.You can see in the pic below.

Here's the second piston.

And here's the bottom piston.

The top cyn dosen't looked scored up where that part of the piston is.


What should I do next? Do you all think 10lb difference is too much compression loss in the top cyl? Should I get a new head gasket and look for another problem? Would over heating make it lose power and act like it was starved for fuel?
I ain't worried about a little ruff idle if thats the only thing a 10lb compression loss will do.
I really don't think its got many hours on the rebuild the cylinder walls look pretty good.Not really scared up or nothing.

[This message was edited by MD Hook on 07-15-03 at 03:13 PM.]

[This message was edited by MD Hook on 07-15-03 at 03:14 PM.]

[MD Hook]

I forgot to tell you all that after she loses power she idles fine but will only power back up for a few seconds then it falls on its face again. When I held my hand in the hot water discharge it's about the pemp you would expect outta the water heater at home. Its got a good stream shooting outta the pee hole as well.

[jtburf]

Any chance you have changed the fuel pump, primer bulb,,,,the 10 pound difference is fine there should only be a 10% difference so thats ok did you acutally check the temp before you tore it apart?... it look's like it very rich it need's a decarb job how's the carb's are they full of crud and water in there .... several thing's to consider ...John

[MD Hook]

Jt,
I didn't check the temp.I don't know how hot it suppose to run but you can hold you hand under the discharge. The carbs seem to be fine. I didn't pull'm and check but I thought it ran and idled too good to be a carb problem. At what temp should it run at?Yep I put a new fuel pump and primer bulb on it as well as the rest of the fuel lines,

[MD Hook]

Dang this board moves fast.Anyone else got an opinion?I pulled the thermostat.It was a little coroded and didn't look as it was replaced recently.I put it in a pan of water w/ a candiy thermomitor on the stove and it started to open about 150.

[theoldwizard]

First, I'm no expert on this engine, but ...

Before tearing off the cylinder head how close did you inspect the carbs ? Could be just a clogged high speed jet. Also could be a crankcase seal problem. Those are hard to detect without spcial tools.

Next time before you pull the head off of any engine, do a leak down test. It is a much more accurate way of telling the "quality" of the piston/ring/cylinder (and valves if it is a 4 stroke) seal than a simple compression test.

[MD Hook]

Wizard,
Never heard of a leak down test but I'm gonna check into it.Thanks for the tip. I might be able to borrow or rent the tool somewhere.I thought maybe we could get a year or so outta this motor and not have to do a lot of major repair but that might not be the case.We pulled the carbs last night and my buddie[John) is taking them to work to clean'm and check'm out. He and I are working on it and pretty much just leaning the ropes of outboards.We collected up several over the winter for real cheap or free. We never did a total rebuild but may attempt it on this motor.
Again thanks to the advice guys!We need all the help we can get.

[chip]

MD Hook,

A fuel restriction could cause those symptoms. What kind of gas tank are you using? If you're using an inboard tank, there could be debris in it causing this problem. As the motor runs at high rpm it sucks up junk into the pickup inside the gas tank, the motor bogs. At low rpm there's not enough suction to keep the debris against the pickup. This happened to me once.

You can try connecting the motor to an external gas tank and see if that helps.

good luck. chip.

[MD Hook]

Chip,
That and the fuel pump was my first thought.
Its an inboard tank.I took the tank out and cleaned it.The vent is open too. A external tank didn't help.

[KStigall]

I had '76 75hp Johnson Stinger that did the same thing a few years ago.
A pain in the a$$ for about half a day.
Had a mechanic in Emerald Isle, NC go over it with me. He's a great guy, sat on the boat and tinkered with the carbs. Tried different gas tanks, compression tests......
His statement, "If it was me I'd get a good fuel line with a GOOD pump ball and save that crap from Wal-Mart as a backup. Those valves around the ball can screw ya."
Slipped across the river and picked up an OMC line, cripes cost twice as much as the TEMPO.... haven't a problem since. Well until click, click, clack, clack, clank, clank ........SHE_IT

[MD Hook]

Thought of that too.The external tank had a different line and bulb on it.

[WildeOne]

The cylinder with part of the piston gone is significant. At least that cylinder is running too lean and has burned away part of the piston. I would clean/rebuild all of the carbs. By the way, running on muffs or at low power in the water doesn't tell anything about the fuel system on an outboard, other than it's getting SOME fuel. A too lean condition means the engine will self destruct unless corrected.

There are other possible causes which should be checked out first (air leak anywhere from the tank pickup to the carbs, plugged vent-but you already checked that, air leak in the crankcase).

I assume you have a shop manual ... they are very helpful in trouble shooting a specific engine.

Good luck.

___________________________
a bad day fishing is always better than a day at work
___________________________
Jim

[This message was edited by WildeOne on 07-16-03 at 10:30 AM.]

[MD Hook]

The plug in that cylinder had a slightly darker insulator but they were all pretty much slightly oily and a little black. Maybe john will find something wrong with he carbs when he checks them.

[MD Hook]

Found a little dirt in one carb.Still thinking. May be a electrical problem under heat. Gotta get a head gasket and thermostat/impeller.Put it back together and see what gives I guess.

[drogerso]

Check the fuel pump diaphram. That motor depends on crankcase pressure to operate the fuel pump, and if the diapram has a hole or tear in it, it is not going to pump enough fuel. It is easy to get to, remove the cover from the pump and the diaphram is right there. Should only take a couple of minutes to get to it and check.

Dan
'2-N-TOW'

[drogerso]

One other thing; a bad fuel pump would also cause the rich condition on the bottom cylinder. The pump is straight across from the bottom cylinder, so it will suck fuel straight through the hole in the diaphram. This would also cause a lean condition on the top cylinder, resulting in the burning you are seeing on the edge of the piston.


Dan
'2-N-TOW'

[MD Hook]

Already replaced the fuel pump.I'm putting it back together this week and see what we got.Maybe cleaning the carbs did the trick.I'll let you all know