1984 Evinrude 70hp

[loose rivet]

I was given an 84 Evinrude 70hp the other day, its missing its lower unit and a few other bits but the owner said it ran fine before but had very low power. He told me its got bad compression. I did a compression check, and got 107-111-108 top to bottom. I connected a water hose to the intake tube and it fired right up and seems to run good. What kind of compression readings should I see out of one of these? I realize that they all need to be equal, but what do most read when tested?
I can hunt down a lower unit for it, but don't want to bother if this thing is near the end of its life.
Its a clean motor, with no signs of saltwater use and its got power tilt/trim.
I found another 70hp with a bad power head for $200, the lower is supposed to be still good.

 

[JB]

Re: 1984 Evinrude 70hp

Those compression numbers are acceptable, LR. Indicated psi depend a lot on the gauge and technique.A different gauge and technique might produce numbers around 125. The important number is the span of the cylinders being below 10%.

If the engine is, in fact, low in power it isn't because of compression.

 

[slowleak]

Re: 1984 Evinrude 70hp

I've got two 70hp Johnson motors, one a 1984, the other a 1985, the 1984 tests at 120 psi on all cylinders, the 1985 tests at 141 psi. Both have very low hours on them, I've owned the 1984 since new. The 1985 came from a buddies old bass boat.
The 1985 will run circles around my 1984, it always did.
Back when I first bought it I had it back to the dealer many times after it couldn't keep up with my buddies identical boat with the same prop. We both bought our boats the same day, mine was a few months older and I got a 'deal' on a left over motor.

The 1984 however sounds better, the 1985 is noisier at higher RPMs. I'd venture to guess neither of mine have more than 300 hours on them. The 1985 is a VRO motor without power tilt, the 1984 is a premix motor without power tilt if that matters.

 

[loose rivet]

Re: 1984 Evinrude 70hp

I went ahead and picked up the parts motor, got it for $100 flat. The lower unit looks good and the motor shows no sign of saltwater use.
Just for kicks I check the 'bad' power head on the parts motor for compression and got 151/21/147 pounds top to bottom. The middle spark plug is smashed. Its a VRO motor and the VRO pump is bypassed. The parts motor is a 1985 model Johnson.

I swapped installed a new water pump,changed the gear lube, then installed the used lower unit and gave it a test run on the muffs. It shifted and ran fine. When I put it in a tank of water, I had issues getting it to start, it took me choking the carbs by hand to get it to fire, and then it wouldn't idle. I put it back on muffs and it ran fine so I let it idle a bit and gain some heat in the block.
After it warmed up, I shut it down and rechecked the compression, I got 131-123-142 out of it. The bottom plug looked black and wet with beads of water. When I cranked the motor with the plugs out I got a couple spurts of clear water out of the plug hole.
The one thing that didn't make sense to me was that the plug was black, not white like a washed down cylinder and a bore scope shows no cylinder damage that I can see. A close look at the old plugs does show a bit of rust on one electrode, I'm pretty sure that came from the bottom cylinder. I didn't think much of it since the motor has been sitting a while and I saw no other signs of water.

Where should I check next? I would think that a head gasket would lower my compression, especially when hot. Maybe an exhaust side issue? With the compression readings being good, and the cylinder with the water reading as one of the best, its not sounding like a head gasket. It also run fine on muffs, it only acts up in a tank. When its running on the muffs, I get a nice steady tell tale stream, and some light spray out the upper exhaust ports, so its getting water, but I suppose not enough to make the problem surface?