1994 15 HP Johnson hard to start and rough idle

[uofa314]

Hi, I have a 1994 15 HP Johnson, model # J15RERE, that is hard to start and has a rough idle. It runs great if I can get it started and get it above an idle. It will push my 14' jon boat about 19 mph (2 mph faster than my 1986 15 HP Evinrude).

My dad traded up this motor and gave it to me, so I don't know its history. It had been sitting for a while. So far, I have rebuilt the carburetor (have taken it back apart to make sure I didn't miss anything), replaced the thermostat (someone had removed it), replaced the water pump impeller, replaced spark plugs, and changed the oil in the lower unit. Both spark plugs have good spark. I know some of that has nothing to do with the hard start, but wanted to list everything.

I have adjusted the low speed fuel mixture screw and tried it at all different settings. It starts ok while on muffs, but is very hard to start when back pressure is added when the motor is in a barrel. The exhaust relief port and muffler that are on the back of the motor are clean.

One thing I haven't done yet, is to check the compression. I have ordered a compression tester, but it hasn't come in yet.

Other than possible low compression, is there anything that I'm missing that I could try?

 

[racerone]

Missing thermostat is a red flag to me.------Correct , no point doing anything until you have compression values.----Post them here.----The 1994 15 HP has bigger displacement beginning in 1993 model year.

 

[uofa314]

Missing thermostat is a red flag to me.------Correct , no point doing anything until you have compression values.----Post them here.----The 1994 15 HP has bigger displacement beginning in 1993 model year.

It was leaking around the thermostat cover, so I pulled the cover off and was surprised to see that the thermostat was missing. Maybe that didn't cause too much damage.

 

[uofa314]

The compression testing kit came in today, so I measured the compression on the motor. I used a cordless drill on slow and then fast speed to turn the flywheel to check the compression. This is what I got:

Top cylinder:
Drill in slow speed: 65 psig
Drill in fast speed: 105 psig

Bottom cylinder:
Drill in slow speed: 60 psig
Drill in fast speed: 95 psig

I then measured the compression of my 1986 15 HP Evinrude that runs good, using the same test kit and drill, to get a comparison.

1986 15HP Evinrude compression:
Top cylinder:
Drill in slow speed: 85 psig
Drill in fast speed: 105 psig

Bottom cylinder:
Drill in slow speed: 90 psig
Drill in fast speed: 105 psig

So, I then removed the head from the 1994 motor with low compression. I did not see any signs of the head gasket leaking. The cylinders look pretty smooth, but the pistons are very sloppy in the cylinders. I guess the pistons and cylinders have this much wear?

At this point, the motor may not be worth fixing. I may do it anyway just to have something to tinker with.

 

[racerone]

The compression results confirm the " red flag "-----Sounds like it overheated and scored the piston.----Jammed rings in the grooves.---New pistons O / S are big $$.

 

[uofa314]

I looked at the motor a little more and the pistons and cylinders look pretty good except there is a big gap (about 0.030") between the cylinder wall and the piston on both cylinders. The rings are not stuck and make up the gap. I measured the cylinder ID and got 2.380" when it should be 2.375". The pistons are about 2.350" OD. To me, it seems like there is just lots of wear and most of the wear happened to the pistons. I wonder if it ran a long time without the thermostat and running cold accelerated the wear rate?
I will probably go ahead and get it bored 0.020" over, even though it will cost more than the motor is worth. Gives me something to tinker with.

I have never rebuilt an engine. For 0.020" over, do I just take the block to a machine shop and ask them to bore it to 2.395" (2.375" + 0.020")? What is the tolerance on that dimension?

 

[racerone]

Pistons are sized / measured at the bottom of the skirt.-----Even a new piston has about 0.010" taper