Johnson CD-25R Losing Power

[TboltBeardie]

Hi everyone, this is my first post and I come to you with a predicament.
I recently bought a 1968 Johnson CD-25R 6HP outboard motor. The person who sold it to me had rebuilt the carburetor, replaced the impeller, replaced the points and plugs, and installed a new fuel pump. Basically a lake ready motor.
I took it out on the water, pushing an '87 Coleman Crawdad 11 foot flat bottom. It ran like a top for a couple of hours, but during the latter half of the outing, the engine was completely gutless. It became impossible to start and when in gear, would hardly get above idle speed at WOT. Putting it in neutral and revving it up to clear it out would buy me about a minute of half-decent performance, but it would eventually dwindle back down to a point where it was barely able to stay running.
I did some research and came to the conclusion that it was down a hole, so I pulled the flywheel and checked the ignition system. Points and coils all looked brand new. Readjusted the points just in case they were out of adjustment. Making very strong spark. Ran a compression test and both cylinders are making 70 PSI. A bit low compared to spec, but they're equal, so I'm assuming it's due to normal wear. Adjusting the points seemed to improve things a bit, enough so that I found that the motor doesn't start to have issues until I do any trolling. When I avoid low speed operation, the motor seems to do fine. After talking with a local marine shop, I discovered that I made the mistake of using Pennzoil marine 2 stroke oil (rookie mistake, I know), which put me on the chase for fouled plugs. With a fresh tank of gas with quicksilver oil and a new set of J6C plugs, I took it back out on the water. It performed about the same, maybe a little better, but ended up on the brink of stalling just like before.
Yesterday I set up a makeshift test tank, got the motor warmed up, and seafoamed it in the hopes of freeing up a potentially stuck ring. I ended up doing this three times over the course of the evening, and the performance improved significantly. Compression remains at 70 PSI, possibly 75. Now it can idle in gear for an extended period of time without stalling or hesitating when I whack the throttle. I'm hoping that the seafoam works, but I am still pricing new parts for a rebuild if it ultimately needs it. Maybe busting the glaze and installing some fresh rings is what it needs.
I have no problem shelling out $250 to rebuild this motor, since getting another motor is out of my price range, but with Labor Day weekend coming up, I don't have time to do an overhaul just yet. I don't want to open the engine up if it doesn't need it, but if it does, I will.
My question to you: am I on the right track with low compression? Or should I be looking at other causes? Any input would be greatly appreciated.

 

[racerone]

The original head gasket have been updated with a much better quality one with a metal rings.----Cheap at about $20.00 for a SIERRA one.----Lets you inspect cylinder walls at the same time.

 

[F_R]

I seriously doubt it needs a rebuild. Do a cylinder drop test. Sounds like it probably is running on one cylinder

BTW, there is nothing wrong with Pennzoil. He has his opinion, I have mine.

 

[oldboat1]

Ain't the oil -- unless maybe you are using too much or not enough, Run a 50:1 mix. Check the operating temp (should never be too hot to touch). There is a thermostat on top of the head, and that may be due for replacement. Make sure bypass openings are clear in the t.stat cavity. Sounds like you may be revisiting repairs the PO says he made. Remove the carb and disassemble/soak, then use carb spray and perhaps a soft wire in every opening. Measure spark with an open air adjustable tester (cheap). If there are cracks in the coil coatings, replace the coils.

 

[TboltBeardie]

UPDATE:

I pulled the head off yesterday, and didn't find anything obviously wrong with the head gasket or the mating surfaces of the block/head, and no obvious damage to the cylinder walls. They aren't completely pristine, but they aren't heavily scored or pitted. After some further inspection, however, I did find that the top exhaust side bolt hole has a helicoil in it, so I'm clearly not the first person to have a kick at the cat with this motor, so there may be a good chance that it's been having issues for a while. I also found that the thermostat was stuck open. I don't know if it's the root cause of the issue, but surely it isn't helping the situation.
I have a new head gasket and thermostat on the way, and hopefully that solves my issue.

 

[racerone]

Every other one of those thermostats that I look at seems to have failed in the open state.----Very common.

 

[TboltBeardie]

UPDATE:

So I ended up completely disassembling the powerhead, and I'm glad I did. The top piston had a patch of heavy scoring on one side - so heavy that it displaced enough material to impinge the bottom ring. It had obviously been this way for a long time, as there was a section of ring that wasn't touching the cylinder wall and still had the break-in grooves present. There wasn't any damage to the cylinder walls, aside from some light pitting that honed out without any trouble.

I (somehow) managed to find two sets of new old stock rings (P/N 378432) on Ebay, as well as a used piston (P/N 386692) that was in decent shape. I also ordered a gasket set (P/N 377828), and opted to rebuild the powerhead.

The crankshaft was in very good shape, as were the bearings in the block. The cylinders were in decent shape - no scoring or ring lip. Just glazed with a little bit of light pitting. The old rings had end gaps ranging from 0.020'' to 0.022'', and had lost much of their springiness. The bottom connecting rod bearings were in acceptable shape, so I cleaned and reused them. The reed valves looked good, sealing against their respective surfaces.

One thing that I found to be very concerning, however, is that most of the inside of the engine had been painted, including the inside of the crankcase, the transfer ports, and even the combustion chamber in the head! I don't know if Johnson/Evinrude/OMC did things like this (I know that a lot of older engines recieved a coating of glyptal paint on the interior surfaces to prevent oil from permeating the metal, but that mainly applies to cast iron), but it does strike me as being very odd. Some of the oil drain ports near the crank bearings were even completely obstructed by paint. Acetone and a toothbrush made quick work of the paint, though. I made sure that every port and gallery I could find was clear.

After honing the cylinders, all of the new rings' end gaps measured right at 0.005''. All mating surfaces were cleaned, appropriate gasket sealer was applied, and all fasteners were torqued to spec. Reassembly went smoothly.

Compression came up some, around 80 PSI, but I anticipate that it will increase as the rings continue to bed in. I tanked the engine and let it run in gear at various speeds for a few hours, constantly monitoring the temperature of the discharge cooling water.

After letting the engine break in, I took it out to the lake. Significant improvement, but not 100%. It ran like a top, but took its sweet time to get up to speed. After doing some more reading, I finally caved and opened up the carb (I had left it alone up until this point since the guy who sold me the motor had rebuilt the carb). Everything was brand new and super clean. I referred to the parts diagram and lo and behold, there was something missing. The gasket that sits on the end of the main jet at the bottom of the bowl (P/N 344037). Lucky enough, said gasket was among the handful of extras from my gasket set.

Installed the gasket and went back out to the lake. Night and day difference. Immediate throttle response, and I got ALL the RPM. It's like a completely different motor now.

Looking back, the missing gasket in the carb was likely the root cause of this engine running like a turd, but there's no doubt in my mind that it was in dire need of a rebuild. At least she'll be good for another 50 years or so.

 

[Willyclay]

Attaboy!

 

[racerone]

I have a number of these motors.------I usually take them apart without trying to start them-------Most have had 5 or 6 owners and folks do amazing maintenance and non maintenance !