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Tom Hawk

Contributing Member
To All:

I have a 1971, 20 HP Johnson, Model 20R71S. It has been in storage for almost 20 years. It looks almost new. I have been working on the motor, boat and trailer for a couple of years in my spare time. I am now retired and ready to enjoy life!

Before using the motor, I replaced all seals in the gear case and installed a new water pump impeller and all O-rings associated with both sections and I put in new plugs. Once finished, I filled the gear case with oil and tested the motor in a barrel filled with water. Everything looked fine. There is no "pee hole" on this motor but the water outlet/exhaust relief was putting out a lot of water. Everything looked and sounded fine.

I took the boat out. At dockside all was OK. The motor ran good with plenty of cooling water. I ran at idle in a no-wake zone for a couple of hours and then went into open water. I opened her up and all was good for a few minutes then the motor started to slow down and I noticed that it smelled hot. I checked the water outlet - NO WATER. I shut it off immediately and got a tow back to the dock.

I just finished pulling the gear case off and checked the water pump impeller. The keyed hub had separated from the rubber impeller so that was the problem. Is this a very common occurrence?

I can, and will, install a new impeller but what should I do before I try to start the motor. I hope I haven't damaged it. The motor is free (not seized) , and sounds OK when cranked over (plug wires removed). I just hope I haven't ruined the power head. I am tempted to install a new water pump impeller and start it up but I would like to know if I should check anything before I do so. If the power head needs attention, I will probably take it to a repair shop.

Thanks for any advice.

Tom
 
You can do a compression test if you want. Usually if you shut it off while it was still running you should be ok. I would take the water pump back to where you got it see if they will warranty it. That definitely is not a common occurence
 
flyingscott,

Thanks. I do not have the gauge to do a compression test. What is the down side of trying to start it after replacing the water pump impeller? I bought the impeller from Marine Engine. I bought it in 2012 but did not use the motor until Monday of this week. They'll probably not replace it since it's been three (3) years but, I'll ask.

Thanks for your help.

Tom
 
Really no downside to starting it. Those are really good motors if the power head is damaged though you will probably be better off finding a different motor. When you install the new impeller coat the pump housing with a light coat of Vaseline. And always start it with water in the pump in a barrel you want the water level above the pump housing
 
Factory impellers are the best replacement parts.---------Never a problem with factory impellers.---------I rarely if ever use aftermarket for that part !!!
 
flyingscott,

When I installed the impeller that failed, I had coated it with a light coating of grease and I had it submerged to normal level when I started it in the water barrel. I have located a compression gauge that I can borrow. I should be able to get compression numbers on Friday. How much compression should I expect to see if the power head is OK? I have seen reference to numbers in the 100 psi range. How low is too low?

Thanks,

Tom
 
racerone,

Thanks for your comment. According to the part number and the paperwork that I got from Marine Engine, the part that I used was Johnson (OMC) original. If the compression looks good, I guess I'll try another one.

Thanks again,

Tom
 
Tom Hawk said:
flyingscott,

When I installed the impeller that failed, I had coated it with a light coating of grease and I had it submerged to normal level when I started it in the water barrel. I have located a compression gauge that I can borrow. I should be able to get compression numbers on Friday. How much compression should I expect to see if the power head is OK? I have seen reference to numbers in the 100 psi range. How low is too low?

Thanks,

Tom
Click to expand...

Does the impeller look like it has been attacked, softening, sticky, brittle, swelling? Elastomers are selective in the type of hydrocarbons they are compatible with. I don't know what OMC uses, but likely Nitrile or EPDM. Nitrile does ok with petroleum greases, but EP does not.
 
I hate it when that happens (hub separation). Yes, we used to see a lot of it back in the '70's era. Not just OMC either. Merc was notorious for that. Anyhoo, how hot did it get? Did it discolor the paint? If not, you may get lucky. In any case, any motor that has run hot should have the head gasket and exhaust cover gaskets replaced. Or you can count on them leaking. While you have the exhaust cover off, you can peer in and see the side of the pistons. Heat damage will show as scoring on the skirts and stuck rings.
 
fdrgator,

The exterior of the power head looks new. No discoloration at all. All the paint is intact. It did not run hot for very long. The engine started to slow down, I looked at the exhaust - saw no water and shut her down. It had a "hot" odor. I am still trying to get my hands on a compression tester. As soon as I get it, I'll take the readings and I'll send the numbers out so you guys can tell me if I should install a new impeller and try to start it up.

Thanks to all who have tried to bail me out of this mess.

Tom
 
To All,

I just checked the compression on the power head. I did not start the engine as I have not replaced the water pump impeller. I just cranked it over, cold. The compression readings were: Top Cylinder, 115 psi, Bottom Cylinder, 105 psi. Based on those readings, I plan to purchase and install a water pump impeller and then test the engine in a barrel. If it runs OK, I'm going to try it on the boat.

I would appreciate any comments on this approach.

Thanks to all.

Tom
 
kimcrwbr1,

Thanks! I have ordered the necessary parts. I'll let you and others know how I end up. In case I have another problem, can you or others suggest a good shop for having the cylinders bored and also a source for two (2) .030 over sized pistons. Each site I look at shows the over sized pistons as being obsolete. Since I have already had a problem with the power head, I would like to latch onto a couple of pistons. I'll need them one day. The '71 Johnson belonged to my Papa and I sure want to keep it in service. I have decided that if it needs to be rebuilt, I'm going to do it myself. I have rebuilt many, many, 4-stroke engines but have never done a 2-stroke. At 69 years old, it's time to learn!

Thanks to all!

Regards,

Tom
 
kimcrwbr1,

I don't intend to purchase any additional parts until and unless I determine that the engine is bad. I'm just going to install the impeller and see if it runs OK. I only want the information on additional parts for future work, if necessary.

You suggested giving it a "good decarb". How is that done?

Thanks!!!!

Tom
 
kimcrwbr1,

As soon as I get the impeller replaced , I will do the decarb. What compression numbers are acceptable and what differential is acceptable between cylinders?

Thank you and others for all the help.

Tom
 
The motor was manual start but I put a starter on it. Thanks for the information. I will proceed according to your advice.

Thanks,

Tom
 
fdrgator and kimcrwbr1,

Here's the latest. I installed the new water pump impeller. The engine started. The water pump worked well, as plenty of water was coming out of the water outlet/exhaust relief. However the engine did not run smooth. I took the engine cover off and noticed water coming out between the head and the bottom cylinder. As fdrgator suggested, the head gasket was shot. I took the head off. The cylinders look great - no scores or scratches visible. I guess the next step is to pull the power head. Replace all gaskets, head gasket, exhaust cover gasket, thermostat gasket, and new thermostat. If the cylinders and the pistons look good, I guess I'll use a cylinder hone and break the glaze on the cylinder walls and install new rings. Should I replace all bearings while I'm in the thing?

What a mess!

Tom
 
How did you go from a possible warped head to a rebuild. People overheat motors all the time you are over thinking this way to much. Fix the head make sure its not warped with a good straight edge if it is you can resurface it. Take a piece of glass tape a full sheet of 220 grit sand paper to it and move the head in a figure 8 pattern until the whole mating surface is shiny.
 
flyingscott,

The head is flat. I already checked it on a piece of glass with 220 grit paper. Should I just replace the head gasket and go?

Tom
 
Yes Your motor is in good shape has good compression. . From everything you told us the motor is in good shape. Once you have the head gasket on redo your compression test cold
 
Last edited:
As you guys suggested, I guess I am "overthinking" this problem.

I will order a head gasket and will replace it. While I'm at it, should I replace the exhaust cover gaskets and the bypass cover gaskets or only the head gasket?

Thanks,

Tom
 
kimcrwbr1,

I forgot one thing. Yesterday when I tested the engine after installing the water pump impeller, there was water coming out of two holes on the forward side of the exhaust housing, just above the lower mounts. The two holes connect with the channel that the drive shaft passes through. I had installed new O-rings on the exhaust housing spacer. This morning I verified that the exhaust housing spacer was installed correctly and that the lower end of the water tube was properly inserted into the water pump grommet and that the upper end of the tube was in its socket. How did water find its way into that area of the engine? Is that normal?

I know I'm driving you guys nuts but I sure appreciate the help.

Tom
 
Those holes connect directly to the channel (passageway) that the drive shaft passes through. I verified that this morning. The two holes enter the drive shaft passageway just above the exhaust housing spacer. The water and exhaust gasses are supposed to be outside that area, mixed together, and discharged via the under water exhaust outlet. The water I saw coming out of those two holes has to be coming from the engine section and down the hollow area around the drive shaft. The hollow area terminates where the exhaust housing spacer seals to the water pump with an O-ring. The water that I saw yesterday, coming from the two holes, had no exhaust in it. It was clean, steady and slightly warm. Maybe it's OK.

Tom

Tom
 
If the impeller is EPDM Rubber then putting oil or grease of any kind on it and letting it set for any length of time is a bad mistake. EPDM softens and swells when in contact with oil and grease. I assemble mine with liquid dishwashing detergent so that I don't have any problems with any rubber seals in the engine. If you still have your old impeller you can set a piece of it in a cup with oil in it for a day and observe the swelling and softening. On your engine there is a water intake on the forward surface of the skeg. Make sure that the screen is not blocked with sand or paint, etc.. The propeller pushes water into this intake when underway thus assisting the water pump when the engine needs the most cooling.
 
foxstan,

The intake screen is clean and open. Thanks for the tip on grease's affect on the impeller. My old impeller (the one that failed) is solid and firm, it just separated from the hub! The intake screen is open. Based on my test yesterday after replacing the impeller, the water supply to the pump is good and I had great flow.
I'm now planning to replace the head gasket and see what follows. I am, however, concerned about the water that's coming out of the two holes in the front of the exhaust housing. I don't understand how water gets into the cavity that contains the drive shaft unless it's coming out the top of the water pump housing where the drive shaft enters (and passes through) the pump assembly. Maybe that's normal.

Tom
 
To All:

Thanks to y'all, I think I'm back in business. Here's the latest. I installed a new head gasket and a new thermostat with gasket. The compression (electric start) is 112 psi, top cylinder, and 109 psi, bottom cylinder. The engine started with no problems and ran good. I had plenty of water coming out of the water outlet/exhaust relief. After running a few minutes in a barrel, I noticed a little water seeping from around three (3) of the bolts that hold the exhaust cover in place. The exhaust cover has never been removed (both gaskets are original). I tightened all the bolts on the exhaust cover. The leak stopped. Unless there could be some internal water leak in the exhaust cover portion of the engine, I guess I'm good to go. Is there any need to replace the two (2) exhaust cover gaskets? If not, I'm satisfied that all is OK.

Thanks to all of you for the help.

Tom
 
kimcrwbr1

Thanks to you and all who have guided me through this issue. I sincerely appreciate the advice and the encouragement. I will follow your instructions

Sincerely,

Tom
 
To All:

Because of the water leak around some of the exhaust cover bolts, I pulled the exhaust cover and replaced the gaskets. While the cover was off, I could see a portion of the pistons through the exhaust ports. I noticed slight scoring on area about 1/8" wide along the skirt of the bottom piston. I closed every thing up and intend to use the engine as-is. Am I making a mistake or should I hone the cylinder and replace the piston and rings?

Thanks,

Tom
 
Mistake or conscious decision? There is a practical limit to how much the engine costs and is therefore worth. Honing will dress a cylinder up so that rings can wear in properly but is not a great way to really remove metal from a cylinder's walls to install oversize pistons if you can find them. How much do gaskets, piston, rings, etc cost and how much is your time worth? You could run the engine to destruction and spend the money it would cost to fix this one up on the next engine. If you enjoy working on engines as I do then you might strip the engine block bare and go at the cylinder, fixing the engine, and write the expense off to personal enjoyment and hobby. It would be wise to consider how much you will spend to get it running before you dig in and compare that to the cost of procuring another engine in your area that does not have these flaws. If you do button it up the way it is and go to the lake are you going to be really disappointed if the engine seizes up and leaves you stranded?
 
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