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1966 40 hp Electromatic Seahorse - Question about rusted out Heat Exchanger

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livetofish64

Contributing Member
Recently my son purchased a vintage 16' Orlando Clipper with a Johnson 40 hp (Model RKL-28). The compression seemed ok (105 psi & 95 psi). With this compression should I expect the boat to run slow? It seems to be responsive but it just can't seem to get over 15 miles per hour. I de-carbed the engine and cleaned the carb. I replaced the water pump and thermostat as well as replaced the throttle cable. The engine idles well and is responsive at full throttle but just seems like it wants to go faster but can't. I removed the port side exhaust panel and noticed the heat exchanger was rusted out near the bottom piston. Could this cause it to over heat and affect the top-end performance? I hope someone has some advice. Thanks
 
The heat exchanger would not cause overheat.---It certainly would allow a bit of exhaust to be drawn into the carburetor and that will certainly have an effect on power.----16' is fairly big for a motor like that.--What is the pitch of the prop ?
 
Thank you very much for your insight. I am not sure of the pitch but I will check it and respond back when I return from Chicago this weekend.
 
My Son's boat is not as streamlined as yours but it looks similar. The hull is in good condition and we had three people in the boat totaling about 550 lbs. The boat also had two batteries and an electric trolling motor, plus all our fishing equipment and two small anchors. We did try different trim angles which helped a bit but did not make a huge change. Maybe a good tune-up will help, changing the wires, points, condensor and coils would be a good idea?
 
Thank you for your reply and knowledge about the heat exchanger. I have a new one on order and did initially experience problems with the motor stalling from too much smoke generated inside the cowling. I was also running the oil mixture at 25:1 but found that this motor only needed 50:1. This help reduce the smoke dramatically. I will check the prop pitch when I get back from Chicago this weekend. What would be a good pitch to maximize speed?
 
Before you spend another penny I would be looking at the pistons and rings on both sides.----Should be able to see them with the exhaust cover off and with the bypass covers off on the other side.----Broken rings were fairly common given your compression readings.
 
I have the same motor on a 1963 Thompson Bros. 16.5 foot wood hull boat with full decks etc. weighing approx. 1500 lbs. I get a top speed of 25 MPH according to GPS with a stock prop. I would not run a 50:1 mixture in the engine as it will wear faster. I keep mine at 40:1 according to advice I found on this site and have experienced no problems so far. These old engines run beautifully with a little TLC and respect for their age.
 
This is awsome information from everyone. Thanks for the tips and I will let my Son know what to expect and the next steps. I did remove the cover exposing the pistons. No cracked rings but I did notice some scoring on the lower piston. I'm hoping I don't have to replace the pistons and rings.
 
That explains lower compression and possibly the reason for lack of power.----------These engines gave OMC a good reputation.-They were rugged , simple and easy to maintain.----------Much more backyard friendly than new motors today.
 
Kimcrwbr1, thank you for the information. To be safe I will tell my Son to start using the 40:1 ratio. I did however see a Johnson advertisment that stated this particular engine was rated at 50:1 do to the improved internal bearings. However, design changes can sometimes become a problem. We will give it a good tune-up for the next season and see what happens. Thank you again for sharing your experience.
 
I also feel the corroded heat exchanger was the cause of excessive heat resulting in the scoring of the lower piston (the point of corrosion). Do you know where I could purchase new pistons for this engine. Marineengine.com shows they are unavailable. Any ideas??
 
livetofish64 said:
I also feel the corroded heat exchanger was the cause of excessive heat resulting in the scoring of the lower piston (the point of corrosion). Do you know where I could purchase new pistons for this engine. Marineengine.com shows they are unavailable. Any ideas??
Click to expand...
Where do you live?
 
The heat exhanger warms up air for the " hot air " automatic choke and has very little to do with the piston scoring !----The air goes in the one open fitting and to the carburetor via that steel tube.-------------Lots of used parts for that motor available 2 hrs north of Port Huron.
 
I believe you and I am sure you are correct when you say the heat exchanger does not usually contribute to the condition of the piston but in this case it seems the slight scoring is directly in front of the rusted out area and it seems to me that the pressures generated might have forced rusty metal from the heat exchanger into this area of the piston. This is just speculation of course. It just seems very coinsidental and makes sense to me. Thank you again for the part supplier contacts. I may need to use them in the future.
 
Thanks again 426hemi. I am not sure I will need to purchase the pistons. What I thought was scoring might actually be carbon build-up. I am going to try using Sea-Foam since most everyone seems to like it. I have put some in the gas tank and run the engine in the driveway and it really started to smoke, buring up that carbon. I also installed the new heat exchanger but water started spraying out the top port where the elbow and tube come out that connects to the carb. Is this normal? I did not see water coming out before. Please advise.
 
The spraying water seemed to slow down as I tighted the nut conecting the heat exchanger to the outer cover plate, but water still leaks there a bit. I'm afraid to tighten it down too tight. Maybe in time it will stop on its own and maybe this is normal. Anyway the engine seems to run strong. The boat doesn't go as fast as I thought it should just because I am used to my 16 foot Boston Whaler with a 115 hp Merc (45 mph).
 
I reused the gasket attached to the block but changed the gasket on the exhaust cover. I guess I should have changed both. Do you think water is getting into the pistons?
 
Can anyone tell me where the high speed jet is located in the carb?? I may have missed this during the the breakdown and cleaning. Is there any adjustment to this jet?
 
Can someone help me with a question about the Magneto ignition for this motor (RKL-28) 40 hp. I discovered that after pulling the Flywheel that only one side had magenets inside it. Would this cause uneven compression (110 & 100 psi)? Would this condition affect timing? Would it further affect the performance at top speed? Finally, is it possible to purchase magnets for the Flywheel or do I have to purchase a new one?
 
Thanks for responding kimcrwb1. I did a spark test and the gap was less than 1/4 inch on the lower cylinder but they were both firing and passes the cylinder drop test. It just seems to me that the magnets on the flywheel need to be on both sides to create a strong magnetic field. So this doesn't have anything to do with timing or top end performance? Also the manual for this motor states that the compression values should not vary from the highest cylinder by more that 5 psi or something major is wrong with the engine and needs corrected before attempting to tune the engine. The prop seems to be the right pitch as it matches the one sold by marineengine.com. Anyway, I ordered a new flywheel from marineengine.com...maybe I should have waited to read your post. Oh well...if you want to lie to me and tell me it will improve the starting and overall performance I would feel better...lol.
 
Thank you very much for your experience. I really appreciate everything. I checked the resistance on the coils and got 3.3 ohms on the black coil and 4.0 ohms on the green coil. I looked on YouTube and saw a video showing that the coils should read 4.0 - 6.0 ohms. Is this true? If so I will replace both coils.
 
The armature plate has a some movement but doesn't feel sloppy. It turns at WOT but seems a little tight. I think I will take your advise and remove the plate and clean the area well and replace the ring and bracket anyway. I already have new condensor and points on order. If you think the ohms on the coils are too low I will get new ones as well. Please let me know.
 
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