Johnson 40 HP - Loses Power When Warm

[Bay Boater]

I have a Johnson 40 HP - late 60's or early 70's. Sorry, do not have the number just now but can get it if needed.<br /><br />The motor runs smooth and strong when first started cold - will run good for perhaps 15 to 30 minutes. Will then bog down, run a bit rough, and lose power as if I had pulled the throttle back to 1/2 or 1/3. If I go ahead and then pull the throttle back, it will continue to run ok but will only putt along. Still starts ok too, just no power.<br /><br />After it has cooled off, will go thorugh the same sequence again - run strong at first then bog down. I have confirmed when it is acting up and the lever is full forward, the carburetor throttle valve is full open.<br /><br />Have also been through the carburetor. Seems clean and tight. Fuel pump seems good also - have been through it. Have also tried bypassing the vacuum switch with no effect. Also tried pumping the fuel line bulb while acting up - also no effect. Adjusting main jet setting has no effect.<br /><br />I'm wondering if maybe one or both condensers (capacitors) in the magneto are bad and act up only when warm. Would it make sense they would run ok at low speed but act up at high speed? I don't have a puller so have not had the flywheel off yet.<br /><br />What else could cause the motor to act like this? If you need more detail on model no. etc. let me know.<br /><br />Thanks,<br /><br />Bill

 

[MadeCoolCom]

Re: Johnson 40 HP - Loses Power When Warm

Hi bill you said it ran bad after getting warm. Can you please tell us that you checked the cooling system on this engine and it 's working like it should. Then we know that's not the problem because that would be first thing to look at if you think the temperature is causing it not to run good are seen to suggest to us that the temperature had something to do whit it not running good by telling us that it's getting warm and runs sluggish when does get warm.

 

[OBJ]

Re: Johnson 40 HP - Loses Power When Warm

Bill....What MCC said. And do post the model number.

 

[Bay Boater]

Re: Johnson 40 HP - Loses Power When Warm

As best I can tell, the cooling system is ok. I ran it in a trash can before putting the boat in and the exhaust spit out a hot water spray after a minute or so. The thermostat housing got warm ok. Also, after running for its 20 or 30 minutes and then acting up, when I pulled the cover off to look at it, nothing seemed to be cooking. No discolored paint and nothing was too hot to touch (although some things could only be touched for a second or so, but no burns). On top of that, when it loses power, it still putts along for extended periods with no apparent difficulty other than the power loss.<br /><br />I thought about the cooling system and was worried about seizing something up, but I think if the power loss was something like a hot piston dragging down against the cylinder, it wouldn't run for much longer. Also, when I have shut it down during the problem and restarted, it spins up ok. By the way, this is a pull start only motor and if something were dragging down I think I'd notice - but my experience here is limited. Hot or cold, running right or wrong, I'm usually getting one-pull starts.<br /><br />Other than checking for hot water spray out the exhaust and making sure nothing is baking the paint on the motor, what other check should I run on the cooling system to be sure?<br /><br />When it loses power after warming up, if the speed control is full forward, it runs noticeably rough. When I pull back on the control and let it putt along, it smooths out. Advancing the control roughens it again.<br /><br />The boat's anchored out in the bay just now - will get motor model number etc. tomorrow and post it then.<br /><br />Thanks to all.

 

[Bay Boater]

Re: Johnson 40 HP - Loses Power When Warm

OK - The model number is RDS22. Serial No. 1997613. Hope this tells you something helpful.<br /><br />I ran it back around to the ramp today - have it in the back yard where I can look at it better. It's about a four and a half mile run to the ramp. Motor ran kind of ok for three and a half, but a little rough and after the first mile or so. Not full power but enough to plane (15 foot fiberglass runabout). The last mile all I could do was putt along - motor ran steady but obviously missing - kind of a random stutter. Pulled the cover at the end of the run and nothing seemed overheated. Could easily touch every part of the motor without getting burned but it was plenty warm to the touch. Seemed normal.<br /><br />OK guys. I'm pretty puzzled. The only thing I can think to do is go exploring in the magneto and just change out the condensers on principle. The ones in there now are only about four years old. So are the points. My previous motor was a 1958 35 HP Evinrude Lark and the condensers and points lasted 12 years and were still ok when I gave it away. If there is a condenser problem, will I see any telltale signs in the points?? Any other obscure thing I'm overlooking?<br /><br />Thanks for any advice (including scuttling it - I may disagree but I'll understand).

 

[Paul Moir]

Re: Johnson 40 HP - Loses Power When Warm

I think you'll find that the magneto is where your problem is. Since you seem to be suddenly loosing 1/2+ throttle, it sounds an awful lot like your dropping a cylinder. If the magneto components (particuallairly the ignition coils) are original, they're shot. Ones of that era end up cracked and arc over or absorb moisture causing all sorts of weird spark problems, usually above starting rpms.<br />When you pull the flywheel, you'll find there's very little difference between your 40 and 35. Your 40 is a 1960 engine, by the way.<br />You can test for a dropping cylinder. When it starts acting badly, pull each of your sparkplug leads. If pulling one causes the engine to die, you know that that cylinder was firing.<br />Condensors are rarely bad, but normally get changed out anyway. All the ignition parts are cheap and avalable from NAPA autoparts from their marine catalog. Your looking for a 18-5006 (points & condensors) and two 18-5181 (coils). I'd bet real money your sparkplug wires are still good. They're not like automotive ones.<br />Good luck!

 

[Solittle]

Re: Johnson 40 HP - Loses Power When Warm

Replace the impeller and t-stats to be on the safe side.

 

[Bay Boater]

Re: Johnson 40 HP - Loses Power When Warm

Problem solved. Made the 4 1/2 mile run back from the ramp and later ran a couple miles each way up and down the bay. The motor did nothing but run. Was able to hold an easy plane with the speed control pulled back off WOT and even down a few rpms from that.<br /><br />As Paul supposed, the problem was in the magneto, one cylinder dropping out. But it was not the coil. It was the condenser after all but not in a way any of us ever guessed.<br /><br />One of the condenser cans became separated from the little tab-bracket it's screwed down by. There are a few spot welds there that broke. When the can happened to be touching the bracket or magneto plate and was grounding out, the motor ran OK. When it got shaken around or whatever and did not make good contact, that cylinder started dropping out. I can only guess something about the temperature or run time had something to do with how well it made contact, doing ok at first then losing it.<br /><br />The other condenser had spot welds on its bracket but was also brazed on. So were the two condensers in the NAPA tune up kit I bought (thanks, Paul, for the reference). So I wound up saving about $50 for the two coils, although I have a question about them.<br /><br />They are the same make as this condenser with the non-brazed bracket. All I can see here is a cost-cutting move (eliminating the braze operation) which really kills the quality of the part. I'm wondering if the coils are equally cheap and will need replacing soon.<br /><br />Both the coils and the bad condenser are marked with a slanted "M" which is surrounded by a slanted, slightly rounded box. Anyone have any experience with this brand (whatever it is)? Should I keep watch on these relatively new coils? Or is my bad experience just a fluke?

 

[DJ]

Re: Johnson 40 HP - Loses Power When Warm

BB,<br /><br />That trademark you describe sounds like the old OMC logo. Those would be original, or original equipment manufacture, replacements. If that's the case, those are as good as you're going to get.<br /><br />Congrat's on your diagnosis. Great job!