motor runs great for 1/2 hour, then loses power

[huntandhook]

I'm having some problems with my 1979 40HP Johnson outboard model # 40R79E. The motor starts very well, idles and runs excellent for about 1/2 hour, then it loses power and will only run at low throttle. When I open up the throttle more than about 1/4, the motor loses power and stalls. I checked the breaker points and they are clean and properly gapped. I have the correct sparkplugs and proper plug gap. The fuel pump is also working fine. I also disconnected all safety switches and the runaway cutout switch and this has not solved the problem. The motor has externally mounted ignition coils, could one of these be the cause of the problem? I don't know what else to check. I don't understand why the motor initially runs as good as it ever has, and then starts to run terrible after a while. If anyone has some advice for me on what could be causing this problem, it would be greatly appreciated. <br /><br />Thanks for the help.<br />Phaedo

 

[Walker]

Re: motor runs great for 1/2 hour, then loses power

It very well could be a coil. You need to do a spark test while its acting up.

 

[kevin88]

Re: motor runs great for 1/2 hour, then loses power

The stator could also be leaking to ground when the engine warms up. This will prevent the motor from running above idle but won't affect the idle speed.<br /><br />Do a search for stator tests in this forum to see how to test yours.

 

[huntandhook]

Re: motor runs great for 1/2 hour, then loses power

Thanks for the advice. I don't think the motor has a stator, but there are some coils under the flywheel (I think they are for charging). Is there any way to test these?

 

[Walker]

Re: motor runs great for 1/2 hour, then loses power

First you need to find out if you are indeed losing spark to one cylinder when it acts up.<br />And yes, your motor has a stator. It is incorporated in with the charge coils unter the flywheel. 2 of those coils run the ignition.<br />Are any of the coils cracked and leaking?

 

[huntandhook]

Re: motor runs great for 1/2 hour, then loses power

I didn't notice any cracks or leaks on any of the coils under the flywheel. I'll take the boat out tomorrow and test the spark when its acting up as you suggested. Do I need a special tool to check the spark, or can I just pull the plug and see if a spark jumps when I crank the motor? Thanks for the help.

 

[kevin88]

Re: motor runs great for 1/2 hour, then loses power

It's my understanding from reading some other posts on this site that 1 coil under the flywheel runs the ignition system (Power pack, ignition coils, etc.) while at an idle. But when the motor RPM's are increased some energy is used from the charging coils to power the ignition. This is why I suggested that your charging system may be acting up when the engine warms.

 

[huntandhook]

Re: motor runs great for 1/2 hour, then loses power

Upon closer inspection of the exterior ignition coils I noticed that one of them was cracked and there was a dried up dark reside along the crack. I guess that means the coil is no good even though it sparked well when the motor was cold. I'm going to replace the coil and see if the motor quits acting up. If this fails to solve the problem I'll check the charging coils as kevin88 suggested. Unforunately I have discovered a new problem, but I'll save it for a new post.<br />Thanks for all the help guys.