1975 70hp Johnson losing power at WOT

[70smithy]

I have a 1975 70hp Johnson on a 16' Starcraft American BR.

I have had several issues with this motor that I have worked through as needed but came upon a new one this past Sunday.

At WOT the motor will lose power and slow down on its own. It doesn't stall and when I put it back in neutral and idle for 20-30 seconds, I can throttle up normally with no problem. Primer bulb hard the whole time. In general, I have had trouble with wet plugs and a skipping idle, but this is a whole different ballgame.

This seems to have started after a particularly large wake from another boat sent me airborne earlier in the day. Coincidence?

 

[oldcatamount]

Re: 1975 70hp Johnson losing power at WOT

Sounds like it's a bad ignition coil to me. Old coils will break down when they get warmed up. Trouble is, which one is going bad? You say you've had wet plugs and poor idle as well so, that's pointing more and more to the coils getting weak. Testing a coil (with a muti tester) is misleading at best so, it's difficult to pinpoint the problem but, if it were my motor, I would replace the ignition coils.

 

[Daviet]

Re: 1975 70hp Johnson losing power at WOT

Easy to check the coils, use an inductive timing light and check eack coil and see if you loose spark on any of them. Have you checked the high speed orifices for restrictions?

 

[70smithy]

Re: 1975 70hp Johnson losing power at WOT

Old Catamount, I just replaced all three coils, as one had a hole in it and another was cracked. That being said, I did not like the "style" of the new boots and plug wires, as they had to be assembled. I think I need to thoroughly check the ignition system for proper function. What is a good way to truly test for spark, other than the tried and true "hold it near the block" method?

Daviet, do you have any sort of guidance you can offer on the proper way to check the coils with a timing light?

Also, I did a "quickie" carb tear down and everything seemed crystal clear, but I am going to go back in this weekend and REALLY clean and rebuild them.

 

[Daviet]

Re: 1975 70hp Johnson losing power at WOT

An inductive timing light is the type that clamps over the plug wire, no need to remove the plug wire from the plug. Have someone run the boat for you and when the engine acts up start checking each plug wire for spark, the timing light will light up each time the plug wire passes a spark. Check all plug wires for spark, if they are all sparking steadily, then the ign system should be good, move on to the fuel system.

 

[70smithy]

Re: 1975 70hp Johnson losing power at WOT

Ok, an update. Tested the spark, first with a borrowed timing light and again with a cheap Harbor Freight inline tester (all I could find locally) and got spark on all cylinders. Wasn't an adjustable tester so I am not sure how strong. Moved on to thorough carb cleaning. While I was doing that, my wife found that the rectifier had a wire that was badly corroded. Since we had purchased a "just in case" spare, she changed that out while I finished up the carbs.

Next day, took it on the water and decarbed with seafoam. Swapped the plugs for fresh ones and boat shot out of the gate but soon slowed down. Not the complete loss of power as before though. Also, idle got rough again. Noticed as I was adjusting idle to stay running that the flywheel was real hot, almost too hot to touch(?). Took it home, frustrated. While flushing motor, noticed that spark was leaking around boots like crazy. Pulled boots and cleaned insulators with achohol and it helped, but not a lot. Bought new boots, that problem was solved. Now for the ignition system testing...

All I could secure to do the tests was a multimeter with peak hold. I had read about a substitute for measuring DVA with a peak voltage meter was to take a reading of AC volts x 1.41, so that's what I used to get the following numbers: (all of the resistance numbers checked out so they are not included)

Stator output - 110 V
Timer base (to ground) - 165 V
Power pack output to ignition coils - #1 - 66v, #2 - 65v, #3 - 63v

The last numbers were the toughest to test as I had to crank the motor with the coil wire disconnected in order to test the output so the motor was chugging on two cylinders.

Any feedback on my results would be great. I am trying real hard to get to the bottom of this.