'73 Johnson 115 Loss Of Power

[RLMarz]

She's old but she still runs...most of the time... She sucks at cold starting so it was a bit of a pain as usual on Saturday. Ran fine on the muffs. Started up fine at the ramp, and idled out. Easily got on plane and ran 2/3 to WOT for 10 minutes or so, until I felt a loss of power and started to come off plane. Brought her to idle and she quit immediately. Immediately started and throttled in neutral, then put her in gear and ran for another 5 minutes or so. This time when I started to lose power, instead of bringing her to idle I held her at WOT. Eventually quit, and when I tried to restart she would quit anytime I put her in gear, and eventually wouldn't start at all. She would turn over fine, just simply wouldn't kick. Hello TowBoatUS...

New plugs as of a week or two ago, water separator had no visible water when emptied, fresh fuel with 50:1 mixture and Sta-Bil, new fuel lines, carbs rebuilt last year, brand new coils and stator last year. This did happen once last season, however it was corrected with the first course of action; back to idle and restart.

Any ideas at all? I'll be going down today (Unfortunately in the driveway) and running over the systems before trying to start her again. One would think ignition (She heats up and quits) but it's a gradual loss of power, not immediate, and I'm usually able to immediately restart her. Much appreciated.

 

[BonairII]

Start with the basics. Check compression first....then check spark on all cyls with an adjustable spark tester. Probably going to end up being a bad coil(s).

If you have spark and good compression....then check to make sure you're getting fuel.

 

[racerone]

Agreed , a compression test is the first thing to do.

 

[emdsapmgr]

Check to be sure the engine is not overheating. Use a laser temp gun to verify the cyl head temps when the engine starts to slow. The vernatherm (thermostat) on that engine should open between 115 and 130 degrees. I'd think it should run maybe up to 140 after idling for a while in a warm lake. Cooler when the engine is running at speed-on plane.

 

[RLMarz]

I discovered two bad plugs (one reoccurring) with bend electrodes. Need to check the piston visually to make sure I don't see any mechanical evidence, but carbon buildup could explain it. Does it sound plausible that losing one plug the first time and bogged and the second the second time explained the loss of power and eventually inability to restart?

 

[RLMarz]

One piston had a decent amount of carbon buildup on it explaining the piston strike. The opposing cylinder however was clean but still had piston strike. I am going to proceed to pull the heads to see if anything is loose in there causing the piston strike on the non-carboned cylinder. If not well, too much time and money to be worth fixing.