Thread: 1970's Johnson 25 horse losing power

Sure sounds like you are on the right track. A fuel starved 2 cycle engine is not good. When it runs out of fuel it leans out and can cause damage to cylinder or piston. That applies to any 2 stroke that runs on mixture, chain saws and weed whips also. If oil injected maybe not a problem. The 2 Stihl dealers near me say that 90% of chain saws that have cylinder damage have been run out of fuel.

Greg has a good point...I like my 2 strokes to smoke a little...no matter what they are. A little smoke means plenty of oil in my book!

Good point, I thought that too much oil might foul the plug. What is the proper fuel:oil mixture for a 70's motor? 50:1? 32:1?, 16:1? I am running 50:1. Please give me some input.

50:1...yes to much oil will cause plugs to foul...it will also cause engines to coke up (get carbon build up on rings and piston tops). BUT...its cheaper to run a little seafoam and change a plug a little more often than worry about enough oil or possible be little lean on oil. Thats why I like a little smoke at idle.

More so than too much oil...the majority of boats are propped with to high of a pitch and prop care is not taken seriously enough. Remember an outboard doesn't have a transmission to ease through gears to get to high. You are in essence starting in OD or 5th gear and getting up to speed. All engines have a rpm range that the manufacturer recommends. The easiest way to check this is to have your avg load in your boat and run it to WOT. Where ever that RPM is at is where you are in that range. I would recommend with an avg load you be in the middle to upper middle of that range. That way when you have your buddy and his or her stuff in the boat you'll still be in that range. If you drop below that range you are lugging the engine. Whether it sounds like it is lugging or not...its still lugging and slowly building up carbon and shortening the life of your plugs.

Scrapper - Thank you kindly for your confirmation on 50:1 I hope to have time maybe on Sunday to launch the boat and put it thru its paces close to the launch ramp. I may take a picture of the richness setting and see what you think about it.

I personally run 40;1 in my 60's Evinrude 6hp and Eska 7 1/2hp. It smokes a little but not excessive. Yes, too much oil in your mixture can and will foul your plugs and make idling rough. When mixing your fuel and oil, take great care in measuring your oil. My chainsaw and string trimmer run on 50:1. Buy the best oil you can find to mix with your gas. I think it money well spent.

Thank you

I just saw your thread. You've been given good advice about the grey lines. Take off the connector and see if you can pump the primer and get plenty of fuel out. The liner of the hose will come loose and block the lines. If you're getting plenty of fuel to the carb---take the fuel line loose from the carb and pull over the engine or turn it over with the electric start if it has it. You should see fuel pulsing out of the hose. If not, blocked line or bad fuel pump. If you have fuel pulsing and your carb is clean, then you may have a coil getting warm and cutting spark to one cylinder. What year motor is it? There's a big difference in ignition systems from early 70's to mid-70's to late 70's. An overheat can also commonly cause a power loss. The next time it does this, sprinkle some water on the cylinder head----it should not boil!
Just some suggestions,
JBJ