1975 Evinrude 70hp diagnostics

[skinners1]

Recently purchased 1975 70hp Evinrude with problems... Previous owner couldn't give me alot of details as I suspect he had just purchased and could not solve the problem so was selling.

This motor starts immediately and runs great on the muffs. Once under water it is a different story. No tach installed so i am working strictly by ear. I can get it to start and idle using the choke and warmup lever. Once we start to throttle though she dies out.

Diagnostics so far:

Compression on all 3 cylinders, 120, 120, 120

Spark good on all 3 cylinders (tested with tool, and the old fashioned way, believe me, they have spark!)

Carbs have been pulled and are as clean as if they were just installed (everything looks new), high speed jets are clearly open, leads me to believe that they were rebuilt recently. I have yet to pull the high speed jets though, haven't found the right tool.

New tank and line with bulb that does seem to hold pressure

Fuel pump appears to be fine, using the squeeze test with bulb, fuel flows through efficiently

Bottom plug appears to be clean as the other 2 are slightly discolored, leads me to believe that the bottom cylinder may not be getting fuel/oil? I plan to pull the fuel line that feeds the bottom carb from the middle carb tonight to inspect.

Any other thoughts or ideas.

 

[jtexas]

Re: 1975 Evinrude 70hp diagnostics

You may have the idle speed set too low. should be about 850-900 in gear in the water. a cheap yet useful inductive digital tach for diagnostic purposes: www.tinytach.com

follow the "carb cleaning and rebuilding" link from the "top secret file" at the top of this forum for instructions on fabricating the jet removal tool.

I bet you already made sure the throttles are parallel, and choke plates stay open except when the choke is engaged.

that's exactly what I would do.

 

[Jet Wrench]

Re: 1975 Evinrude 70hp diagnostics

There is no idle adjust on the carbs for a 75 Rude. More than likely, you have some crap in the jet. Pull the carbs and clean the jet with a piece of stainless .024 safety wire.

Jet Wrench

 

[Jet Wrench]

Re: 1975 Evinrude 70hp diagnostics

If the bulb is not holding pressure, you need to check all of the lines starting at the tank to the engine connection. engine connection to the fuel pump, fuel pump to the carb fuel "T" and the "T' to the carbs. I had a similar issue with mine, I was losing the prime at the carb "T" causing erratic idling and stalling. Replace any questionable lines! The system needs to be air tight in order for the fuel pump and carb floats to work correctly.

Jet Wrench

 

[skinners1]

Re: 1975 Evinrude 70hp diagnostics

Pulled the bottom carb last night and inspected good. Needle was sticking, pulled and cleaned thoroughly with cleaner and appears to be moving freely now. Did the "blowing air" test when upside down and right side up. I still need to pull the jets and clean. I plan to go ahead and pull the other 2 carbs and check, and also test all fuel lines and "T" around the carbs.

Could this have been the problem?

 

[jtexas]

Re: 1975 Evinrude 70hp diagnostics

Float sticks closed, that carb will run dry. This motor will idle surprisingly well on 2 cylinders, especially on the muffs. You might not even be able to detect it if you aren't accustomed to it.

Carb kits, about $12 apiece and as you know by now, removal & reinstallation is about 80% of the work involved in a rebuild...I'm just saying...

You really need a manual for the link & sync step-by-step, but this should get you going in the meantime: idle set screw is identified below (it's a later model than yours but the important parts are the same) -- shifting at over 1000 RPM is not recommended -- without a tach, you'll have to balance between too low to run, and too high to shift. Carb throttle valves are closed at idle, and idle speed is controlled entirely with timing advance. Use the black thumbwheel to adjust cable length if necessary, to get the throttle lever butted up against the idle set screw.

Notice the mark on the throttle cam -- it needs to hit that roller right on the mark. The roller can be adjusted if necessary.

Use the WOT stop screw to make the throttle valves horizontal (not beyond) at full throttle.

You didn't ask, but I'll throw this out there anyway:
Water pump impeller should be changed every couple years and gear oil should be changed annually. That's the only maintenance I do on a schedule, other than inspecting the spark plugs and fuel lines couple times a year.