1976 70hp Evinrude Missing at high RPM...among other things.

[Ken_R]

Hey everyone,
My son and I spent the last year resurrecting an 18' fiberglass boat and have finally gotten around to the engine. As far as I can tell it has been run once for a few minutes since 2004.
So far I've replaced the fuel tank, fuel supply line and fittings. Rebuilt the fuel pump and replaced the fuel lines and vacuum line. There was one problem where the engine would start out WOT but quickly bog down to where I could barely keep the engine running. I could make about 5 kts back to the pier with the engine fully choked. I went in today and pulled all three carbs and sure enough two of the float bowls were full of dark brown chunks that looked like bark. I was able to blow everything clear with carb cleaner and compressed air (keyboard cleaner since I don't have a compressor.)
So the newest problem...I got it back up to top speed then it started bogging down again and stalled. Once I got it running again, it sounded strong but once I tried to go to WOT it started vibrating and rumbling. Sounds like one of the cylinders isn't firing. It almost sounded like the carbs blew clear but now its missing. On previous outings it bogged down before the engine really warmed up so I don't know if the cylinder was missing before or not.
I checked for a spark when I checked engine compression earlier. On two I was able to get a spark through the plug wire insulation but not the third. I pulled all three plugs, one at a time, and all three plugs were sparking. I don't know how to check the strength of the sparks though, I could use some help there.

BTW all three compressions were good, 120+/-2 psi. I'm running 87 octane unleaded mixed 50:1. I've added Sta-bil for the ethanol and Star-Brite to help clean the system.

Another issue I noticed today is that the voltage meter when the engine is running barely gets above 12 volts.

Obviously I have some electrical problems now and would greatly appreciate some help on where to start. Is there a way to check coil assemblies to see if they are failing, maybe a resistance check across the transformer? Would a failing coil cause the drop in charging voltage or do I need to look at a new stator assembly? The middle spark plug wire seems very loose in the transformer, I can twist it around like in a socket rather than a soldered connection. I have no idea if that is normal or not but the other two don't do that. I've also noticed a mustard colored powder on the outside of all three transformers but do not smell anything strange.

Thanks for your time,
Ken and Donovan.

 

[tbull]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

You did go ahead and rebuild those carbs right? And not just blow out with compressed air?

 

[Ken_R]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

I didn't rebuild them yet. I pulled each, and cleaned the bowls out and let them soak in cleaner for a while (20-ish minutes). They looked like they had been pretty recently rebuilt since all the gaskets were good and the internals appeared clean. I ran cleaner through all the orifices though and showed good flow everywhere. Honestly though I've never done any carb work before so unless it was obviously gouged I wouldn't know the difference. Is there a way to check the floats though? They appeared solid but I'm not sure if they are hollow floats or solid rubber.

R/
Ken

 

[Ken_R]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

I just did some quick research on the carb float and see that my next step is definitely to rebuild the carbs. The side effects from a leaking float sound exactly like some of the issues we've been having, no acceleration, bogging down in a turn or after hitting a wake, etc.

Will that cause it to misfire on a cylinder though?

I'm also still wondering about that low charging voltage...

Thanks for help so far, I'll let you know how the carb rebuild goes.

R/
Ken.

 

[tbull]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

Rebuilding the carbs is very easy and relatively cheap, even if you have no experience. Best to go ahead and get a manual for your engine also, you cant really tell the condition of a carb by looking down in the bowls. Its the small Jets and orifices that get gummed up, this site has a great step by step on how to do it, a manual will as well. I would not run that engine again until you replace the floats with a carb kit, make sure the carb kit you get comes with floats, or purchase them seperatley. Those floats getting stuck can cause you all kinds of problems. As far as the back fire, is it sneezing or backfiring? I am no expert just learning along the way, but you may be running to rich due to the condition of the carbs causing that side effect.

 

[tbull]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

I believe your engine has a rectfier, non-regulated charging system, may want to run a test on the rectifier if your engine has one. There is also a section on here by Joe Reeves on testing a rectifier. May be the reason for your low charging voltage. Have you checked the voltage at your battery with the engine off and the engine running to see if its a higher reading?

 

[Ken_R]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

It definitely has a rectifier, I'll check out the test and see what I find. I have the shop manual already but haven't gone through the troubleshooting steps yet since I'm not sure if it's electrical or fuel related.

It's not that the engine is backfiring. When I rev up above about 1/4 power the engine sound gets very deep (like its in low gear if it had one), it starts to vibrate excessively and makes a coughing noise. Before I cleaned it out, the only way to keep it running was to hold the choke down and then I made about 5kts to the pier. Now it will only run in low throttle. Sounds like the rebuild and float replacement will fix it though.


I have an indash voltage meter connected to the switch panel bus so I can read whichever battery is connected. With the Engine off its at 12 volts and with the engine running it only appears to go up a little bit. I'm planning to clean all of the electrical contacts and replace any bad wires. I'm hoping it's just a series of bad connections bringing the voltage down. The rectifier test should help narrow it down.

Thanks for the help again,
Ken

 

[tbull]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

I never trust my dash voltage meter, for some reason its never right. I usually rely on my depth finder to give me an accurate reading for voltage, if you have one use that and see if it shows any different. After I replaced my rectifier, it was very easy to monitor the charge on my batteries by watching the voltage gauge on the depth finder.

 

[Ken_R]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

I checked the rectifier and it's blown or getting there fast. The three wires read 172, 174 and 363 Ohms in one direction and 0 Ohms the other direction.

 

[tbull]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

Unless I am loosing my mind or something, I could have sworn that a reading in one direction and no reading in the other direction would indicate good diodes inside the rectifier. I may have to look at Joe Reeves testing procedure again, or in my manual. I am a little confused here because it sounds like your rectifier is fine..I may be mistaken though???

 

[tbull]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

Here is the article, I am not sure about the OHM's readings, but getting your reading in one direction and not the other means it is working, see below. You do have the small 3 cable Rectifier correct?

Small Rectifier Test, by Joe Reeves

--------------------------------------------------------------------------------

Perhaps your engine does not have the water cooled regulator/rectifer but rather incorporates the smaller 3 wire rectifier which sits just forward of a vertical electrical strip on the starboard side of the engine.

If so, a failed rectifier would knock out the tachometer. See the following.



Remove the rectifier wires from the terminal block. Using a ohm meter, connect the black lead of the ohm meter to the rectifier base (ground), then one by one, connect the red lead of the ohm meter to the yellow, yellow/gray, then the red wire (some rectifiers may also have a fourth yellow/blue wire. If so connect to that also). Now, reverse the ohm meter leads and check those same wires again. You should get a reading in one direction, and none at all in the other direction.

Now, connect the black lead of the ohm meter to the red wire. One by one, connect the red lead of the ohm meter to the yellow, yellow/gray, and if present, the yellow/blue wire. Then reverse the leads, checking the wires again. Once more, you should get a reading in one direction and none in the other.

Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires.

Any deviation from the "Reading", "No Reading" as above indicates a faulty rectifier. Note that a rectifier will not tolerate reverse polarity. Simply touching the battery with the cables in the reverse order or hooking up a battery charger backwards will blow the diodes in the rectifier assy immediately.

 

[Ken_R]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

I'm not sure on this one either. I used the procedure in the shop manual which is about the same as this one except it says you should see infinite in one direction and zero in the other. Any reading other than that indicates a bad rectifier.

Either way, the DC(+) wire was missing insulation in two places and the wire is brittle with several broken strands visible. It won't hurt in the long run.

I'm trying something else though first, I caught another idea from a couple other posts. Several people around the web have recommended replacing the rectifier with a full wave bridge rectifier from Radio Shack. Turns out I can get essentially the same thing for $3.29 instead of $38.00.

The radio shack one is rated for 50amps and 200V, my only concern is if the stator will produce enough voltage to excite a rectifier rated for 200V.

 

[Ken_R]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

Rebuild complete. It wasn't too tough over all. I got it back out on the water and it ran great. Twice though, after throttling back up from neutral it stalled. Once I let it run for about 30 seconds, I throttled up slow to WOT and it ran fine again. Once I had it running I could throttle down and up again with no problems. Any idea what could have caused something like that?


BTW, the rectifier from Radio Shack works like a champ and I saved $35. I'm seeing 13V now so at least that one looks fixed for now. I think the missing earlier might have been caused by the fact that it was running off the battery and the battery was losing voltage.

 

[Ken_R]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

Here's a side view of the Radio Shack rectifier installed. Red wires are the DC and Yellow are the AC. DC(-) grounds to the engine.

 

[Ken_R]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

Here's a shot from the top looking down so you can see the blade connections and side markings.

 

[Ken_R]

Re: 1976 70hp Evinrude Missing at high RPM...among other things.

To finish this thread out for everyone. I started by saying it was missing. Turns out that was just a side effect of the bad rectifier. As the battery started dying the plug with the most resistance lost spark and made it feel like it was missing.