1973 evinrude norseman 40hp problems

Jetstar73

Cadet
Joined
Oct 4, 2015
Messages
10
Hey everyone, I have been lurking this forum for quite sometime but still can't figure out what's wrong with my 1973 evinrude norseman 40hp outboard. The motor had been sitting dormant for approximately the last 14 years after the floor in my 73 Peterborough jet star had rotted away. Last year I finally had a new transom and floor put in the boat, although the guy who did it put some cracks right through the Fiberglass trying to get the old rusted screws out of the trim. Anyways, I put it in the water and couldn't get it to start after changing everything but the electrical. I then took it to a marine place down the road from the cottage where they replaced all the electrical components. Got it back and the motor started, although now if I revved at around half throttle or a bit more in neutral the motor would start running away and start hiccuping and wouldn't throttle down when the butterfly valve is closed. When it does this I turn the key off and it kills the motor. So other than that the motor seemed to run ok at full throttle for a while. About a week later I'm half way across the lake and a cylinder drops out on me. After narrowly making it back to the cottage I haul out the boat and take it back to the place who fixed the electrical. When I get it back I asked what was wrong and he says something about a loose ground connection or what not. So I put it back on the water for a bit and now when I get it up to 3/4 or higher on the throttle the motor starts shaking like crazy like its misfiring or something. The motor would seem to settle down a bit and smooth out after it warmed up for 20 minutes or so. Sometimes you would gun it and it would shake for a couple seconds and then smooth out again. Also, at times it would be running smooth until you took a sharp turn and it would start shaking again. I also noticed when you would slow down quickly and come off the plane it would make an awful growling sound that I guess I would describe as ventilation or cavitation? Despite my better judgement, I continued to run it this way for the remainder of the 2014 season. Fast forward to the beginning of the 2015 season and lo and behold she won't start again. Some mice did get under the hood in the off season but I didn't see obvious damage. I decided to just redo the magneto plate again with all brp components. At this time I did notice there was no continuity between the magneto plate and the power head which I thought was strange. I then noticed that there was a wire that was tie wrapped and connected to the power head that appeared that it was at one time secured to the mag plate for a good ground connection. I ended up reattaching this wire to the magneto plate to ensure a solid ground to complete the circuit. Finally got the boat back in the water near the end of this season and got it running but still had the runaway problem when revving a bit in neutral and still have the motor shaking at 3/4 throttle and beyond. Again, if the motor is allowed to warm up for at least 20 minutes I can get it to eventually smooth out a bit but obviously somethings not right. I believe the boat is heavier since it was redone which if anything would just put the prop lower in the water. I've considered everything from an misfiring problem to a spun prop to ventilation issues but have done nothing but confuse myself. I haven't actually marked the prop yet so that could still be on the table? I just find it strange all these problems happened at once so I figured it's all related? I also suspect the first time I got the motor back that the ground between the magneto plate and power head was connected and the second time they decided to leave it disconnected and that's what he was talking about when he said a loose ground. The fact the motor works slightly better when it warms up and gets rid of the misfire problems may also offer a clue to what's happening? Maybe the grease the mag plate is sitting on is insulating it a bit from the power head but as it warms up the grease thins out a bit allowing a better connection? I have no idea at this point. I take it a spun prop wouldn't cause intermittent bite problems, it would either bite or completely fail? I uploaded some videos on YouTube to help diagnose the problem and perhaps the members here who are far more experienced than myself will be able to definitively tell me what the problem relates to. I appreciate any help you can give me, thanks!
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,051
What pitch prop are you running on the JETSTAR.------Have you looked at using a timing light to observe ignition behavior at full throttle ?----And is the " cut out switch " installed and working as designed ??
 
Last edited:

F_R

Supreme Mariner
Joined
Jul 7, 2006
Messages
28,195
No Title

I also highly suspect the vacuum cut-out switch is missing or defeated. It is supposed to be operational only at slow throttle settings. At fast throttle settings, the safety switch disconnects it. Trouble is, people think they don't need all that "stuff" and rewire it or toss it. If the vacuum cut-out does not disconnect at fast throttle settings, it can cause a high speed miss.

As for the magneto armature plate not grounding, that year had an improved mounting system, whereby the plate is mounted on wear-resistant but non-conductive parts. Thus the ground wire. It must be connected, or there is no spark voltage return to the coils. That will make it miss.

I am attaching a 40hp wiring diagram. Yeah, I know, it is the wrong year. What I want you to do is study the wiring for the vacuum cut-out switch and safety switch. If yours is not correct, fix it.

EDIT: Looking at the diagram, it may be a bit misleading. The two white wires connect to one corner of the square metal switch body---not to the plastic mounting plate

EDIT EDIT: The purpose of the vacuum cut-out switch is to prevent run-away at fast idle or above in neutral, as you are experiencing. That is the reason we suspect yours is missing or disconnected--or something.
 

Attachments

  • photo234594.jpg
    photo234594.jpg
    231.1 KB · Views: 4
Last edited:
G

Guest

Guest
Jetstar;

There is also something you need to be aware of regarding that 1973 model year 40hp.

I had a 1973 Johnson 40 that I bought new.

When this particular design came out in 1972, they used a simple O-Ring for the crankshaft/drive shaft spline that connects the powerhead to the driveshaft in the leg of the motor. This proved inadequate and resulted in saltwater getting in the spline and corroding it enough so the spline is weakened and fails.

The end result is the the crankshaft "spinning" around the drive shaft due to the spline being destroyed over a short period of time, resulting in either a new crankshaft replacement, drive shaft replacement or both.

This might be your "bite" problem.

OMC quietly revised the crankshaft/ driveshaft spline seal design in 1974, to alleviate this flaw. (This is why you always find old big twin 40's like this that are 1974 and later, or 1971 and earlier models. This problem was only inherent on the 1972 and 1973 40's.

OMC had to redesign the bottom of the block casting to accept the new seal in 1974, to my knowledge, there was no "recall" on the 72's and 73's

I ended up replacing the crank in my 1973 40 once (in 1975) and the driveshaft (in 1977). So the spline went bad twice on a 4 year old motor.

There is no bigger fan of OMC products than me, but this was one ground ball that OMC missed, but they fixed it the following model year.
 
G

Guest

Guest
After seeing the video, I noticed your powerhead shaking erratically under full power......Thats what my 73 Johnson 40 did when the spline was shot.
 

Jetstar73

Cadet
Joined
Oct 4, 2015
Messages
10
Thanks CaptErich, that is a possibility I didn't even consider. Not sure if it explains the runaway condition in neutral though...especially with no load for slippage on the motor.

F_R I tore apart the vacuum cutout switch and it seemed to be functioning ok. Also, the wiring is correct and I can't see anyone messing with that wiring anyways at the shop? It's good to know that this motor does in fact require a ground wire from the plate to the power head.

Racerone, I intended to use a timing light or at least the spark plug testers that light up but never got the opportunity before the end of the season. This will be the first thing I do come next spring/summer. If the throttle cam was out of sync could that also cause the motor to shake/lose bite at higher rpms??
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,051
Observation of the timing light might pinpoint which cylinder has a miss.------If that was the case the repairs would be easier to figure out !
 
Top