Last year, when I restored my boat and engine, I got an NOS 1960 Evinrude 40 powerhead for my 1959 Lark engine. (I also got a 1960 Lark hood to make it look correct).Anyway, I had a problem last season with the motor cutting out at the very end of the throttle run. Just at the last 1/4 inch, the engine would lose power.Went over the carb (tried several) and ignition many times, all is according to spec.Just ocurred to me that the engine has a switch that senses vacuum and shorts one of the two coils as a precaution if, for example, the prop comes out of the water and there is a potential for over-revving. Remember that this is magneto ignition, so there are no distributor electronics to monitor, so this vacuum cut-out was probably as sophistocated as the engineers could get.Could this be the cause of my high speed cut-out? Could the vacuum diaphram fail or weaken? It probably has the original 1959 daiphram. Could the 35 hp switch be incorrect for the 40 hp powerhead?Would be nice to try to trouble shoot this now, in the off season...
Darn rights it could cause your problem. Easy to prove, just disconnect wires from it and retest engine at WOT. There are other issues like point gap, point spring tension, condenser value etc. that could affect it as well.....
J.B.,Right now, I'm on vacation in Florida. My Clymer's J/E 1956-72 Manual and 1959 and 1960 Evinrude dealer's manuals are back in Virginia. Will be back home in 10 days.Do you know of cases where the switch actuates prematurely? Is this type of switch even used in newer engines?
The diaphram acts against a spring. The spring can rust out and lose its tension over time. This would allow earlier governing........as i say, disconnect the wires going to it and retest. If problem solved, pull it apart and inspect....Good luck!
The vacuum switch was not for normal over-revving, it was in case the motor "ran-away" in neutral. That motor could rev up very high in neutral and pulling the throttle back would not slow it down. The vacuum switch would cut out 1 cylinder when the intake vacuum was very high with the motor racing and the throttle closed.A common problem on those 2 cylinder motors was the spark plug wires arcing underneath when the magneto plate was at full advance.
Hello Captain:What was the result of your switch problem?My vacuum cut-out switch stopped working last year. The weather just warmed up enough to check it out last weekend. I have a 1968 100 hp Evinrude. My engine quit all together, and only restarted breafly one time, a few days later.Thanks.Jim
Hello Lark40,You might check the throttle plates arn't going past horizonal when you go to full throttle this could cause the engine to slow down,because that is effectively again closing the throttle plates.If you suspect the vacumn switch do like Schematic said, that should effectively eliminate that switch.
Jim,The vacuum switch on your 68 100hp is a safety switch so if someone turned the flywheel by hand, or moved the spark advance plate, while the key was turned ON, the motor would not start and cause an injury. The ignition system does not receive 12 volts until the motor cranks over, building up crankcase vacuum that pulls the diaphragm in and closes the contacts. Then 12V is sent to the power pack to generate the spark.
The last V4 magneto was in '67 (first in 1958) on the smaller V4's with manual shift. Electic shift motors used the the battery and coil ignition with belt driven distributors. The '67 and '68 100 had power packs. The '67 still used the distributor and toothed belt while the '68 had those under the flywheel.
Thanks for that explanation Seahorse! I knew that the Clymer manuel was incorrect for this engine, but I didn't know how the switch was being used. Last night I cleaned the contacts and added a bypass switch so that if it failed again I could flip the bypass switch and keep going. Sounds like I could do away with the vacuum switch all together.Does the safty switch under the flywheel go bad and cause problems very often?
Gee, my January posting has risen again.I will probably be able to get my boat in the water within the next week (getting close to 70 today).Since I have a "kill switch" on the boat's dash to ground both points, I'll probably just disable the vacuum switch if it turns out to be the problem during testing. Thanks for the advice. This is a great board!