50 Merc Switchbox

[Laotzu]

Are there switches in the switchbox?? Must be if you go by the name. Anyway, the motor will drop from 4k to 3k rpm. it still runs smooth, no missing, but can't exceed 3k rmp. then it seems to kick back in up to 4k on its own. The reason for the switch's question is.... if I pull the cover and tap the switchbox lightly, it fixes the problem. I'm thinking a switch inside there is cutting off power to one of the spark plugs.<br /><br />Any suggestions welcomed.

 

[Barnacle_Bill]

Re: 50 Merc Switchbox

They are electronic switches, no moving parts. Check all the connections and make sure they are clean and tight. Is 4K the best it will do.? I believe that motor should top out around 5300 RPM.

 

[Laotzu]

Re: 50 Merc Switchbox

IBNFSHN, no, but I usally cruise at 4K rpm. It'll do over 5-6K like you said. My thought was loose connections too, but I checked and re-crimped all of the wires coming from the box, and didn't seem to do any good. I also pulled all the plugs after an episode, and they all looked the same, ie: no excessive oil. This started soon after I replaced the plugs, I may have a bad plug. I guess I'll keep playing with it.

 

[Laddies]

Re: 50 Merc Switchbox

You have a selidium rectifier switch that is sticking inthe box, not unlike condensers use to, it is sealed in the box and must be replaced as a unit---Bob

 

[Laotzu]

Re: 50 Merc Switchbox

Laddie,Ouch, I was afraid of something like that. Fixing it by tapping on the box kind of indicated a problem inside the box. Thanks for you help.

 

[Laotzu]

Re: 50 Merc Switchbox

Bob, had the boat out today and it went into the usual routine. Cut in and out. Tapping the S/box area did no good. However, went I put it in neutral, and use the fast idle lever (just for a few seconds) it gets up to 5K plus with no problem. Why would it work ok when its out of gear? Wouldn't the selidium rectifier react the same regardless?

 

[Laddies]

Re: 50 Merc Switchbox

I really don't have a answer for that inless the increase in voltage made it function? Bob

 

[Laotzu]

Re: 50 Merc Switchbox

I think you were right Bob. As it turns out, I should have mentioned that the prior owner had a red stator upgrade installed. It comes with a module that has a full wave rectifier, and shunt voltage regulator. It straps to the side of the switchbox. When I cut it loose, I seen that it had melted around a plastic screw, which I assume was a filler port for the black stuff. Only the blue wire to the switchbox is connected since it derives full power at idle speed as well as high speed. Got a new one special order. Cost only an arm rather than an arm & a leg if it was the switchbox. Now just need to install and give her a test ride.

 

[Laotzu]

Re: 50 Merc Switchbox

The module wasn't it. I took it into a shop and they replaced some parts in the carburators, checked the fuel pump, fuel filter, reset the carburators. Commpression good, power to the switchbox 300 volts (in spec), power to coils fine. Replaced the spark plugs. Spark to plugs strong at idle. Of course it wouldn't repeat my problem in the tank. They said the next step would be to replace the switchbox. They would have to take it on the lake (2-3hours labor)to be sure. I decided to do it myself. Got a CDI switchbox and tried it out today. Ran good for about 1/2 hour, then....back to 2800 rpms tops. Pulled the plugs at home, and the electrodes are black. Ran it in the driveway in the dark, no leakage from the plug wires. <br />Output to battery is 14 volts, so with the test they did on the stator output to the switchbox, can I assume the stator is ok? Could the trigger cause this problem?? It's the only thing electrical left.

 

[Laotzu]

Re: 50 Merc Switchbox

Just ran a resistance test on the leads from the stator. Reading was 644 ohms. The manual says a red stator should read out at 660 to 720 ohms. Also tested out ok for shorts. Any relationship between resistance and output? Could this be a problem or is close good enough?

 

[Laotzu]

Re: 50 Merc Switchbox

Problem solved. Got a flywheel puller on E-Bay, and pulled the flywheel. The trigger switch had a fine coat of rust (iron oxide)and one of the magnets had a fairly thick coat around the raised portioin of the magnet. Like it accumulated over time. Cleaned it with a solvent and went out on the lake. Works fine now. I figure the iron oxide was interferring with the triggers operation to cause a condition similiar to being out of time, thus the loss of power.<br /><br />Has anyone ever come across this before?? Sounds bizzare to me, but something to remember if all else fails.