Merc 500 rpm problem

[dutchman89]

Hi guys,

i`m going crazy over my 500 thunderbold 50 hp. When she is cold she runs like hell (5500-6000). I then get the idea that i`m using all of the 50 hp. But when she is warm she holds back and won`t turn more then 4700 rpm with a lot less power. What can this be? Is it an compression isue? Or spark plug issue?

When i pull out the plugs the lower once ( cil. 3 and 4) are nice and clean but the upper ones (cil 1 and 2) are wet and shows oil burns.

I ordered 4 new spark plugs, how this helps. I also cleaned the carbs out, reinstalled the venturi`s (previous owner let them out somehow..) and set the carbs to when she is running best.

What could this be? I`m running out of ideas...:facepalm:

 

[dutchman89]

Re: Merc 500 rpm problem

a little update: I pulled out all of the spark plugs and all of them look a bit wet and oil burned. Only one of them (the 3th cil.) is totally clean and looks like new. Prehaps that cilinder isn`t firing when the engine turnes warm?

 

[oldman570]

Re: Merc 500 rpm problem

Dose sound as tho you have a electrial problem with it. Depending on the year of the motor will narrow down some factors. Some older motors only had one coil and the distrubure cap and rotor could be the trouble while others had 4 coils that can cause trouble. What year motor is it or what is the SR# of the motor?
Oldman570

 

[dutchman89]

Re: Merc 500 rpm problem

Thanks for the reply!

Sorry should have mentioned that. The sr# is: 7079266. It is made in Toronta canada it says (don`t know if that has anything to do with it..but for the information) It is an thunderbold ignition with 4 coils. I think the year is late 70`s or begin 80`s. Is it possible for such an electrical problem to occur when the motor is heating up?

 

[oldman570]

Re: Merc 500 rpm problem

The coils on the motor will heat up as they are being used and one can go open or lose ground from heat expansion. Switching the #3 coil with another one will tell if the coil is the trouble. If the miss goes to the other plug the I would replace that coil. If not then the trigger, switchbox, or stator could be the problem, and will need to be tested. also check the wires where they go under the flywheel for any sign of them worn from use and touching anywhere.
Oldman570

 

[dutchman89]

Re: Merc 500 rpm problem

Thanks Oldman570. I will try that, good advice! any other advice on how to test the trigger, switchbox and stator?

 

[dutchman89]

Re: Merc 500 rpm problem

Found the problem. One of the coils is toasted. It was full of cracks and fell apart. It also didnt created any sparks. Runs the same with or without the sparkplug connected. So it was running on 3 cilinders. Just to let you know. Tomorrow new parts will arive, cant wait!

 

[dutchman89]

Re: Merc 500 rpm problem

A little update with an photo. This is not how you want your coil to look. Anybody else had this problem?

 

[dutchman89]

Re: Merc 500 rpm problem

Hello guys,

well new parts arrived and are installed. The coil was not the problem. It still isn`t producing a spark on the 3th cilinder. I`m going crazy!! I called an mechanic and he said that the switch box is broken. Could this be the reason for the problem? I`m a little scared of ordering the part (300 euro`s here in Holland) because if it is not the problem I wasted 300 bucks. What could it be the switch box, or could it also be the stator, or trigger. The other cilinders are running great.

Please advice

 

[oldman570]

Re: Merc 500 rpm problem

That coil is really shot. More than likely the switchbox is bad now also. Testing the trigger,coils, and stator should be done before anymore items are bought. You will need a multymeter (ohm meter) to test those and a manual that tells how to do the test. OME manuals are best and others can be bought online or at a dealer usually for less than $20. and should be a must for anyone wanting to work on thier motor. I have seen switchboxes on Ebay that have sold for alot less than a new one from a dealer, but some are used and you have to be watchful as they cannot be tested unless they are on a running motor. A search here on the fourm for testing the stator coils, and trigger will probbly get the info needed also. If you have already talked to a mechanic and he has tested everything, then the switchbox is the problem. Those will go bad if the motor is turned over or run without the plug and everything being grounded.

 

[dutchman89]

Re: Merc 500 rpm problem

Thanks oldman! Tommorow I will out to the mechanic tommorow and he has a spare one. We will put it on and test it. He really thinks it is the switchbox. The cost is 120 it is a second handed but when it works its fine by me. How did my switchbox end up being broken? Is it oke to just put another in or will this also go bad due to the same error? Can the coil caused my box to breakdown?

 

[dutchman89]

Re: Merc 500 rpm problem

Oke,

so today we put on another switchbox and she fired at all cilinders. She runs perfect!!

Problem solved it was all due to a bad coil and a broken switchbox.

Just in time for the holiday of next week.

 

[oldman570]

Re: Merc 500 rpm problem

Glad the new coil an switchbox cured your trouble. The electrial system on the motor is very touchy when it comes to the electric going to ground. The coil is probbly what took out the switchbox. You never want to turn the motor over without having ALL the plugs or plug wires grounded. Alwasys disconnect the battery cables when charging the battery. Just touching the wrong battery post with one of the wrong cables can take out the rectifier and cause the battery not to get a charge when running the motor. Battery cable swithces can be installed on the battery to make things easier for disconnecting from the cables.
Have Fun,
Oldman570