75 hp mercury middle spark plug weak

[topwaterjunky]

Why? I'm not saying it is a problem, but what makes think it's not.

my compression is 120 on all 3 cylinders. I Just bought a compression tester today and all 3 read 120

 

[ondarvr]

Good deal, takes that possible issue away.

 

[topwaterjunky]

Why don't you quit "......messing around with my motor today...", get some new fuel line, flush out what's left (squeeze the bulb and ensure clean fuel only is ready to be introduced into your "new" fuel lines, and be done with it?

I bought all new fuel lines today and I'm replacing them all right now with new. Sorry, I know I'm aggravating as hell. I just need to start listening to yall instead of thinking I know what I'm doing.

 

[Texasmark]

I bought all new fuel lines today and I'm replacing them all right now with new. Sorry, I know I'm aggravating as hell. I just need to start listening to yall instead of thinking I know what I'm doing.

Gotta laugh at that comment. I too have that problem sometimes. Ask for help and then go about my business disregarding it.....well, sometimes. We'll get-r-fixed.

Mark

 

[topwaterjunky]

ok guys I pulled all the carbs and all 3 carbs were clean. Everything that could come off the carbs came off and was cleaned. I replaced all the fuel lines. Took it to water and still only reaching 3200 rpm's. Brought it back home and checked the coils. The coils all read 975 ohms and no cracks or anything with any of them. My compression earlier today was 120 on all 3 cylinders. Anybody have any lue what I need to do next??

 

[topwaterjunky]

I also hooked it up to the muffs and checked the spark plugs and noticed the middle spark plug is not firing at all.

 

[ondarvr]

Swap a coil with the cylinder not firing and see if the problem follows the coil. Testing the readings on a coil doesn't mean much.

 

[topwaterjunky]

so can a coil be bad but still read good??

 

[ondarvr]

so can a coil be bad but still read good??

How it tests out on a multi meter doesn't mean much, it needs to be tested under load to determine if it works correctly, and then it can still fail when when it heats up.

 

[topwaterjunky]

ok thanks, I'm just trying to understand this as I go. I will try this and get back with you. Thanks again for the advice.

 

[Texasmark]

I used to test them with a square wave single pulse. The'd ramp up storing energy during the pulse and when it disappeared they were supposed to ring out 11 detectible oscillations. If you only got 1 definitely dead and a few, going dead. Next test is voltage breakdown. You can't test that with an ohmmeter. The coil has a voltage applied across the windings and so many windings. If 10kv and 1k turns, it's 10v per turn. If any turn in the coil fails to achieve it's 10v most of the energy in the coil will be used to prop up that wire. Obviously if that's happening, you are short on your output volt amps when the spark plug fires. Nature of the beast.

 

[Faztbullet]

Index the flywheel and check timing on each cylinder as a bad switchbox can cause this. If bias circuit is bad it can cause the switchbox to fire before internal capacitor is fully charged......

 

[topwaterjunky]

ok guys, I swapped the wires around on my coils and my problem moved to the top cylinder from the middle and the middle cylinder started firing. I moved all the wires back and the problem went back to the middle cylinder not sparking. I did notice the box above the coils is cracked. I'm assuming this is my switch box.

 

[Texasmark]

Looks like you found your smoking gun.

 

[topwaterjunky]

is there a way to test the switchbox to make sure this is my problem? I'm about 95% sure it's my problem but I hate to spend $200 on a new one and it be something more in depth like the stator or do you think the switchbox is 100% my problem?

 

[topwaterjunky]

ok fellas, I got me a new switch box today and got it all put in after work. Hooked to the boat and headed to the lake and she ran like a top. I hammered down on the throttle and she came out of the hole like a fire was lit under her tail. I appreciate all the good advise and guidance in what steps to take to diagnose and fix this problem. I went from knowing nothing about a boat motor to being pretty comfortable with it now. I still don't know everything but I learned quite a bit asking yall questions and seeing your answers. Thanks so much for everything!!

 

[Texasmark]

Glad you got her running. Mission accomplished.

 

[topwaterjunky]

Yes sir! Thread closed!!!