75 hp mercury middle spark plug weak

[topwaterjunky]

I have been having problems with my boat not reaching high RPM. I have a 1995 75 HP mercury and I found the middle spark plug has a weaker spark than the top and bottom spark and it is dry as a bone too. The test was done with a spark plug tester. Can someone please tell me where to go from here??

 

[topwaterjunky]

could this be due to a bad coil or could it be as simple as a spark plug wire or am I totally in left field with this? Any advice would be appreciated because I try to fix things myself if at all possible, just need advice which way to go.

 

[Texasmark]

Weak spark and dry as a bone are black and white.......weak spark should represent weak/late in the firing cycle combustion making for lots of unburned residue which should load up the cylinder and plug with oily residue. Dry to me means that your mid carb is too lean. First I would go through the carbs and ensure that the linkage ensures that all 3 butterflies open and close in synchronism. Then I'd do a carb tear down, and rebuild with a kit looking for contamination....if black specks , your engine's fuel lines are deteriorating and need to be replaced.

When you have finished that, get the engine running and carb low speed jets adjusted along with the link and sync for the ignition timing and all that.

Then if you still have a weak spark problem, your plug should be wet this time and you can pursue your ignition problem.

That's the way I'd do it.

 

[topwaterjunky]

I pulled and checked all 3 carbs about 3 or 4 weeks ago and all 3 was clean as a whistle. I have looked at all the fuel lines when I replaced the fuel filter and rebuilt the fuel pump and they all looked good, no cracks nowhere and they too were clean and smooth on the inside and out. Can a fuel line be breaking down with no visible evidence?? I'm asking because I really don't know. These boat motors are like reading Japanese to me. I'm just trying to learn as I go where I can gather knowledge for later uses. I pulled the lower unit yesterday before I found the spark problem and it all looked good. I just can't understand how I get a bogging problem, it goes away for a few days after I change the fuel filter, fuel pump and blow the carbs out and run sea foam thru the gas with non ethanol gas and then come back but not as bad. When I say not as bad, when it first happened, I could only get 1800rpm at WOT, when it started again, now I can get 3200rpm at WOT. It will come up on plane eventually but I don't have the top end speed.

 

[mercster]

could be a bad coil or ignition wire. To check the coil to that cylinder, locate and remove it and peal back the rubber cover. The ferit bar should not be cracked in any places and there should be no melting of plastic. The ground wire should be making good contact. switch an ignition wire from another cylinder and see if the problem changes to that cylinder.

 

[mercster]

In addition, if the fuel lines are old they can break down inside and send post filter globs of rubber to the carbs clogging the needle valves. If old, replace all the fuel lines, get fuel line pre packaged at advanced auto parts and cut to size. its like a stroke waiting to happen.

 

[Texasmark]

In addition, if the fuel lines are old they can break down inside and send post filter globs of rubber to the carbs clogging the needle valves. If old, replace all the fuel lines, get fuel line pre packaged at advanced auto parts and cut to size. its like a stroke waiting to happen.

This happened to me on my 2002 90 triple at about 10 years. Clogged the high speed jet (intermittently) on #2.

 

[DavidMoore]

Agree with all previous advise.
Important to sort the carbs out before solving the weak spark issue. Did you go through the full link and sync after the carb rebuild, mixture screw set 1 1/2 turns out from lightly seated as a starting point?

 

[topwaterjunky]

No sir I didn't go thru the link and sync. I appreciate everyone's input. Looks like I will be tearing down the carbs again and soaking overnight and see what that does as soon as I get a full weekend off at work. I appreciate all you that's trying to help. I will post once I get the carbs done.

 

[Texasmark]

Don't forget the compressed air blowout part. I wouldn't have found my little chip had I not squirted the bottom of the bowl with shop air. It was invisible prior to that....wedged up inside the jet housing.

 

[topwaterjunky]

I appreciate all the advice you all are giving me. I'm hoping to tear down the carbs labor day weekend unless I can get a couple days during this next week. I will be sure and give an update afterwards. Thanks so much guys!!

 

[topwaterjunky]

ok fellas I finally got some time to take my boat out today after work. I worked on it a few afternoons and I pulled the carbs and cleaned them and they were all still spotless. I didn't find not one piece of small debris in any of them. I put those back on and while I had them off I went ahead and disconnected my VRO. Nothing wrong with it but I've just never been a big fan of them especially since my alarm didnt work for the low level. I pulled the fuel pump and took it apart and it all looked good. I went ahead and bought ANOTHER fuel filter and put it in. I pulled the lower unit and replaced the water pump and might I say it's pissing like a 5 year old now. I took it to the lake and still the same old thing. I'm just not getting that punch at WOT. I'm only able to get up to around 32mph and pushing 3200rpm's. When I took it out of the water I took the cover off the motor and put my hand on the back of the motor and it was just warm to the touch. Does anyone have anymore advice I can try because I'm bound and determined to fix this thing myself and I've seen on here alot of yall know your stuff when it comes to boat motors. I just need to dig a little deeper I guess. By the way, I checked all the plugs afterwards and now all 3 are showing sign of firing.

 

[DavidMoore]

Well if your sure your getting fuel and spark your going to need to rule out the compression. Don't get hung up on the values because gauges tend not to be accurate, the lowest needs to be within about 10% of the highest.
Let's hope it's not a compression problem.

 

[topwaterjunky]

I'm going to go back through all the fuel lines again and make sure I didn't miss anything. I'm also going to rebuild the carbs even though they all looked good, I was told today that just one little grain of sand in the high speed jets could cause this bogging problem and since I don't know either way, I'm gonna go ahead and do it to rule that out. I know it's something small I just haven't put my hand on yet. I'm also going to double up my wire ties on my fuel lines. I don't think I have a compression problem, I'm still thinking fuel problem so I'm going to gamble with it and start buying new parts if it has to do with the fuel.

 

[mgjtkt]

When going thru the carbs, look at everything with a magnifying light. It's hard to see into the small holes.

 

[Texasmark]

What about your fuel supply lines and bulb. How old are they? Several posters and I too have had delaminating external fuel line problems and the delaminations clog the line and prevent high flow rates when needed. Still have the fuel filter you changed? If so cut it open and if still using the grey OEM Quicksilver feed line, look for tan chunks of junk, larger ones are arc shaped like the inside diameter of the fuel line.

Next time you're out, and want more power, squeeze the bulb and see if things pick up.....needing to do that is bad fuel pump or clogged line/squeeze bulb. Some folks say that after market bulbs cause them problems. Use the OEM bulb which I always did.

 

[Texasmark]

When going thru the carbs, look at everything with a magnifying light. It's hard to see into the small holes.

That's why I said don't be stingy with compressed air. That's how I accidentally found my #2 clog just when I thought I had done everything and had no foreign objects in the bow.

 

[ondarvr]

I don't think I have a compression problem.

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

 

[topwaterjunky]

I got to messing around with my motor today. I had the cowl off and for no reason pumped the bulb and I heard a faint hissing sound on one of the fuel lines. I wire tied that line up and pumped the bulb about an hour later and didn't hear it anymore. I went back after about 4 hours and the bulb was still very firm. I'm going to take it for a run tomorrow and see what happens.The hissing sound wasn't coming from the fuel pump side but was from the opposite sidend of the motor. It was the lines that are vertical and connect to the carbs on the opposite side of the main fuel lines.

 

[Texasmark]

I got to messing around with my motor today. I had the cowl off and for no reason pumped the bulb and I heard a faint hissing sound on one of the fuel lines. I wire tied that line up and pumped the bulb about an hour later and didn't hear it anymore. I went back after about 4 hours and the bulb was still very firm. I'm going to take it for a run tomorrow and see what happens.The hissing sound wasn't coming from the fuel pump side but was from the opposite sidend of the motor. It was the lines that are vertical and connect to the carbs on the opposite side of the main fuel lines.

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?