Logo
FULL SITE MENU

Mercury 125 spark issue?? Need some help!!

M

Mattc8821

Member
2003 merc 125 saltwAter
compression 120+ in all 4 cyls
motor was getting about 4500rpms at about 30mph max. I have 17p 3 blade prop , 20ft sea fox dual console.

put new plugs in, took for a ride up canal a couple times . Pulled plugs, and cyl 4 spark plug looked literally brand new. No color on it whatsoever. Cyl 1-3 perfect color on plugs. First thing I did was pull #4 carb to make sure it was clean. Went through it and gave good cleaning. Bowl fills with fuel when primed.

Next I checked all spark plug wires with volt meter-all good. I also switched #4 coil with another. Took for spin and plug still looks brand new. No color at all. That cyl does not seem to be firing when I'm running it. But when I pull plug out and hold it against head and turn motor over, it does produce a spark.

I pulled plug off of cyl 4 coil and traced all wires . I checked for continuity in all of them and all tested good. WhAt else can I test??
 
You might have water getting in that # 4 cylinder, or you might have a bad plug--it happens. Try a new plug and, if that doesn't do it, look for water droplets on the plug after idling.

Jeff
 
Plug was brand new. Last night I switched some cdms around and checked spark on all cyls. I put new plug wires and boots on. The plug on that questionable cyl seemed to have slightly weaker spark than the other 3 plugs. Even when trying it on diff wires. It was still a blue spark but didn't seem as strong. Tonight I triple checked that carb and the reeds and both are all clear. Thinking about takin for a ride tomorrow or Friday and seeing what happens . Still stuck between whether or not it's a fuel or spark issue
 
Leave it on the trailer, bring it up to 2500 RPM, Take a squirt bottle with premix and squirt it in carb and it it takes off it fuel problem if not its a CDM module.


[FONT=Verdana,Arial,Tahoma,Calibri,Geneva,sans-serif]https://www.go2marine.com/productcenters/electrical/merc-force.pdf[/FONT]
 
Had it out this past weekend. Took it down canal and had it up to about 3500rpm, sprayed some premix into carb throat and no change in speed or rpms. I also changed around cdms and it still made no difference
 
I saw similar post that relates exactly to my issue. I went through motor again yesterday and found one of the yellow wires coming from the stator to be burnt. The connection and like 3 inches of wire. Also had my motor running yesterday and pulled plug wires off each cyl to look at how far spark jumps. First three cyls had good distance. Last cyl had barely any jump. I had to put the boot practically on the spark plug for there to be any spark.
 
#31
mattc882I , Yesterday 04:40 PM

Still have not figured out this issue.


Im getting a good spark on all 4 cylinders. Compression is 120+ on all 4. Boat starts and runs fine but only gets up to about 4200rpms. Cyl 1-3 are running good, but when I pull plug wire off of cyl 4 , there is no diff in how it runs. Spark plug also tells me that cyl is not performing how it should. I sprayed fuel into carb throats when running and no difference in performance. I also put spark light on cyl 4 and watched it when I ran it and it was lighting up, telling me it was still getting spark. I swapped cdms around. No difference. New plugs and plug wires. No difference. Changed plugs around to diff cyls, no difference. I pulled carb apart multiple times and it is perfect. I know for a fact it isn't carb issue. I pulled reeds out and physically looked at them and they are perfect. I ran motor in driveway and I covered each carb with a clean towel. Each carb had good suction. Carbs 1&2 (carbs that idle) acted accordingly when covering. 3&4 also acted the same when covering them. I'd assume if there was a carb issue with #4 , it would react differently than #3 when covering, but they had exact same reaction. Idle slowed down a little bit, spit some fuel out towards towel, and caught back up and smoothed out.

Theres an electrical component underneath the oil tank towards bottom of the motor with a blue/yellow wire, purple, and brown. Is that the rev limiter? And could that be something that could cause an issue like this?

There are 2 relays on this motor on the same side. If one of these were faulty or bad, could that affect engine performance like I'm experiencing? I thought these were primarily for trim but could be wrong. These motors seem really simple but I just can't figure this thing out.​


 
Mattc8821 said:
I actually haven't tried to rev that high in neutral or reverse. If it does rev past 4500, what would that mean?
Click to expand...
DO NOT DO THAT TEST EVER!!!!!! You can easily damage the engine that way. A bad piece of advice for any motor!!!!!! Have you checked the carb? If the accelerator pump is leaking by it will flood the cylinder. The accelerator pumps work on the bottom 2 carbs only. You Need to buy a factory manual for your motor because the fuel system is a little tricky on those.
 
Last edited:
flyingscott said:
DO NOT DO THAT TEST EVER!!!!!! You can easily damage the engine that way. A bad piece of advice for any motor!!!!!! Have you checked the carb? If the accelerator pump is leaking by it will flood the cylinder. The accelerator pumps work on the bottom 2 carbs only. You Need to buy a factory manual for your motor because the fuel system is a little tricky on those.
Click to expand...


Yeah I've been through that carb 3 times because I kept having doubts about it but I know it's 100% clear. So I put new plugs in and after I ran it, cyl 4 plug had no color on it as if that cyl wasn't getting any fuel. The plug looked brand new. If there was issue with accelerator pump, could that affect fuel to that carb in high rpms? I thought accel pump was just for when it was at low rpms
 
Mattc8821 said:
I actually haven't tried to rev that high in neutral or reverse. If it does rev past 4500, what would that mean?
Click to expand...

AS mentioned this apparently is a no no although I am not sure why a quick rev past 4500 rpm could possibly destroy an engine or hitting above 4500rpm while in reverse, but you learn something new everyday! Not talking about holding it red line here for extended period, check it and down.

If it does go over you have something else going on, get away from spark, do you have an internal or external gas tank?
 
X96mnn said:
AS mentioned this apparently is a no no although I am not sure why a quick rev past 4500 rpm could possibly destroy an engine or hitting above 4500rpm while in reverse, but you learn something new everyday! Not talking about holding it red line here for extended period, check it and down.

If it does go over you have something else going on, get away from spark, do you have an internal or external gas tank?
Click to expand...

Its internal. I was curious if it had something to do with that, but all 4 carbs are getting fuel when primed. And ball stays hard.
 
The anti siphon valve can stick causing pressure issues, although the ball is hard the flow could be restricted if stuck. You can try running with the gas cap off or an external tank if you know someone who has one.
 
flyingscott said:
DO NOT DO THAT TEST EVER!!!!!! You can easily damage the engine that way. A bad piece of advice for any motor!!!!!! Have you checked the carb? If the accelerator pump is leaking by it will flood the cylinder. The accelerator pumps work on the bottom 2 carbs only. You Need to buy a factory manual for your motor because the fuel system is a little tricky on those.
Click to expand...

I just pulled accelerator pump apart and the diaphragm is all cracked. Could this have any type of effect on motor in higher rpms?
 
Mattc8821 said:
I just pulled accelerator pump apart and the diaphragm is all cracked. Could this have any type of effect on motor in higher rpms?
Click to expand...

Have boat in the water now. Tried to open gas cap and run it. No change. Still not running on that #4 cyl. I also unplugged the rev limiter. No change. Still only getting around 4K rpms. I gave it a quick rev in neutral and it revved past 4K .
 
X96mnn said:
AS mentioned this apparently is a no no although I am not sure why a quick rev past 4500 rpm could possibly destroy an engine or hitting above 4500rpm while in reverse, but you learn something new everyday! Not talking about holding it red line here for extended period, check it and down.

If it does go over you have something else going on, get away from spark, do you have an internal or external gas tank?
Click to expand...
Broken piston skirts
Runaway ignition
Holes in the bolck from inside parts coming out.
It is a useless test that tells you nothing. 2 cylinders can run to 4500 rpm!!
Running in reverse same as above.
 
I got the same engine with similar issues
cannot exceed 4200 RPM when engaged ,In neutral RPM can get up to 6000 RPM,
I did replace my fuel pump kit few months ago and the hose from the fuel pump to the filter with regular fuel line without any bend on it as the original , I think this created a fuel restriction , replaced the fuel line with original and testing schedule this week end
 
Trophy125 said:
I got the same engine with similar issues
cannot exceed 4200 RPM when engaged ,In neutral RPM can get up to 6000 RPM,
I did replace my fuel pump kit few months ago and the hose from the fuel pump to the filter with regular fuel line without any bend on it as the original , I think this created a fuel restriction , replaced the fuel line with original and testing schedule this week end
Click to expand...

Let me know if that ends up being the issue after you test. I found out my #4 cylinder wasn't firing even though it has spark. My carb is freshly rebuilt with no restrictions. I tried every test imaginable. I think I narrowed it down to getting some water in that cylinder. I'm currently waiting for gaskets for exhaust cover and water jacket on back of motor. I'll know after they're installed and I take for test run- hopefully this weekend also
 
Back
Top