1993 Mercury 25 HP wont run under load- on one cylinder (i think)

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DEFLYCASTER

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Ok Gents I neeeed help.

First- I am not a mechanic. I have tools, a brain and some analytic skills but no real engine experience.... so please give me as much detail as you have patience for. Any suggestions are really appreciated

I have scoured threads but cannot quite find my issue.

1. he motor runs great in driveway with muffs.
2. In pond it runs at low speed, and every tens mins or so it will randomly "jump" up in speed then die off. It acts like it wants to go at full throttle but just wont hold. ONe time it stayed full throttle..... never again
3. The carb was just rebuilt professionally. as was diaphram and fuel filter.
4. today I pulled top plug, shocked myself and it stayed running. I then pulled bottom plug and it dies immediately so it seems to be running on one cylinder.
5. Again it will open wide in driveway.. is that normal?
6. How can I figure out if it is a fuel vs electrical vs who knows issue with this cylinder?
7. I do not have a spark tester but can get one if needed.
8. Will it help to change plug and wires?

Thank you so much for any help you are willing to give.....
 

cincinnati

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Jul 10, 2013
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Re: 1993 Mercury 25 HP wont run under load- on one cylinder (i think)

Ok Gents I neeeed help.

First- I am not a mechanic. I have tools, a brain and some analytic skills but no real engine experience.... so please give me as much detail as you have patience for. Any suggestions are really appreciated

I have scoured threads but cannot quite find my issue.

1. he motor runs great in driveway with muffs.
2. In pond it runs at low speed, and every tens mins or so it will randomly "jump" up in speed then die off. It acts like it wants to go at full throttle but just wont hold. ONe time it stayed full throttle..... never again
3. The carb was just rebuilt professionally. as was diaphram and fuel filter.
4. today I pulled top plug, shocked myself and it stayed running. I then pulled bottom plug and it dies immediately so it seems to be running on one cylinder.
5. Again it will open wide in driveway.. is that normal?
6. How can I figure out if it is a fuel vs electrical vs who knows issue with this cylinder?
7. I do not have a spark tester but can get one if needed.
8. Will it help to change plug and wires?

Thank you so much for any help you are willing to give.....

I nominate the ignition switchbox! (A fuel problem would affect both cylinders since a single carb feeds both.)

Had a similar, very random problem w/my 25 ('88 vintage). Acted like a fouled plug under load, which would occasionally clear out, usually after the motor got hot while lugging on one cylinder. The issue worsened until the motor would not always start.

And yes, I also got shocked pulling plug caps while it was running. Asked my fishing partner to please beat me w/an oar if I thought about trying it again.

You can test the stator, ignition trigger & coils w/an ordinary ohmeter. If they test OK, as mine did, the switchbox - for which there was no test in the shop manual - is the next candidate. It's a fairly expensive, but easily replaced, item & instantly cured my problem.

Good luck!
 

quicktach

Petty Officer 1st Class
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Aug 24, 2012
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394
Make sure the switch box has a good ground before you go ahead and replace it.
 
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