miss fire

[martin47]

mercury 115hp . I have been out in the boat a few times in the last few weeks and all has been well.last sunday i developed a miss fire ???? when i was idleing.......it then stayed with me . i have changed the plugs with new and the problem is still there ???? the ht leads seem to be fine ??????

 

[redjmp]

Re: miss fire

What year is your motor?

 

[martin47]

Re: miss fire

1982 ...........

 

[redjmp]

Re: miss fire

Well the first thing to do is check for spark on all cylinders and identify any that aren't firing.
If you run engine for 10 minutes under load you can tell any that aren't firing because they will be cold and clean with gas while others will be warm and show signs of deposits forming. Then you work backwards up the line from there.
Also make sure that gas is getting to all cylinders. Any cold plugs must be wet otherwise no gas is getting through.

 

[studlymandingo]

Re: miss fire

You can purchase a spark gap tester at a local auto parts store for around $7. This will give you a visual on your spark at idle. There are a number of things that can cause spark to drop from faulty wires, switchboxes, your stator, or a number of other things. The first thing to determine is whether or not this is the problem by testing spark, then you can begin to test for the cause.​

 

[martin47]

Re: miss fire

i tested all the plugs for a spark and they all seem ok . but ? when i ran the motor and pulled the leads off one at a time the missed beat was clear..except number 6 the drop in power was very slight compeared to the other 5 .

 

[studlymandingo]

Re: miss fire

How did yo test for spark?

If you still have steady, strong spark on that cylinder, you will need to test the compression.​

 

[martin47]

Re: miss fire

i took the plug out atached it back to the lead and held on the side of the head then ran the motor ??????

 

[redjmp]

Re: miss fire

That only tells you that is has some spark. You need to tell if it has good spark under load. Get yourself a cheap tester or hook up a timing light to #6.
Does that plug heat up as much as the others after cruising for 10 minutes?
Is it cleaner than the others?
Try swapping that coil into a different cylinder and retest. Do the symptoms follow the coil? Check that coil for any cracks in the plastic. Peel back the rubber casing and have a good look. Replace if cracked.

 

[martin47]

Re: miss fire

i am taking her out weekend i will try what you said and get a light ,,,,thanks

 

[j_martin]

Re: miss fire

Do a compression check before you go out. You may have a failing ring or piston that hasn't done much damage yet. If that's true and you continue to run it, you could do real serious damage.

If compression is even, then troubleshoot the spark and gas.

hope it helps
John