Plug reading

[delgrad59]

Need help. I have five plugs that look like the one on the right and one that is all black thick oily coverage. Gapless plugs 2.4 mariner. Where should I start? Motor runs but backfires intermittently. Revs right up. 326507

[delgrad59]

#4 cylinder.

[pointer]

Revs up on hose or in water?
Do you have a spark checker? I'm not a V6 Merc / Mariner expert, looks to me like you have an ignition problem not firing that plug. Backfire could be intermittent spark on that plug, or the unburnt fuel from #4 is getting out the exhaust port and ignited by exhaust from another cylinder.
If you have the testing tools, the ignition troubleshooting process can be found in factory service manual, or CDI electronics online...
pointer

[racer]

If that came from cylinder that operates your lift pump could be pump leaking.

[delgrad59]

Thanks will run a compression check and verify spark. I have the cdi spark block. Maybe weak or intermittent spark on that cylinder? I ran the stator and trigger test in the clamer manual and all checked out before. Maybe it burnt out since the check

[delgrad59]

Compression was fine 130 on that hole. I did go and check the timing on each cylinder and found that #3 is firing with #5 as well. All other cylinders are fine at idle and full advance timing. Would this cause the difference in the plug reading on #4. Is this a switch box issue? I had changed the inner switch box a few weeks ago because #5 wasn't firing. Could I have had two bad boxes? They are used. Just odd that I didn't have fire on 5 previously now I get 3 and 5 firing on hole 3. Hole 5 fires at the right time. Any suggestions?

[Dave Strong]

You don't have a DVA meter I assume. So swap the switch boxs and see if the issue changes cyls.

Dave