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77 mercury 850 with thunderbolt ing. good spark on 1,2, & 4.

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77 mercury 850 with...

77 mercury 850 with thunderbolt ing. good spark on 1,2, & 4 . # 3 spark is good for a quick moment then weak wont go to plain. acts like starving for fuel . stator checks out, trigger checks ok, new plugs, about to give up. boat mechanics around here dot know whats wrong with it

Mechanic's Assistant: Sometimes boat issues seem like they'll be really complicated, but end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Is there anything else important you think the Marine Mechanic should know?

Carbs been rebuilt

Submitted: 1 year ago.Category: Boat
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Answered in 10 hours by:
8/1/2017
Marine Mechanic: Jeff G., Marine Mechanic replied 1 year ago
Jeff G.
Jeff G., Marine Mechanic
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Experience: 42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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Hi, how are you determining spark while running? What makes you think #3 isn't firing?

Can you give me a little history on the engine. Is this something that just started, a long time problem? Is this the first time use this year? Thanks

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Customer reply replied 1 year ago
using spark tester. can see spark is good at first than gets very weak. i used it in april asnd worked normal for 2 weeks , then started to run bad. took to shop and he adjusted carbs. ran good. went to lake 2 weeks ago and ran like now. took to 2 differant shops and they said dont know whats wrong
Marine Mechanic: Jeff G., Marine Mechanic replied 1 year ago

Your engine is using 4 individual coils. Try swapping on of the other coils and see if the problem moves with the coil or stays on that cylinder. When you remove the coil there should be a fine stranded wire between the rubber case and the mounting block. Gently clean the wire and using a small wire toothbrush clean the mounting area of any grease, corrosion etc. Then retest.

Very possibly one coil is breaking down and overheating internally.

If you have a inductive timing light you can use it to monitor the ignition while running. Just look at the flashes from the light, should be bright and consistent.

Post back what your results are.

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