# Mercury Outboard losing RPM's



## Carphunter (Jan 30, 2018)

Hey guys, My 1980 Merc 25 loses rpm and speed after warm up. When cold I idle a bit then
putt for a bit then wot. The motor will gain to max rpm and speed for about a minute then lose rpm and about 1.5 mph. It will run fine to my fishing spot, cool off, and the same thing happens
when I start it up and move. Any ideas? Why would it slow down when at full operating temp?
Thanks in advance!


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## Guest (Aug 22, 2018)

Sounds like fuel delivery problem to me. Check your entire fuel system over starting with fuel line quick connect. Check pick up in tank, all hoses, is there an anti siphon valve on the pick up at tank?, rebuild/replace fuel pump. While the last one was mentiined last and may not be your problem, I reccomend to everyone to rebuild/replace the fuel pump every few years! Ya can’t walk on water and the cost of doing the job is cheeeeeap insurance.


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## Carphunter (Jan 30, 2018)

Hi Boatbrains thank you for your reply. Carb was rebuilt and fuel pump as well with new diaphrams
at beginning of season. Pick up in tank is clear. No siphon at pick up. Quick connect at motor rebuilt
at beginning of season. This problem only happens when motor is warm and run at wot. Pumping
primer ball does not help. Only two feet of fuel line from external fuel tank to motor. I know it is only 1.5 mph but everything helps at this altitude, 6000 ft.


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## Guest (Aug 23, 2018)

She still may be running a little lean at wot. Other tgings to check would be compression when warm, you could have a coil or trigger acting up when warm also.


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## Carphunter (Jan 30, 2018)

Thanks Boatbrains, I did not think to richen and have only done compression test when cold, new coils installed two months ago. Trigger?


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## Guest (Aug 23, 2018)

Carphunter said:


> Thanks Boatbrains, I did not think to richen and have only done compression test when cold, new coils installed two months ago. Trigger?


Timer base/trigger coil directs/ releases spark charge from capacitor. Charge coil in stator produces the voltage. Sometimes these will act a fool when they heat up, and can be a bear to diagnose until they break.


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## Carphunter (Jan 30, 2018)

Thanks for clarifying that!


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## DONAGHUE (Nov 18, 2017)

Fuel tank not venting ???


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## Carphunter (Jan 30, 2018)

Good idea Donaghue, but tank vents fine Checked that already! I am leaning tward too lean a mixture. I have not had a chance to test this theory out yet. Will let you guys know when I do!


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