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1966 Mercury 500 Slowly loses power after WOT

S

Sasqwatch

New member
1966 MERCURY 500 “NEED ADVISE PLEASE”

Starts great and goes to WOT,planes out nicely……gets up to about 30 mph and then starts slowly losing power! It doesn’t stall or hesitate just starts slowly losing power to about 10 mph.



Tried new gas tank

New fuel line with pump

New fuel

Tried pumping while it starts going slower …doesn’t affect it

New plugs

I did just replace the whole magneto assembly…purchased it on eBay

New bearings ,points, cap,rotor,and plug wires

Not sure what’s going on.

I had to replace the magneto because the bearings went bad along with the coil….before all that it ran fine
 
Replaced water pump impeller ?----Set points at 0.010"?----Factory plug wires with metal core ?----Mixing oil at 50:1 ratio?
 
Fuel tank not venting properly, try running with the fuel cap cracked open. Obstruction in the fuel tank pick up, try a different tank. Coil defective after it warms up. You don't mention what happens next; does the engine run poorly the rest of the day? Do you shut the engine off, let it cool down and repeats the same issue after running at wide open? More information would be helpful.
 
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