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1974 Mercury 500 thunderbolt 50hp idles poorly, stalls going into gear

R

ratherbfishin

New member
74 Blue band mercury 500 4 cyl outbrd 50 hp. Idle is not great, stalls when going into gear in the water. Can get it to run at speed by revving, even get on plane just fine, but then stalls when go to neutral. I tested the motor by pulling the plug wires off one at a time with the engine running. With either of the lower two spark wires off, the engine speed doesn't change, but with one of the upper wires removed the engine speed drops by half and/or dies. Seems like cyl #3 and #4 aren't firing. I've replaced a lot recently as maintenance and trying to fix this including: engine wiring harness, fuel pump, cleaned carbs to the bowls and replaced lower carb needle valve and gasket. New spark plugs, tested the spark current. Fresh gas, new gas bulb and replaced a couple lines. Compression test is just fine as well. Driving me nuts here! Symptoms remain. Runs okay on the muffs in the driveway, but can't get off the launch ramp without dying. Anyone got some ideas what to do next? Thanks for help. :confused:
 
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Top carb is screwed up. How did you "clean" it? Spraying carb cleaner at it won't do it. It has to be completely dissembled and soaked in cleaner.

Jeff
 
I think you mean the bottom carb? the top cylinders actually are contributing to engine speed, whereas bottom cylinders seem to not matter if spark wire is disconnected. And I did just clean out down below the float and thats it. I havent removed the rest of the carb (the part with the jets and the idle screw) from the front of the engine. I could give that a go next. How do I rule out that this isn't an internal engine problem like reed valves?
 
Sure sounds like a plugged carburetor !--------------You must be absolutely sure that carburetors are clean ( take them apart !! ) so that you do not risk engine damage.--------No fuel going into the bottom cylinders means no oil either.
 
If the bottom crankcase seals are bad it will run great on a hose. If they are sucking a little water it will not run at idle but will run top end once cleaned out.Check carb first and then plugs to see if has moisture on them and its best to test in a barrel or lake
 
Take the carbs all apart and soak them till they are clean, put back together and enjoy. The biggest reason most older motors don't run well is from dirty carbs. New ethanol gas with old rubber fuel lines are not a good combo. Old gas turns to gummy sludge blocking jets and ports. If you just took the bowl off you are missing most of the carb.
 
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