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Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4724
Experience:  42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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I have a 1978 mercury 700 70hp outboard it will only run on full chok

Customer Question

I have a 1978 mercury 700 70hp outboard it will only run on full choke if I try to advanced the throttle the engine will die carburetors and fuel filter are spotless, I can only increase the RPM by increasing the choke
Submitted: 1 year ago.
Category: Boat
Expert:  Jeff G. replied 1 year ago.
Welcome,this question may need more than a single answer. Please post back with additional questions on this at any time.
You still have a fuel problem. Either the carb is not opening properly, the idle mixture is not set properly or there is something in the carbs.
Any time the carbs come apart you need to put in new gaskets particularly the bowl gasket. if not it could suck air internally.
The main plug for the carb is in the front of the float bowl. Remove it and screwed into the end of it is the high speed jet. This must be absolutely clean and open all the way. The main nozzle must also be clean. There are small holes in the nozzle that need to mix with air.
On the front of the carb port side of the opening is the idle mixture screw. Make sure this screw is not turned in all the way. Turn it in slowly, counting the turns until lightly seated. Then turn it out about 2 turns. Once in the water you can fine tune it.
The fact you have to choke it points to not enough fuel going through the carb. choking it increases the fuel to air ratio to allow it to start but is overly rich and inprecise so they usually won't stay running, certainly don't want to idle.
Do the things I outlined and I'll bet the engine runs fine.
However I would also recommend a compression test as bad compression will not allow the engine to develop enough air pressure to allow the fuel and air to flow and mix properly.
I'd do a spark test of each cylinder too.
Let me know how it works out and we can go from there is necessary.
Thanks

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