1975 Mercury 20 hp throttle problems

boatster

Petty Officer 1st Class
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Jun 13, 2004
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331
Hi <br />my 1975 Mercury 200 20 hp has a very rough idle. Even if I adjust the idle screw you can't really discern a performance difference. Also When I try to throttle it up the engine will begin to bog. The only way I can throttle it up is by closing the choke 80% then throttling up then once throttled up opening the choke. I'm assuming when I throttle up its not getting enough gas. Thus by closing the choke I am making it draw more gas? Can someone diagnose this? Is it the fuel pump or something wrong with the carb?? Any advice suggestions appreciated. Thanks.<br /><br />Adam
 

Chinewalker

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Aug 19, 2001
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8,902
Re: 1975 Mercury 20 hp throttle problems

Hi Adam,<br /> Sounds like your carburetor is in need of a thorough cleaning and rebuilding. You cannot clean it by simply spraying something down the throat - it must come apart. Get the carb kit from a Merc dealer, borrow a manual from the local library (or pick one up from here at Iboats) and go through it. Generous use of compressed air, carb cleaner and paper-clips should get you back on track....<br />- Scott
 

boatster

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Jun 13, 2004
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331
Re: 1975 Mercury 20 hp throttle problems

Thanks Chinewalker.<br /><br />Your probably right. I did a search under the keyword "bog" and it seems like alot of people either have fuel delivery problems or the carb needed going through. Once I figure it out I'll post my results so that someone else with the same problem can figure it out.<br /><br />Adam
 

LWW

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Jun 6, 2004
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Re: 1975 Mercury 20 hp throttle problems

I got the same problem with my 1972 model 200. the engine likes to be choked even when warm and bogs when trying to throttle up in fwd gear. I can get by it if I squeeze the primer bulb on the fuel line so I'm thinking I have low fuel pressure and will try changing the diaphragm in the fuel pump next (I already took apart the carb and cleaned every bit of it).
 

boatster

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Jun 13, 2004
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Re: 1975 Mercury 20 hp throttle problems

Ok update: I took the carb apart and found the float valve to be sticking in its seat. Since I just got this motor I don't know its service history but it seems either the float valve is defective or someone put an incorrect one. According to the Mercury book there was a float valve update that superceeds (sp?) the old original one. Either way I talked to a Mercury expert and she said that a stuck float valve would cause the throttle up/choke requirement because the engine is getting starved on throttle up. Once you choke the increased vacuum finally pulls the stuck valve down. LW, you might want to check if your float valve is getting stuck. It should be able to fall out of the seat by gravity alone. Anyways I just got my parts today and will be installing and testing my motor tomorrrow. I will give post test results. Hopefully this will come to a conclusion with this hypothesis. Fingers crossed.
 

boatster

Petty Officer 1st Class
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Jun 13, 2004
Messages
331
Re: 1975 Mercury 20 hp throttle problems

Well,<br />it all works sweet now. I put the carb back together and started her up with no problems whatsoever. I can throttle through the range without any hesitation. Anyone having throttle bog problems might want to seriously look into the float valve and pump diaphragms as well as setting the proper float tabs on the float. I also used the rubber and fibreglass gasket together in the one-way valves on the fuel pump. It makes sense that both belong: the fibreglass holds the rubber in place and gives it a bias towards staying closed and the rubber makes a clean valve closure due to its suppleness. Engine purs like a kitten. Good luck to those with similar problems.
 
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