1995 Mercury Sportjet 120hp bottom carb problem

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yesizmatt

Seaman Apprentice
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Sep 30, 2013
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I have been working on a 1995 mercury sportjet 120hp and have good spark and compression on all cylinders. The engine hasn't been running very good and after doing tests I found out that the bottom 2 cylinders are not getting fuel, since the spark plugs are dry. This leads me to believe something is wrong with the bottom carb. I have recently cleaned both carbs but am currently re cleaning the bottom one. When I took the carb off the motor, the bowl was full of gas, so it is getting fuel inside of it. I'm guessing its one of the jets but the all seem clean and clear of debris. Another thing I have noticed is if the engine sits for a day it will fire up the first time almost as if a bit of gas moves into the cylinders slowly, but once it starts and that gas it burned up, the carb wont supply it with more fuel. Any thoughts?
 
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leonreno

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Mar 17, 2011
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Not an expert or anything but my thought would be a problem with the reed valves. Have you checked the throttle plates to see if they open and close in sync. Don't know if it's possible with your motor but if you can swap the two carburetors you can narrow down the problem.
 
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yesizmatt

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Sep 30, 2013
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The throttle plates open and close in sync. Later today I will try putting the top carb on the bottom just to see if it will get gas into the bottom two cylinders. If the bottom cylinders still are not getting gas, then I assume it will have to be the reed valves.
 
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yesizmatt

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I do not know much about reed valves, can you adjust them if they are not working properly or will they need to be replaced?
 
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leonreno

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Mar 17, 2011
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They have to be replaced, sometimes they crack or chip. If you pull off the carbs they would be behind them. It could be something else also, I am no expert by any means. Do be careful running the engine with no fuel to any cylinders, no fuel means no lubrication, so be careful, might spray some fogging oil in the bottom cylinders until you get fuel into the cylinders or poor in some 2 cycle oil through the spark plug holes mad turn the flywheel.
 
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