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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10979
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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My 1993 9.9 Yamaha outboard just had its thoroughly

Customer Question

My 1993 9.9 Yamaha outboard just had its carburetor thoroughly cleaned. Although it seems to run fine at first, after a short while it starts to race and it will not go into neutral, forward or reverse without great difficulty. It continues to race until it has been turned off for awhile. Any help?
Submitted: 2 years ago.
Category: Boat
Expert:  Tan replied 2 years ago.
Hello. My name is***** is a lean fuel condition. Either fuel starvation or an air leak. Try this pull out the choke when it starts to race. You shouldn't need full choke just some choke try it at different positions. If no change squeeze the primer bulb. Does the engine run slower with these checks? Could I get the complete engine model number please?
Customer: replied 2 years ago.
What could cause fuel starvation?
What could cause an air leak?
Expert:  Tan replied 2 years ago.
Fuel starvation weak fuel pump clogged fuel filter, loose fuel line connection. When you do the tests the one that helps the engine run better will tell us where to look for the cause of the issue.
Customer: replied 2 years ago.
It will take me a couple of days to perform the tests and get back to you.
Expert:  Tan replied 2 years ago.
That's Ok . There is an inline fuel filter that could be clogged. The filter is in the supply line between the fitting on the engine and the fuel pump. Post back with your findings.
Customer: replied 2 years ago.
If the in line fuel filter is blocked how would this cause racing?
Expert:  Tan replied 2 years ago.
Fuel starvation. When an engine starts to run out of fuel it will start revving. This is due to a lean fuel condition. If you are getting air in the system it will do the ssame thing. With air you aren't getting the correct fuel to air ration which causes revving. To much air is the same thing as not enough fuel a lean fuel condition. The filter may not be completely clogged but restricted.
Customer: replied 2 years ago.
That explanation is very helpful. A fuel filter is very cheap and easy to replace. I will replace it after doing your tests and see if there is any difference. Also I could replace the fuel line from the tank to the engine if this is where air might be entering. It is fairly old.
What is your opinion about ethanol in an older engine like mine? Do you recommend additives to mitigate ethanol?
Expert:  Tan replied 2 years ago.
The ethanol will attack all the fuel components on this engine. If you can replace any hard fuel lines this will help. The carburetor has been rebuilt so that should be ok as all the new components are alcohol resistant. Leaving the fuel lines and the fuel pump. Once these are replaced the engine can use ethanol. If you can get away from ethanol great but if not use the ethanol additives only when you are going to be storing the engine. The ethanol doesn't do damage as long as you are using the engine it is long term sitting that the problems occur. I would run the engine out of fuel when storing then you won't have the issue. Post back with findings and questions. The question says open even after you have rated so post back at anytime.

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