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Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10802
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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High RMPS at low speed. I have a XR6 150 Mercury...runs good

Customer Question

High RMPS at low speed.
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: I have a XR6 150 Mercury...runs good but the rpms don't match the speed. Changed the humb...same results. RMPS raise when I use the trim switch.
JA: Is there anything else important you think the Marine Mechanic should know?
Customer: no
Submitted: 10 months ago.
Category: Boat
Expert:  Jason replied 10 months ago.

Hello my name is ***** ***** am I speaking with?

What kind of RPMS and speed are you getting? What kind of boat is the engine mounted to?

Customer: replied 10 months ago.
5 to 6000 rpms at 30-40 mph and I have a TR-186 Triton
Expert:  Jason replied 10 months ago.

What is the story on the engine, has it ever been any different for you in the past, or did you just buy the boat?

Customer: replied 10 months ago.
I've had this boat for over 10 years..brought it new.
Expert:  Jason replied 10 months ago.

What kinds of questions did you have? Are you concerned that the RPMS are to high, or the speed to low? How is it I can help you?

Customer: replied 10 months ago.
RPMS to high. Speed doesn't match. Have to keep the RPms down to keep the motor from stopping when running down the lake.
Expert:  Jason replied 10 months ago.

The first thing to do is to hook up a good accurate shop tachometer to the engine to see if those engine rpms are accurate or not. It might very well just be a bad tachometer not reading correctly. Is that something you can do?

Customer: replied 10 months ago.
Customer: replied 10 months ago.
What if it's not the tach...
Expert:  Jason replied 10 months ago.

If it's not the tach then the propeller has to much slippage in it for some reason. You did say you replaced the hub. What kind of condition is the propeller in otherwise, is it in perfect shape or does it have dings in it?

Customer: replied 10 months ago.
Propeller is in good shape..no dents. Yes replaced hub.
Expert:  Jason replied 10 months ago.

Out of curiosity what did the boat used to get for RPMS and speed?

Customer: replied 10 months ago.
4700 at 50 mph
Expert:  Jason replied 10 months ago.

Is that speed measured by GPS or a boat speedometer?

Customer: replied 10 months ago.
Boat. I have changed spark plugs and installed new tech..starts good and now the idel is very high. How to adjust idel?
Expert:  Tan replied 10 months ago.

Hello. My name is Tan.

What kind of rpms are you talking about at idle? Have you verified the roms of the engine with a known good tach. Does the engine sould like it is turning the rpms the tach says?

Customer: replied 10 months ago.
Expert:  Tan replied 10 months ago.

Check the throttle plates are they completely closed?

Could I get the engine serial number please?

Customer: replied 10 months ago.
5m9xmo2.51co. Yes
Expert:  Tan replied 10 months ago.

That isn't the engine serial number. http://outboardpowerheads.com/serialnumbertag.jpg

Customer: replied 10 months ago.
18116982 04
Expert:  Tan replied 10 months ago.

With engine idling push the choke does it slow down. The only thing that will cause a fast idle is not enough fuel or to much air. Since you have verified the throttle plates are closing that mean you have not enough fuel. This is caused by clogged carburetors in the idle circuit.

Have you tried adjusting the idle jets? Has the engine been sitting?

Customer: replied 10 months ago.
Where are the idel jets located.
Expert:  Tan replied 10 months ago.

#10 If the plastic covers are still on remove them and toss them. There are 2 on each carburetor.http://www.marineengine.com/parts/mercury_marine_parts/static/img/COMMON/2541.gif

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