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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 18750
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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I have a 2005 Yamaha F40 four stroke. Starts great, idles

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Hi I have a 2005 Yamaha F40 four stroke. Starts great, idles great, run good but when I return to idle after running the idle will run up to sometimes as high as 2500 before returning to 800
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. Is there anything else important you think the Marine Mechanic should know?
Customer: No that’s the only issue

Hello my name is ***** ***** am I speaking with? I read what you wrote. Can I get the model number on the outboard as it reads on the ID tag.

Customer: replied 4 months ago.
Customer: replied 4 months ago.
Customer: replied 4 months ago.

HI Michael, okay got it. The engine comes up as a 3 cylinder, 4 stroke, carbureted engine. Just want to make sure that's what you have.

Customer: replied 4 months ago.
Correct with 3 carbs

Got it. I more or less have an idea of what the problem is going to be. But I don't know how much you know about multicarb 4 stroke engines, or engines in general. Have you done any poking around on your own yet, looked the engine over, tried any troubleshooting, or just reading, or anything like that?

Customer: replied 4 months ago.
I have looked around a bit. It was idling down to 5-600 and would die. I adjusted the idle up to 850 after replacing the plugs. Running great starts easy and goes strong but does not like to return to idle
Customer: replied 4 months ago.
Fyi...I had the low idle before I changed the plugs
Customer: replied 4 months ago.
I adjusted the idle screw about 1/8 to 1/4 turn

Got it. How long have you owned this engine for, and does it get into a Yamaha dealer regularly for service? Have you ever personally seen it run or behave correctly?

Customer: replied 4 months ago.
I have owned it for 7 years. It’s had a low idle on startup for a while. The high idle after running has also happened before

Got it. I am attaching a picture. Let me know when you can see it, and I will explain what you are looking at.

Customer: replied 4 months ago.
Looking a the carbs parts explosion pic

In the picture locate #12. #12 is what is called a dash pot. All carbureted engines have a dash pot on them. With any carbureted engine, if you throttle back to quickly, the engine can stall out. What this dashpot does, is when you throttle back quickly, the dashpot prevents the carburetors from closing to quickly. The dashpot holds the carburetors linkage open and then allows that linkage to slowly closes, instead of just slamming shut. If the carbs linkages and throttle plate slams shut quickly, the engine stalls. If the linkages close slowly, the engine returns to and idle without stalling. Ultimately your problem is going to revolve around the carbs. You can have a sticking dashpot, sticking carb linkage, or sticking carb throttle valves. In the shop what we would do is pop the linkages off the carb and first make sure that the linkages, as well as each carburetors throttle valve (the butterflies) are not sticking and jamming up. If they are sticking or jamming up, then you need some oil/lubricant in those areas. If the carbs throttle valves open and close freely, without sticking or binding, then you have a bad dashpot, holding the carbs linkages open for to long, and not allowing the throttle valves in the carbs to slowly close. Does that all make sense to you. I do want to make sure you are happy with my service before you go. If you had a further or more specific question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

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Thank you

Customer: replied 4 months ago.
Will the carbs need to be readjusted after R&R the linkage? How to a solve a bad dashpost?
Customer: replied 4 months ago.
Will the Carbs need to be readjusted after removing and replacing the linkage. If it’s the a dash pot problem how do I fix it.

Good morning, sorry for the delay. When you replied back last night I had already logged off for the evening. Yes If the carbs come off they do need to be resyncronized as per the factory service manual. There is a special tool to do that called a Morgan Carb Tune. All shops have these tools. Was there anything else?

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