Mercury 3.5HP 2009 4 stroke just idles

A

AAK

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TECHNICAL Contributor
Feb 14, 2009
135
Perry Hall , Md
Boat Info
2000 38 Sundancer
Engines
7.4 Horizon FWC
I've got a 2009 Mercury 3.5 HP 4 stroke outboard with about 15 hours run time on it that I bought awhile back. It starts right up and idles but if I turn the throttle up it stops running almost immediately. If I pull the choke all the way out while it is idling it will run at high RPM's. Push the choke back in and it will only idle.

I checked the gas filter in the fuel supply hose at the shutoff valve and it was clean (I even left it out to see if that was the problem). I took the carb off to clean it and took out both jets to clean them. The only thing I haven't taken apart is the needle seat on the bowl float. It looks like it is moving freely with no problem.


Any ideas on what's wrong or what to try next?

Thanks,
Al
 
Sounds like an air leak, maybe. Check carb mounting bolts/nuts or any hoses attached to carb, also look at carb gasket for damage.
 
Carb is gummed up or something is not working it is hard to clean them right. I had the same problem. Is it dumping fuel into the water? If so make sure you change the oil as well. I now run mine dry every time, by disconnecting the fuel line and running it until it dies.
 
Last edited:
Sloburn & Skolbe,

Thanks for the replies.

The gaskets look great and all of the hoses are on the carb. No fuel in the water, I was running the engine in a bucket of water to see if it was dumping anything.

All of the mounting bolts are good and no cracks, leaks, or damage on the carb.

On you tube, google, etc.. the vast majority of problems with these motors are ethanol fuel related. I'm thinking now that I might have to just let the carb sit over night in some carb cleaner and then blow it out with compressed air. Maybe the spray carb cleaner didn't get it clean enough.

Keep the responses coming!!!!!!
 
You will need to drill out the plug covering the low speed idle air mix screw. There are videos on YouTube on how to do this.


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H20NUT,

OK, I give, now I'm somewhat confused, the low speed idle is the only spot this motor is running right with no choke on. In other words it's idling perfect. It's when I try to increase the RPMs it dies unless I also use the choke. I hope you are right but could you help me understand how this works to stop this problem? I do appreciate the help, but I just want to make sure I understand what's going on.
 
I got frustrated with mine and ended up buying a new carb and now run it dry every time. The new carb was $80 and runs perfect.
 
skolbe said:
I got frustrated with mine and ended up buying a new carb and now run it dry every time. The new carb was $80 and runs perfect.
Click to expand...

That's what I do on my equipment when its been sitting and I can't get it running within an hour trying. Not always new either. My lawnmower or my mercury friends have given me several working used carbs through the years.

If you can get it for less than what an hour at a shop costs, it seems a no brainer. Just save the old one and fix it in the winter. Then you can swap it when it happens again.
 
I am not sure on the technical side of how it works. We have done 2 6hp Yamaha's and 3 3.5 Mercury's. All needed the choke just as you described. The only outboard that I can't get to run is my 4hp Yamaha. It's been cleaned 4 times, blown out and even put in an ultrasonic cleaner, still no go...


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H20NUT, In other words drill the cap out, remove the low speed idle air screw, clean it with some carb spray and put it back together. I wasn't trying to say it wouldn't work, I just wanted to know why it would work. I can handle "because it does" LOL Thanks Again
 
skolbe, Can you tell me where you found the carb for $80. I'm finding much higher prices. Thanks Al
 
Ok... Because it does! LOL. Not always as my 4 hp has proven that wrong.... Figures it works for everyone else I have done this for, but not me.

I am seeing prices over $200 for carbs...


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I'm seeing $200 for the carb too. I forgot to ask you, when you drilled/took the cover off did you put anything over the hole? Supposedly you buy a carb rebuild kit for like $50 and get a cover. I really don't want to ge ta rebuild kit for just a cover.
 
H20NUT,

I drilled the brass plug out, took the adjustment screw out, cleaned everything with carb cleaner, put it back together and it will still only idle. The only thing I can figure is the carb must still have a passage way partially blocked. I'm thinking it needs to be soaked in a bucket of carb cleaner over night and then blow compressed air through it.

Anybody else have any ideas on what to try next?????

Thanks in advance
 
Same issue with mine right now. Cleaned and blown out numerous times as well as a dip in the ultrasonic. Time for a new carb I would say...


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H20NUT, I'm going to try an overnight soak (with the Berryman's B-12 cleaner), then compressed air and hope for the best. I would really like to know what's going on with this carb since I'll bet it's nothing more than an ethanol gas problem that I haven't taken care of yet. Of course I'm still looking for the $80 carb LOL Let me know if you find one!!!!!! I found a couple of posts (on other web sites) with our similar problem and the B-12 soak worked for them even though they had already tried spray carb cleaner with no luck like us.
 
H20NUT, I have the carb apart and soaking it in the B-12 now as I'm typing this reply. I put it in the cleaner around 11:45AM today. I'm going to let it soak at least 24 hours and then blow compressed air in it and let it soak some more. I'm hoping this it. I'll let you know when I'm done one way or the other. I'm still looking for an $80 carb!!!!!!!!!!!!!!!!!!!!!
 
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