Merc 3.5 help needed.

Hello fellow forum members.

I am hoping you can shed some light on my dilemma?

Warning ?. long post.

I have a love/hate relationship with my 2008 Merc 3.5 - 4 stroke outboard. Well to be honest I don't love it at all ?. but I digress. I bought it in 2009. It is used exclusively on my little sailboat to get in/out of the harbour. That's it. Very short running time. Sometimes if the wind goes slack I have to motor in but in reality it isn't used much. I might go out a couple times a week in the summer months. I live in the Pacific northwest so summers are short. It is laid up for the winter from approximately October to June. So all in all it has had very little running time since purchase, but for such little running time, it has had, (and still has), a lot if issues. It just doesn't run well period, and never has from the get-go, but from my research into the subject some of that may have been my fault? Although I do rinse it every time after use in fresh water, I haven't been draining the carb. Also, I have on occasion been using older (but not ancient) gas. I am not very mechanically inclined with motors but I am now retired with time on my hands and I want to learn. I want to be able to fix this motor as best I can or at least get it running correctly, or I will have to get rid of it. It just isn't reliable. I live a bit remote and taking it to the dealer is both expensive and time consuming. I want to learn the basics and do things myself. I am sure many of you can appreciate that mindset? Since I don't have a mechanical background I have to do the research and ask questions on forums like this in the hope that some individuals will shed the knowledge I need. I "think" at the juncture that I may need to clean the carb? I am not sure what else is causing these problems?

So a small bit of history?.. last year, it would run for a bit then stall. It was impossible to restart until about 5 minutes had passed. Took it to the dealer (under warranty - whew) and it was deemed that the valves had carboned up?? What, how could this be after literally less than 5 hours total running time??? Anyways after 4 MONTHS in the shop it was finally repaired (the head gaskets were on back order). This left a very tainted taste in my mouth about Mercs and their turnaround time for repairs. And yes, once opened up they said the valves were gummed up and gave themselves a pat on the back. I still don't know why that happened? So now, this summer, it is acting up again in a similar fashion, but of course I am now out of warranty. Wah. It is not stalling after a few minutes of running but if I stop it for any reason I can't restart it for at least 5 minutes. It will stall on occasion. Yesterday, for the first time since I've owned it, I went for a long motor trip 1.5 hrs each way steady running. Again, if I stopped it, 5 minutes to wait till restart. After this trip I noticed the oil was very dark in the inspection window. Today I changed the oil (it was black, not sure why), replaced the plug, emptied the gas tank completely, put a mix of gas and SeaFoam in, and ran it. It did start and seems to idle fine but here's the issue. If I give it just a "titch" of throttle past idle it seems to cough and wheeze like it wants to stall. It sort of kicks into the next level of having more gas and seems to run ok at higher throttle (this is all in the fresh water bucket not on the boat). HOWEVER, if from idle, I QUICKLY turn up the throttle it STALLS EVERY TIME ???? It restarts on one pull at idle speed (no choke required) and it coughs, wheezes, just past idle as described before, and it still stalls every time if I gun the throttle quickly past idle.

SOOOOOOO ?. what is wrong ????

I am willing to take a shot at cleaning the carb (bit scary for me but I think I have seen enough videos to take a shot). Also, I do not know how to properly adjust the jets? I am hoping there might be a couple other things I can try before I need to do the carb? I am toying with the idea of buying a replacement carb so if I screw up the cleaning at least I will have a running motor with a new carb and I can just take the old carb into the shop to let the $70/hr guys put it back together. It's expensive to do this though, but I would learn.

Thanks in advance and sorry for the long post.

PS: I have 2 Honda Generators and a Honda Commercial direct drive lawn mower. They start every time usually on one or two pulls and run perfectly. They sound great. they have never let me down. Just like engines should, but this little outboard motor has been such a P.I.T.A. for me. Exactly the opposite of what I need, "reliability". Please, help me fix this thing?

Oh ... I forgot to mention how delightful it is to try and reconnect the plastic linkage between shutoff valve and red lever underneath. What a piece of design concept that is ???

Re: Merc 3.5 help needed.

After discussion with Mercury Marine Tech Support it points a serious finger at Carburetor cleaning .... so that's my next move.

Going to take a shot at it myself.

Re: Merc 3.5 help needed.

OK ... so .... I cleaned carb. As far as I can tell that thing is "hospital clean" unless I am missing something? My problem still exists. It starts ok. It idles ok, but when you give it a bit of throttle it "hesitates" like it wants to stall, a small cough per se, then it revs up as you increase the throttle and seems to run ok. HOWEVER, if you advance the throttle QUICKLY from idle it STALLS EVERY TIME. Not sure why or what do to next. Please help.

OH, one more thing, on this carb there is no "mixture screw". There is a brass plug in the location where the Pilot Mixture Screw is showing in the manual. This has me a bit confused.

Re: Merc 3.5 help needed.

Hello Beacher,

Just a guess, since i had similar problems on my 4 hp fourstoke yamaha on my sailing boat. It took me a long time to figure it out.
Basically you're probably not using your outboard enough, causing the fuel to gum up in your carb and clog your nozzles. the jet holes are very tiny on my carb. Second result of using it very little and leaving it outisde was that i had lime in my jets. I soaked the jets in vinegar for a night and than poked with a starnd of coperwire carefully through the small hole. Carefull so i would not damage the small orifice. I don't think carb cleaner get's that out.

And the carbon depositis on the valves could be a result of that , getting a too rich mixture (?), but like ai said, just a guess

The pilot screw is probably covered because of epa stuff.

Re: Merc 3.5 help needed.

Thanks Mrteenman,

The local Merc Tech on the phone suspects it might have something to do with the gas mixture (which can't be adjusted due to the plug in the mix screw slot which I found out is an EPA thing). I will also re-clean the carb (it looks pretty darn clean to me but obviously can't tell in the small jets). I didn't soak it, just carbon cleaner sprayed it and spray can compressed air'd it. You're right, I don't use the motor much. Just to get in/out of the dock. I now "condition" all my gas and will really drain that carb and clean it again when I winterize the motor this fall now that I know how to do it.

I'll try soaking the jets in vinigear like you did and carefully poking the holes. Will see if that does the trick? I wonder if Merc sells just the jets or if you have to buy the complete re-build kit?? That would be a quick and easy solution plus I'd have spares.

The good thing about all this is that I can now disassemble and clean that carb. Something I had no clue how to do before. I am thinking that eventually I will obtain "master carb cleaner" status ... lol !~!

Re: Merc 3.5 help needed.


OK ... so the "wind up" is .... several carb cleanings did not fix the guick throttle up stall problem. Local Merc Dealer said it was probably the rich/lean mix ... so I popped out the plug and started to experiment (I do have a service manual btw). I discovered that in fact if the mix was too lean any quick throttle up would stall. The Merc Dealer was right. I backed out the mix screw 2.5 times and the idle started to lower a bit, I settled for 2.25 turns out. Once the motor warms up the quick throttle up no longer stalls. The motor runs better than ever at this juncture, so I think I have solved my problems .... and learned a few things in the process ... which are ....

1) Old gas is bad for these small engines
2) Always "condition" your gas, Sea Foam seems very popular. Merc also makes their own product.
3) Anyone can learn to clean these little carbs. I can now take mine off, apart, clean, re-assemble, and back on in 1/2 hr.
4) The mix ratio is not necessarily correctly set at the factory (Murphy's Law). It may need re-adjustment.

Thanks Mrteenman for your comments.

Re: Merc 3.5 help needed.

OK ... so the "wind up" is .... several carb cleanings did not fix the guick throttle up stall problem. Local Merc Dealer said it was probably the rich/lean mix ... so I popped out the plug and started to experiment (I do have a service manual btw). I discovered that in fact if the mix was too lean any quick throttle up would stall. The Merc Dealer was right. I backed out the mix screw 2.5 times and the idle started to lower a bit, I settled for 2.25 turns out. Once the motor warms up the quick throttle up no longer stalls. The motor runs better than ever at this juncture, so I think I have solved my problems .... and learned a few things in the process ... which are ....

1) Old gas is bad for these small engines
2) Always "condition" your gas, Sea Foam seems very popular. Merc also makes their own product.
3) Anyone can learn to clean these little carbs. I can now take mine off, apart, clean, re-assemble, and back on in 1/2 hr.
4) The mix ratio is not necessarily correctly set at the factory (Murphy's Law). It may need re-adjustment.

Thanks Mrteenman for your comments.