Is a 40hp Yamaha Suppose to have a Rough Idle?

I have a 2004 40hp Yamaha that's new to me this year. I bought it earlier this year and the motor has maybe 30-40 hours on it. Ever since I've owed it the motor has a "lumpy" idle. At up to around 1500 it idles rough but after that start to smooth out and runs fine. I can troll with it at idle and it never stalls, just idles lumpy. Is this a characteristic of this motor?

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

Ruggybuggy said:

I have a 2004 40hp Yamaha that's new to me this year. I bought it earlier this year and the motor has maybe 30-40 hours on it. Ever since I've owed it the motor has a "lumpy" idle. At up to around 1500 it idles rough but after that start to smooth out and runs fine. I can troll with it at idle and it never stalls, just idles lumpy. Is this a characteristic of this motor?

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Is it a 2-stroke or a 4-stroke? I had a mid 90s 40hp 2-stroke yamaha that i found would idle a little rough. I adjusted the idle mixture screw on the carburetors to richen up the mix a little and that helped.

If it's a 4-stroke it should idle very smoothly.

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

Sorry, should have mentioned it's a two stroke. I tried adjusting the idle mixture screw without much luck. What surprised me was there really wasn't a setting that changed the rough idle much or RPM's other than screw in all the way and it would stall (I had it hooked to a tach). Me feeling is that it's running rich because it smooths out at higher rpm's. I was thinking that the enrichment system might be dumping fuel into the system. The few times I had it out it seemed to me that it was a little hard on fuel. Also noticed that while it's idling on the muffs that the white prop has black stains on it.

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

Have you pulled and cleaned the carbs?
If so you need to make sure you adjust the linkage and carbs like the manual says.They need to be synced so all 3 are at the same amount of open at all times.

They are not hard to do and Yamaha recommends cleaning fairly often just to make sure all jets and passages are clean. Usually do not need any parts unless you find a problem. Make sure you do not swap parts between carbs as they are different jets and make sure the 3 carbs are marked and go back where they came from

You can turn the red lever to the off position to close off the enrichment passages to check if that is giving you a problem and you can inspect the diaphragm to see if that is still OK

It took me over a year to finally find out a previous owner had drilled out the main jets in my motor causing on overly rich condition, but that caused a WOT miss.

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

Thanks guys for the replies. I haven't taken the carbs off yet and cleaned them. I just assumed that if I had dirty carbs I would have a lean condition and I would have a problem at WOT.

I've had the boat out only 3 to 4 times since I purchased the motor. I went out a coulple of days ago and had a full tank of fuel (5 gallons) and ran the boat for about 2 hours running at WOT and trolling and used about 4 gallons of fuel. Seems a little much for me comparing it to the 25hp Evinrude that use to be on the boat. I could run that motor a 6-8 hours on 5 gallons.

I'll check the diaphram on the enrichment system and report back. 99yam40 how is your idle? Is it smooth? My idle is so lumpy that it jerks the tiller handle as your trolling. My motor use to be a remote control model that was change to a tiller ('04 Yamaha 40hp were only remote control). As the rpm's start increasing the shaking starts to smooth out and at WOT it runs fine.

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

standard fuel burn for an outboard running at or near its rated output is 1 gallon,per 10 horsepower per hour.
its amazing and its very close on my F150,my 175ETEC and my modified little 9.9 jonny rude.
so at WOT your 40 should smoke through about 4 gallons an hour.
take my F150, at 5700 its burning 14.9 gallons per hour at 42 MPH.
back it down to between 3300 and 3700 and its burning right at 6 GPH between 28 and 32 MPH, wind and current affect it some.
your 3cyl 40 should idle and accelerate very smooth.
wont be like a lexus but way better than the 2cyl 40.
black goo on the prop indicates a poor running cyl and your seeing unburned fuel for whatever reason.

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

So I took the time to take the carbs off this afternoon and found just a small amount of dried fuel in the bottom on the bowls. Before I did this I check the fuel pump to make sure it's not drawing fuel into the crankcase. It was OK. Then checked the enrichment system to ensure that the diaphragm was OK by applying a vacuum. It was fine. One thing I thought was strange and I thought I would ask before putting the carbs back on, why is there rubber plugs covering the idle ports in the fuel bowl? All three carbs has them. The plugs are located right next to the main jet and block the idle jet. I've never seen this before and kind of make the idle jet pointless. Should I leave them out? Is it for emission reasons?

Thanks guys!

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

The plug forces all the fuel to flow through the main jet which controls how much fuel flows during WOT and everything in between. There is a passage between the two jets so I hope you made sure that was clean also along with the passages in the enrichment system.
If the plug was not there then it could pull fuel through both jets at high speed and would run too rich.
I do not know what boat you have but my 16" Xpress SV will get about 8MPG as long as I keep it around 4000-4500 RPM. Even the yamaha performance site shows the WOT GPH at above 5. real close to 6GPH so stay away from 5500RPMs

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

99yam40 said:

The plug forces all the fuel to flow through the main jet which controls how much fuel flows during WOT and everything in between. There is a passage between the two jets so I hope you made sure that was clean also along with the passages in the enrichment system.
If the plug was not there then it could pull fuel through both jets at high speed and would run too rich.
I do not know what boat you have but my 16" Xpress SV will get about 8MPG as long as I keep it around 4000-4500 RPM. Even the yamaha performance site shows the WOT GPH at above 5. real close to 6GPH so stay away from 5500RPMs

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Thanks for the tip on checking the passage between the two jets. I will have a second look. The motor is mounted on a 15' Princecraft fishing boat. I'm not two concern about the GPH, just thought it might be an indication of a rich condition. I installed new plugs today and will remount the carbs and see how it runs. If it still has the lumpy idle I'll post a video of it running.

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

please let us know if you get it fixed, i have a 1996 85 hp yamaha doing the same thing. i removed all three carbs & made sure they were clean & adjusted and still idling rough. mine is more noticeable when in the water idling out of the canal. it won't do it as bad with a hose connected to it.

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

I've think I made it a touch better. Purchase a service manual and did the "link and sync" and seemed to help. I found the ignition timing was off but not by much. I will be give it to the Yamaha dealer on Thursday to balance the carbs. I don't have a manometer so dealer will have to do it for me. I'll let you know how it turns out.

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

manometer is needed on a 4 stroke but not on 2 stroke. at least manual does not call for one. The carbs should have the final adjustment with motor in the water warmed up to proper temp and in gear at idle according to the manual.

What manual did you pick up?

What was you timing set at at idle and were you able to get it into spec?

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

I am really interested in how this story ends. From what I'm reading here, I have the exact same problem on my '04 90 hp TLR.

It was serviced for ethanol issues at a "reputable" marina in Norfolk before my purchase. Since then I found disconnected hoses and linkages and all the carb screws show marks from disassembly.

I found crud in the bowl and the drain screw was plugged. I can't pull the main jets without destroying the carbs. I managed to pull one of the pilot jets and clean it, stripped the head on another and will try the third shortly.

I also found an air adjusting screw on the side of the carb, but my Yamaha factory service manual does not call it out or describe its adjustment. I found a post here for a 4 cylinder two stroke that notes an idle screw adjustment of 5/8 turn from full closed. I'll try that if I can't find other guidance.

Beyond that, I guess I'll have to replace. Maybe I can just get the main body and jets and salvage the other parts.

Re: Is a 40hp Yamaha Suppose to have a Rough Idle?

If you would like some help from others ,then start your own thread stating model # and all other info along with questions. this one is on a 40 hp