Hi. Suzuki S40, new 2014, 3100 miles. Bone stock. So I was riding today. The weather was hot as blazes, humidity way up, ozone bubble, the works. Went for a fairly long ride; the bike needed no warm up and was silky smooth the entire ride. I don't mean the growling/rumbling went away; it's just that there was no slight jerking sometimes or odd Thumper behaviors.

So other type days I'll have to warm the bike and it will be balky for a mile or so before it gets it's first wind.

What gives? Atmospheric conditions affecting carbureted bikes? Just curious. Oh, and I use 89 octane gas.

david

Yep, the surging you experience is because you're running lean. You need to rejet a little richer for a smoother running bike.

Lots of things affect a carb bike. Atmospheric pressure, wind direction ( you might not realise you are riding into a slight side wind but it does make a performance difference) and temperature. Manufacturers tune bikes to run properly at sea level and 24 degrees C. Anything different to that and it won't run perfectly without some resetting! I also think your attitude on the day also makes a difference in how you perceive the bike to be running. I know mine seems to run better on certain days!

Once you get it jetted
Right
It's Still gonna run better or worse depending on temperature,humidity, barometric pressure.
That's why, back in carburetor days, drag racers were forever rejetting.
You're needing to find the jetting that works best for your
Elevation and typical atmospheric conditions.
I'm just smart enough to grasp That.
There are people here who can actually help you do that.
One DAY, ONE, day, mine was just Running like a scalded dawg,
And I was on I 20, on a cement section, and it busted through the 100 MPH mark.. Right conditions,
A carburetor IS a crude fuel metering device. An infernal combustible engine IS a crude animal. Considering how they work it's kinda miraculous that they don't self destruct in about five minutes.
Think abowdit, you've got an aluminium slug on a stick flying up and down in a cylinder. Up at the top, you've got rings. The top ring is designed to press out, it's edge tapered, literally designed to scrape at the cylinder, but somehow, through metallurgy and lubrication wizardry, they don't gall together and screech to a halt. The ring, so well designed, does wipe the cylinder and seal pretty well, is seated by the pressure of combustion, and moves down the cylinder without trying to remove material, it's just beautiful,,, but they Should die,, it's just Too Harsh of an environment and the parts should eat each other.. But, they run and run... And I just love them.

Get yourself jetted.
Find out what your elevation is. Tell us. Where you are, what is your average day like. Summa these guys are fluent in jetting.
I still need a translator.

Hi Batman and JOG. Great comments; thanks. Ok, Raleigh, NC is at 315' above sea level. Our temps are: hot in the summer (high 80's to high 90's, with typically mid-to-high humidity), not too bad generally in the winter (yes, I'll ride then too) -- 30s to 40s for a few of the colder months. I just guessing from experience here; no scientific data to back it up other than the sea level height.

The bike seems to run better in high heat/high humidity days. A little less so on those dryer warm days we get occasionally What do you think? Serious rejetting needed or something less onerous.

I typically stay in the Raleigh area with a planned trip to the mountains sometime relatively soon

So I'm interested in a rejetting kit that would accommodate my riding conditions.Thanks.

By the may. I did mention the S40 is bone stock, with the exception of a battery tender. Thanks for the information.

david

The temperature and humidity affect the way an engine runs, and cold dry air makes the most power when the engine is jetted properly.  Folks who race are constantly changing the jetting to work with the atmospheric conditions during the race.

You don't have to be that fussy, and a good jetting change will work in most conditions just fine - I don't know of anybody that changes jetting to suit the weather on a street bike.

I ride in very similar conditions as you 500 - 800 elevation.  A #150 main jet, # 50 or #52.5 pilot jet, and 3 washers on the needle are a very good place to start.  If you have a lean surge just off idle.....then remove 1 washer on the needle and use 2 washers.

I have had this combination at the TN/NC rides and when you get up to 5,000 feet it might be running a bit rich - but not so much that you need to make any changes for that 15 minutes that you are up at that elevation!

(The one exception that I might consider about rejetting - is that if you live at 6,000 feet and above and have jetted for those conditions....it might be worth a jetting change if you are going on a long tour to a much lower elevation, as the bike will be running leaner when you drop down to the low elevations).