cold air = lean?

I took my moped out for a run today, since it was sunny out and seemed like a nice day to go out. So i bundled up and the 'ped started right up. I let it run for a few minutes, and it seemed to be idling fine...but when i started riding it around, the acceleration became kind of erratic. I know colder air = more oxygen..so is it possible that the colder air is making the mix too lean? Because if i would stand in one spot for a while..it seemed to level out a bit..

I never had this problem in my car, but then again my carb wasnt exposed to as much air as a moped would be...

Just wondering if anyone knew?

thanks

--Nick

RE: cold air = lean?

Ron Brown /

Nick,

Cold air = rich, I think, because high altitude = lean.

You do not mention the temperature, but the most likely problem is condensation or icing in the carburetor caused by the additional cooling effect of gas evaporation. Standing still allows engine heat to warm the carb.

If you can rig it, some cars get around this problem by directing warm air from the exhaust manifold to the air intake. I imagine that the cylinder fins would also be warm enough. You may even be able to fashion an air funnel around the carb to warm up the outside of the carb with engine heat.

Good luck,

Ron

RE: cold air = lean?

Cold air is leaner, but just a tiny little bit. Because of the cold, the air is denser and more air can go through the carb while the amount of gas mixed through it wil stay the same. Some high-performance supercharged cars have something called an intercooler, this cools the air just so the engine can take in more air, the cars carburettors are adjusted to perform propperly with the cooler air. Also, when the engine of your bike is still cold, the air will not be warmed up by the carburettor as it passes through and the mixture will be much too lean, that's why the carb has a choke, so you can block the airflow patially to make up for the larger amount of air coming through the carb. As the engine warms up the carburettor, the carburettor will warm up the air passing through, so when you're driving in cold weather the mixture will be a little leaner but not much. This is also the answer to why engines have trouble starting in cold weather.

RE: cold air = lean?

a cold engine will always act alittle strange. i found this happens with my moped when it is cold. once the ped ahs been running for a good hour it stopps doing that totaly. or atleast mine does

RE: cold air = lean?

Yes.. Cold air makes an engine run leaner..(more air molecules, mixing with the same amount of gas equals leaner )

........and they don't run right til they fully warm up.

RE: cold air = lean?

Ron Brown /

Fred,

Am I backwards on my high altitude = lean?

Ron

RE: cold air = lean?

High altitude makes the air thinner, so you'll get less air through the carb wich will make it run richer. Because the air is thinner the air is leaner, but that will make the mix in the carb richer.

RE: cold air = lean?

Yes... higher altitude means less air with the same amount of fuel...which means it will be running a bit richer.

RE: cold air = lean?

Ron Brown /

Is anyone considering the Venturi effect in the carburetor? Lower density air = less pressure drop = less gas through the main jet.

Ron

RE: cold air = lean?

We don't need to.

Extensive testing by motor companies and racers has proven all these facts years ago.

I know people from Michigan who went on a 10 day trail riding adventure vacation to Colorado this summer...taking their own bikes... and they described the richer running characteristics they had when they reached high altitude... less HP in general though.. because of the thin air.

And you describe "less dense air creating less pressure drop..sucking less gas"

Can you show that there is LESS pressure drop at high alt than there is at low alt?

Maybe the pressure drop at both alt's is the same?

Though I have been told that this richer/leaner condition is small (few of those guys of those guys bothered to re-jet)... but the HP loss due to thinner air is more noticeable

RE: cold air = lean?

today i started my bike adn let it run on choke for 5 mins. i get on it adn give it full throthle and it pops off choke like its suposed to and it go about 20 feet then bogs down adn stalls. so i rode it on almost all the way choke but every time i let off the choke it would go for 20 feet and bog then lt would loose power.i ran it for a good 20 mins and still had the same prob. i think the carm might be dirty but it just started doing this. what do you think?

-mike

RE: cold air = lean?

Ron Brown /

Fred,

Thanks, I went back to my copy of Zen and the art of motorcycle maintenance and discovered that his pipes were turning blue when he left the mountains. Forgive me my transgression, it has been 25 years since I read it. Looks more interesting than I remember, I think I will read it again.

Incidentally, with my mind backwards, I was thinking of the problems aircraft have in developing lift in thin air. Obviously is different in carbs.

Thanks again,

Ron

RE: cold air = lean?

i have found that colder air makes my tomos run faster, because it is denser

RE: cold air = lean?

"It just started doing this".... and it's good and cold isn't it?

My 78 Hon PA50 did the same thing on the way to the store with the 90 hp moped guy

That is 2 miles... and it was below freezing... and with the choke on it would run.. and as i went to more throttle the choke would flip off by itself... then it would run about 200 feet and bog and stall... it did that bout 8 times before it warmed up enough to run without choke... it also was still 5mph short of its normal 30mph top speed

If yours will not run without choke after 20 min then it would seem that its dirty inside like you said... restricting the air intake a little would help too.

I think many moped carbs are a little too crude for good running all the time... (mine is.. I think)

jetting

All this discussion (for me) involves changes in altitude OR temp individually.

In other words, a change in altitude without a change in temp...which is almost never the case.

Higher alt (by itself) would require slightly leaner jetting.

Colder temps (by itself) would require slightly richer jetting.

But going up in altitude will almost always result in cooler temps.

So the two effects offset each other to some extent.

That is why all professional race teams carry their own weather stations with them...which have temp, humidity, and barometer measurements... and a book with charts so that they just find the current values and read the necessary changes to be made...so they don't have to guess.

The rest of us ride around with jetting that is good (hopefully) for average conditions.. and a little off for unusual conditions.

RE: jetting

Ron Brown /

Fred,

Thanks for the response, I realize we are all running slightly rich most of the time to allow for variations which could result in a too lean condition.

As for race tuning, I liked a quote attributed to Joey Dunlop, in a obituary type piece in Road Racing World.

Apparently, at some kind of celebrity appearance in Australia, Joey was happily wrenching away on a 2-cycle race bike and the writer asked him how he selected his jetting. His response was "Lean it out until it siezes, then go up one size".

Ron

RE: cold air = lean?

The carb on the Vespa I used to have was crap, it really was the most worthless carburettor I have ever seen. Old Honda's (like my own) are equipped with Keihin carburettors, you can run with them for as long as you want. Keihin carburettors are a lot better then a lot of people think, they want a 27mm Mikuni flatslide or a Dell'orto, and when they've got money they want a Takegawa, Daytona or Kitaco high performance carb, but Keihin belongs right among those three high quality carbs.

I've got a 20mm Keihin carburettor on my Honda SS50, it's originally the carb for a Honda CB125. It runs great, even when it's freezing, and it delivers a lot of power too.

RE: cold air = lean?

well i fixed my own problem. im just let it run longer on choke before i hoped on

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