Any tricks to cold weather starting

33 replies to this topic
  • lewichris

Posted January 03, 2002 - 10:46 PM


does anyone have any tricks to cold weather starting? i am having a hell od a time and we are going to go ride the dunes in Moses Lake WA, soom and i need her to run. Any advice please let me know.

Thank you

  • Taffy

Posted January 03, 2002 - 01:18 PM


yep i've got one! don't listen to people who say that the air is more dense in the winter!

now tell me how much choke you needed and for how long. then tell me what PAJ/PJ/PS you run as well as needle, clip position and MJ.

most of all describe your starting symptoms.


  • lewichris

Posted January 03, 2002 - 01:44 PM


well taffy to be honest i dont know any of the jetting answers cause when i bought the bike it was already jetted for the area so i havent changes a thing. And yes it always runs like a raped ape and it usually starts 1 or 2 kicks. I pull out the choke, do the whole starting ritual with the compression and then kick it over and over again. It dosent seem to fire. everyonce in a while i get something but then nothing. No gas of course with my hand on the handlebars. Keep in mind it is about 20-30 degrees here. Can it be started.

[ January 03, 2002: Message edited by: lewichris ]

  • Guy

Posted January 03, 2002 - 02:01 PM


:) Mine started first kick, and ran fine all day on Mon, temp about -8 deg C give or take, So they run fine in cold weather, Now this might sound very obvious, there any fuel in the tank?, has it gone onto reserve, cos I've been there, Kicking the hell out of a YZ250 for nearly half an hour before someone said..."Is there any fuel in it?" :D

  • KBOO_426

Posted January 03, 2002 - 03:14 PM


My 01 426 dosn't like the cold at all it was +9deg f here and i couldn't get it to start at all. it hadn't been started for appox 3 weeks. I tried to fire her up and it wouldn't fire I could kick the bike all the way though as many times as I wanted to, without the decompession lever pulled. I put it back in the garge and turned on the heater for about 10 min. And she fired on the 2nd kick???? Ran great afterwards but it don't like the cold.

[ January 03, 2002: Message edited by: KBOO 426 ]

  • AK-thumper

Posted January 03, 2002 - 06:19 PM


I picked my bike up last feb (01 WR426). I had a lot of trouble starting it because it was about 20 degrees out. It wouldn't start with the choke on but wouldn't run without the choke(or maybe the other way around, I can't remember). Had to have someone help me start it by operating the choke at just the right time. After changing the jetting to the JD recommended settings it starts fine when cold. Just have to adjust the fuel screw a little. The stock jetting was way lean for me in cool weather at sea level. I have found JDs pilot jet/fuel screw observations to be true, bigger pilot(48) with less screw has better bottom end, stall resistance, and easier starting vs. an equivalent smaller pilot with screw turned out more.

  • Taffy

Posted January 03, 2002 - 11:57 PM


last chance chris.




  • lewichris

Posted January 04, 2002 - 05:41 AM


well if this is my last chance it dosent start at all right now. I got the light to come on a couple of times but it didnt stay running. I noticed this happened when i had the gas on and kicked it a few times and then turned the gas off. it would fire two times in a row but not enough to get going. Would it hlp if i heated the bike up with a heater? Or do you mean how does it run when it is warm.

I ride between 2000'-5000' could you tell me the jetting i need and i will check out what i have in it. Thanks alot. I ride a 1998 WR 400 if that makes any difference. WR timed with a mega max FMF exhaust. anything else you need. thanks for your time.

  • wr400f

Posted January 05, 2002 - 06:47 PM


When my wr400 is cold (35-45 degrees out side) about 3 full pumps on the throttle with full choke fires it first kick. Sometimes it will stall and take one more pump, and it fires right up and idles with full choke. Leave it on full choke until it starts to falter,turn the choke off and it should idle. Sometimes I have to pull the choke out about half way for another minute, then it's fine for "first kick" starting all day. If it's 50 to 60 degrees outside usually 2 pumps fires it first kick. I have the stock jetting (168 main) in it and it runs a tiny bit lean in the cold weather until it warms up good ( approx. 10 minutes) then it's fine. Figure out your bike's "personal" habits and it makes your riding a whole lot more enjoyabe.

  • wr400f

Posted January 05, 2002 - 06:54 PM


I forgot one thing.. I know my bike well enough that if it don't fire by the 4th kick it's the spark plug, so I don't even kick it a 5th time. Keep lots of spark plugs handy, when they go ...they go. Hope it helps.

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  • Rich_in_Orlando

Posted January 05, 2002 - 09:08 PM


Most, if not all, of any cold starting problems I have had are due to my filter being over-oiled. The cold thickens up the filter oil and causes the bike to run/start poorly. Solution: I oil my filter even lighter than normal in cold weather. (Cold weather for Florida is below 50° F.)

Taffy, I just don't see how you can say that colder air isn't denser than warmer air, all other things being equal. You mentioned in another post that the weather map always shows low pressure when it's cold and high pressure when it's warm. That's not my experience. Temperature and pressure in the atmosphere are independent of each other. Air behaves pretty much like an ideal gas. So, air density increases as temperature decreases or as pressure increases. Could you please enlighten me to your thoughts? Am I missing something?

  • Taffy

Posted January 07, 2002 - 04:28 AM


first of all i've just watched a weather chart for the USA on CNN. now there's no if's or but's about this, it showed poor weather over florida and the letter was L and over the rest of the states it said H. so that's a fact.

secondly, in the winter there is more moisture in the air which reduces the amount of all the gases that make up air including oxygen of course.

thirdly in the winter fuel has a job to atomise so you are more likely to have starting troubles and flooding.

fourthly if you are rich in the summer you'll get away with it perhaps but you certainly won't in the winter.

i can't see why anyone is running the rich jetting they're so proud of. i've drawn together a little idea on jetting based upon your starting procedure, however one person in particular and a couple of others whom i've tried to help have proved totally incapable of diseminating their starting procedure so i can't help.

it stands to reason that we should ALL be using choke toi start our bikes and that we should take a period of time before we press it in. i've never been able to start my bike without choke. i've had periods where i needed more or less and even had the dreaded plug fouling as well. but i've come through that period and i'm much the wiser for it.

most of the riders and i have to say they are the americans, just won't go out there and set them up.

we have very few aussies on this site but many of them have got stuck in and so has the "not so swiss" missile. sorry but that's that.

we had the case of a load of lads turn up in the desert for a huge jetting fest and what did we hear of it? nothing, diddly-squeek, FA, zilch, we've had top dragster runners, pro mechanics the lot and they come up with NOTHING.

when their bikes flood. tough, no pity (excepting lack of knowledge to have a go)i've said for long enough that i'm tired. look at my jetting signiture THAT'S where you should be headed with your jetting.

there are loads of lads saying go up from 45 to 48PJ, fine, i've tried to tell you otherwise and none of you listen so carry on.

there are lots of things about nature i find strange. where did jack 'n jill go for their pale of water? bottom of the hill wasn't it? well it has to be doesn't it? after all you'd never find a well on high ground now would you! would you?


  • lewichris

Posted January 07, 2002 - 06:00 AM


well i had time this weekend so i pulled the plug put in a new one and got new gas in it. Startd the third kick. so thanks for the help but now i am on to a new problem. I did a bonehead thing. I turned the oil drain bolt on the bottom of the engin and proceded to strip out the hole. Oh **** huh. so if anyone knows how to fix this please help. I was just going to take it to the yamaha shop and say "fix it".

  • *mike68*

Posted January 07, 2002 - 02:28 PM


I do what everyone says not to do and it works for me.

Pull in decomp lever, twist throttle wide open, kick 10 to 12 times in a row while holding decomp lever and throttle open,.

Pull choke on and follow normal routine. I ride all winter, anything below freezing, no jetting, nop fn around and no problems here in NH


Posted January 07, 2002 - 04:40 PM



Sorry Ol chap....You have much knowledge when it comes to jetting the beast,but are somewhat misguided when it comes to air standards where pressures vs temp's are concerned. Having been a Professional Military Helicopter Pilot for over 20 years, I must respectfully disagree with your assertions. Molecules of Air ,water or solids expand and contract depending on temp not barametric pressure. When it is hot the molecules are bigger thus in the analogy of an aircraft, it takes more force and more pitch to make an aircraft fly....Bigger molecules = thinner air. On the other hand when the air is cold the molecules are smaller (compressed)there are more in the same space (Dense). When the air is cold (Dense) an aircraft requires much less energy to generate the same lift required to become airborne as when it was warmer. The same principal applies to a carb jet. When the air is hot and the molecules are larger (which most applications are set for) The correct amount of air passes through the hole metered in the jet. When that same air is cold or (Compressed) more air molecules pass through the same metered hole changing the dynamic airflow....(Thus the reason some bikes pop under 40 degrees around here but not over 50 degrees) The same goes for fuel....Hotter(Fuel Expands) less fuel....colder (Fuel Compresses)more fuel...going through the same jet. (A simple test is to fill a glass container with gasoline in a warm room ...about 60-70 degrees....Mark the fill line with a crayon or grease put the container in the ice box over night and see what has happened. The gasoline has now compressed and is well below the line. (You always get cheated at the pump in the summer time, unless you fill in the cool of the morning) This is the reason why I foul plugs ONLY in the winter time (since removing the ACV), If I changed my SJ to a 50 or so (60 is Stock) I could more than likely stop fouling plugs for the winter months. In summer I pull out the choke....In winter I crack the throttle once then Kick through...Sometimes Kick through twice. High and Low pressures on a map do not have anywhere near as much effect as the temp associated with them. A low pressure cell at 50 degrees and a High pressure cell at 50 degrees mean absolutely nothing to my WR except I'm more likely to get wet when that Low pressure cell reaches my riding area.

You are still my jetting hero.....

Bonzai :)

  • cycra1

Posted January 07, 2002 - 07:00 PM


well the cold air deffinatly changes the way you shold jet your bike. If you want a quick fix to starting it in the cold here you go.........i've been ridding in 20F give or take for the past few weeks. My bike starts up 1st kick every time. If you got your bike running good, and got good plug color just richen it up a little. I did this by moving my needle up one spot. It mihgt sound weird but a little more gas helps out and my bike also seemd to run better overall!! good luck and have fun, cant wait for spring!!!

  • Taffy

Posted January 07, 2002 - 11:39 PM



i don't doubt that in cooler weather things are denser, despite all i've said i've tried to stick to the point i was making and not sound like a packet of fags (cigs to you boys).

if your point on denser air is right your equations to jetting still don't add up.

in other words the moment you go to apply that knowledge to your bike something goes missing.

first of all what is having the biggest affect? is it the moisture, or the pressure? you can't doubt what i see on TV all the time now can you! after all it's there every night. when the weather is poor it gets an L, thats an LLLLLLLL. how scientists tidy this up i don't know.

but you do hang yourself a little when you say that in the summer you need full choke and in the winter a twist of the wrist (i prefer a flick of the wrist but we shouldn't go into that now!) this indicates that in the winter you DON'T NEED AS MUCH FUEL so have i got that right?

after all you have supplied the chair and the rope!


  • Ynahg

Posted January 08, 2002 - 01:27 AM



Cold air has a low relative humidity, which means it 's not able to absorb much moisture. have you ever heard about a cold humid climate ??? But you must have heard about hot humid climats (tropical, Florida ???).
Hot air is less compact than cold air, which means it can contain less oxigen in a same volume.
Why do engines have more power at sea level than on higher levels ? Because of the air pressure.
Do you run the same MJ at 0°C than at 25°C with a 2-stroke ?? This means I can forget all I learned till now.
At a same altitute colder air contains more oxigen than hot air. If the same volume of air contains more oxigen, you need more fuel to keep the ratio right. (14.7)

[ January 08, 2002: Message edited by: Ynahg ]

  • The_Missile

Posted January 08, 2002 - 10:20 PM


Any of you guy ever do physics ?

TEMP and PRESSURE both have significant effects on gas and liquid volumes (temp only) which is what we are concerned with when we are getting out jetting spot on...right !!!

So if you follow the logic below...hang on we're going back to school :)

BOYLES LAW: If temp is held constant then the volume of the gas is INVERSELY PROPORTIONAL to the change in PRESSURE.
Basically if pressure goes from 1000 mb to 900 mb air volume will increase by 1000/900 = 1.11 and DENSITY will decrease proportionally. Air density is 1.224 kg/m3 at 1013mb at sea level at 15 degrees C. So at 900mb at sea level at 15 C air density will then be 1.087 kg/m3.

In THIS situation with a drop in pressure your carb would be getting 11% less air (O2) than normal so you would be running RICH.

I know this is not the only thing going on so bear with me.

Temperature also has an effect on gas volume which leads us to

CHARLES LAW: For a CONSTANT pressure, the increase in volume is directly proportional to the change in TEMP (in degrees kelvin = Centigrade + 273)). So you heat it up, its volume increases proportionally and vice versa. So 15 degrees C is 288 Kelvin or 59F. When temp drops from 15C to freezing (273 Kelvin or 32 F) the % drop in temp is -5% which decreases volume 5% which increase density 5%.

In this instance where temp is dropping you would be getting 5% MORE air than usual so you would be jetting LEAN.

However temp and pressure are not independant of one another so you have to do both calculations at the same time.

A rough chart follows:

Air DENSITY chart
Mb Pressure
C F 900 1000 1100 1150

-15 5 1.214 1.349 1.484 1.551
-10 14 1.191 1.323 1.455 1.522
-5 23 1.169 1.298 1.428 1.493
0 32 1.147 1.275 1.402 1.466
5 41 1.127 1.252 1.377 1.440
10 50 1.107 1.230 1.353 1.414
15 59 1.087 1.208 1.329 1.390
20 68 1.069 1.188 1.306 1.366
25 77 1.051 1.168 1.285 1.343
30 86 1.034 1.148 1.263 1.321
35 95 1.017 1.130 1.243 1.299

However we're not done because the same principles applies to LIQUIDS. You can pretty much take Pressure out of the equation as its virtually impossible to compress a liquid.

Follows is a density chart for GASOLINE at various temperatures.



Remember gasoline expands and contracts 6 times more than water. TRUE

Formula for the percentage change in volume related to a temperature change of 1 degree celsius starting at 20C is 950 x 10^6 = 0.00950 TRUE

or almost 1% per degree C - which explains the breather hole in our gas tanks !! SO WRONG

C F Density kg/litre
-15 5 1.022
-10 14 0.972
-5 23 0.924
0 32 0.880
5 41 0.838
10 50 0.798
15 59 0.760
20 68 0.724
25 77 0.691
30 86 0.661
35 95 0.634

So then the next step is to combine the two charts to see what happens to the air/fuel ratios/mixture.

Fuel to Air Ratios
Larger number = richer condition.


Mb Pressure
C F 900.00 1000.00 1100.00 1150.00

-15 5 0.84 0.76 0.69 0.66
-10 14 0.82 0.73 0.67 0.64
-5 23 0.79 0.71 0.65 0.62
0 32 0.77 0.69 0.63 0.60
5 41 0.74 0.67 0.61 0.58
10 50 0.72 0.65 0.59 0.56
15 59 0.70 0.63 0.57 0.55
20 68 0.68 0.61 0.55 0.53
25 77 0.66 0.59 0.54 0.51
30 86 0.64 0.58 0.52 0.50
35 95 0.62 0.56 0.51 0.49 it gets colder for a given pressure the air/fuel mixture RICHENS, which means you have to lean the jets. WRONG

So have I proved to you guys that basically colder weather RICHENS. Warmer weather Leans.
Higher Pressure leans and lower pressure richens the mixture. WRONG (OUCH WAS THAT A KICK!!)

Watch the barometer AND the temp guage but REALLY watch the baromter.

One last thing....fluid dynamics at different pressures/temps also have an effect as does the RPM of the engine, the size of the cylinder etc etc. Also....humidity plays its part...RIGHT!!!!

I'm done. I spent 3 hours working on this thing.

[ January 10, 2002: Message edited by: The Missile ]

  • Taffy

Posted January 08, 2002 - 11:59 AM

#20'm .....right!
i'm right!
i'm right!!!
yesssssss i'm right. ha ha i'm right i'm right i'm right.

e i aleyo i'm right!! i'm right!!! i'm right!!!!

of course i couldn't have done this without my parents who i'd like to thank, i'd also like to thank....

mmmmmmmmmmwwwwwwwaaaahhhhhh i love you all :) :D :D

ps; should this site invoke a non-gloat clause i will comply but until then yyyyyeeeeeessssss!!!

even JD got that one wrong!!! yaaahhhhhhoooooo

you're all my children and i love you all.

Taffy :D


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