Carburetor/Jetting cool weather and choke..what I want to do to my bike!


9 replies to this topic
  • dag19

Posted December 26, 2011 - 07:10 AM

#1

Hey guys! So I live in Fort Worth about 700' of elevation, average 35 degrees in the morning, and I ride my '07 WR450 to work everyday. It never fails every morning I fire it up and runs great with the choke on...but once I get a couple miles down the road I turn off the choke and it starts spitting and sputtering while driving down the road around 1/4 throttle. When I open it wide open it runs fine but lower RPM's it does not. So then when I pull the choke back out on it....it runs like a champ.

Then on the flipside in the afternoon when I am coming home and its around 50 degrees out it doesn't need the choke the whole way home. I push it in like normal.

So why do I need the choke for 7 miles of 35 degree riding but it runs great all other times with temps above 50? :bonk:

  • ur2silly

Posted December 26, 2011 - 08:48 AM

#2

Basically, as air gets cooler it becomes more dense. This denser air has more oxygen. Since the bike is sucking in this cooler, oxygen rich air, it requires more fuel to keep the air/fuel ratio correct. By opening the choke you are allowing more fuel into the mixture ratio, thus correcting for the lean condition created by the cooler air.

Normally you would want to replace the jets with larger ones to correct a lean mixture. However, you may be able to get a usable result by opening the air/fuel screw.

  • 2grimjim

Posted December 26, 2011 - 08:59 AM

#3

You didn't mention if you have done any changes to your WR. If it still has stock jetting, that would account for some of your problem.

The '07 WR is an EPA emissions-regulated vehicle as it's delivered to the dealer. Because of this, they are jetted horribly lean from the factory to meet tailpipe emissions standards set by the EPA.

If you (or anyone else) hasen't done any changes, you might want to visit the jeting stickeys.

  • dag19

Posted December 26, 2011 - 02:43 PM

#4

Ok that makes more sense with the way it is acting and how it runs better with the choke on.

As far as changing the jets...I did have it rejetted because I took off all the emissions garb. But when I rejetted it I was at 7200 feet in elevation. But now down here at low altitude it still runs great but just not when its cold out. I would rather not change jets, so do you think I can get enough adjustment out of the air/fuel screw?

  • wanawakeboard

Posted December 26, 2011 - 02:59 PM

#5

probably depends on what jets they used when it was jetted for high altitude, higher altitude means less air (gets thinner as you go up) so they put in smaller jets to give the bike less gas to go with the less air senario, but now your down where there is lots of air and same small jets making the bike run lean, and cold dense air makes the bike run even leaner! thats possibly why the choke (richens the mix) helps...

try the fuel screw but if you go out more than maybe 2 3/4 turns you should consider a bigger pilot jet..

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  • dag19

Posted December 26, 2011 - 03:07 PM

#6

probably depends on what jets they used when it was jetted for high altitude, higher altitude means less air (gets thinner as you go up) so they put in smaller jets to give the bike less gas to go with the less air senario, but now your down where there is lots of air and same small jets making the bike run lean, and cold dense air makes the bike run even leaner! thats possibly why the choke (richens the mix) helps...

try the fuel screw but if you go out more than maybe 2 3/4 turns you should consider a bigger pilot jet..


Ok that all makes sense to me. I will have to try the mixtures screw as it will be the easiest to get at. What's a good number for the pilot jet at this elevation?

  • wanawakeboard

Posted December 26, 2011 - 03:56 PM

#7

I think stock is a 45 or 48 on my dirt version, check online to see what your stock number is for your pilot jet. if you have a pipe, done anything to the air box, or modified anything induction related it might affect what jet to use as well...

  • 2grimjim

Posted December 26, 2011 - 07:09 PM

#8

The real problem is with the stock needle. The diameter of the needle (before the taper) is just too big. I've been an R or S diameter needle where I live (4600 feet elevation) and the stock needle is a T.

Here's a beakdown of needle dimensions for FCR carbs;

A1 (3rd letter from right) Included angle of needle taper L1 (2nd letter from right) Distance fron top groove to beginning of taper** D1 (1st letter from right) Diameter of needle above taper (affects idle to 1/4 throttle)

A1 L1 D1
A * * *
B * 74.65 *
C 0°30' 75.1 *
D 0°45' 75.55 *
E 1°00' 76 *
F 1°15' 76.45 *
G 1°30' 76.9 *
H 1°45' 77.35 2.675
I (not used) * * *
J 2°00' 77.8 2.685
K 2°15' 78.25 2.695
L 2°30' 78.7 2.705
M 2°45' 79.15 2.715
N 3°00' 79.6 2.725
O (not used) * * *
P 80.05 2.735
Q 80.5 2.745
R 80.95 2.755
S 81.4 2.765
T 81.85 2.775
U 82.3 2.785
V 82.75 2.795
W 83.2 2.805
X 83.65 2.815
Y 84.1 2.825
Z * *

  • 2grimjim

Posted December 26, 2011 - 07:22 PM

#9

Well.....that didn't work so good. If anyone is interested, I have an excel spread sheet with a bunch of carb specs and stuff. PM your email and I'll send it.

  • ur2silly

Posted December 27, 2011 - 07:19 PM

#10

Go to the link below. Find someone who rides a bike like yours with the same mods, elevation,temperature, etc. If you mimic their settings it should get you really really close on your jetting. Should definitely be better than what you have right now.

I said before to try the air/fuel screw. I didn't know that your bike had been jetted for above 7000 ft elevation. I would definitely get it jetted properly.

Even though it was running good before it got cold out it was jetted for higher elevation. I would guess that it was running lean then too, just not as noticable until it got colder. If your jetting is too lean it can cause engine damage to your bike.


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I have 2007 WR450 and I am running Main jet:168, Pilot Jet:48, Leak Jet:50, stock pilot air jet, MSR fuel screw 2.5 turns out, Yamaha NFLR needle on 4th clip and Lexx MXE slip-on exhaust with spark arrestor in. I have done all the free mods, removed the snorkle from the air box, YZ throttle stop and I run the stock air filter. I ride in North Carolina in low elevations 400-600 ft. This jetting is spot on for me. When it got cooler out I could tell because it started to pop on decel. I opened the fuel screw 1/2 turn and good to go.




 
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