Rich or Lean???

2 replies to this topic
  • Dune7

Posted June 03, 2006 - 09:58 AM


Please bear with me, I am a bit of a dumbo when it comes to carbs and setting them. I have a 2004 XR650R (ED model ie: European) Stock carb and the bike is stock "uncorked". I live in the UAE ie the desert, at sea level .Winter temp around 25-28 deg C and Summer around 40-44 Deg C.

Now stock jetting on the ED model is Main:#175 , Slow jet: #65 and clip position is 3rd from top position.

If I look at "Erics" jetting charts it has the main at #168 Slow Jet at #65 and Clip in position 2. at high ambient and at around #175 ,#65 pos 3 at cooler ambient temp.

I have just installed a PC T-4 slip on muffler (stock header) and it pops ever so slightly if I deccelerate suddenly (If I brake slowly to a stop ,no popping)
Now that I have confused the crap out of all here are my questions. :D

1) With the stock setup, is my bike going to be lean or rich at higher ambient temperatures.

2) Will it make sense to use "Erics" jetting tables and choose a jetting setup ...say ..halfway between the lower winter temps and the higher summer temps. ie: the bike will run a bit leaner or richer whichever way the weather turns. or am I blowing smoke up my own arse with this??

3) With the stock setup and the the new T4 pipe will the bike be rich or lean ?
as the ambient warms up will it become richer or leaner?

I hope there is someone out there with the patience to get my head around these questions...thanks! :applause:

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  • HawkGT

Posted June 03, 2006 - 11:00 AM


Higher temps = less dense air = your jetting gets rich as the temps go up. In other words, as the temps go up you have to LEAN out your jetting to compensate for the reduction in air density.

Same holds true for higher elevations. High elevations = less dense air = you'll need to lean out the air/fuel ratio to maintain the correct ratio.

Typically, aftermarket exhaust components = more air flow through the engine = the need for RICHER jetting to compensate for the increased air moving through the engine.

Look at is this way: the jetting settings of the carb don't change unless you change them. They will deliver the same level of fuel metering reguardless of the amount of oxygen in the air (air density). If the air is less dense (high temps, high elevations) the carb needs leaner jetting settings to deliever the amount of fuel appropriate for that air. If you do something to increase the amount of air able to flow through the engine (like an exhuast system) than you'll need more fuel (richer jetting settings) to compensate for the increased air.

Hope that made sense....

  • Dune7

Posted June 04, 2006 - 09:01 AM

#3 I get it, hence the smaller jets as the temperature rises. So essentially my bike will be running rich now then with a 175 jet and getting richer as the temperature warms up and the density drops. The 175 jet would mbe kinda o.k for temps arounf 25 deg c and I should now go down to a 168 for the warm temps I hope I got that right! :applause:

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