125-200cc Q's on TMX carb

[edmoto]

I'm working on my jetting, and would like some input and advice on the following:

1) Is there any rough "rule of thumb" about how changing fuel/oil ratios translates to jetting changes? More oil in mix makes the mixture leaner, and less oil on mix makes mixture richer. But does this correspond to an equivalent jetting change? So for sake of argument, I switch from 32:1 to 40:1. Is that like changing a main jet (or some other circuit or circuits in the carb) from a "400" to a "410" for example?

2) How do you capture the fuel that dumps from the float bowl when you take out the 17mm plug to gain access to the main jet for jetting changes? I am just rotating the carb a little to gain access to the plug to remove it, but there is no room for me to stick a funnel or anything else under the carb to catch the fuel that comes out. Am I missing something?

3) When you change the jet needle (for example 6DJ8-60 to 6DJ8-59) does the clip position on one represent a clip position on the next jet needle? So if I was on 5th notch on needle 60, is that the same as 1st notch on needle 59?

4) Which do you think has a greater influence on jetting: temperature or density altitude? I'm at 4700' and just today, as I was working on jetting the temps went from 52 to 58, but my density altitude went from 4990 to 5860. Or am I splitting hairs? I tweaked my settings as the day wore on, so I have my opinion, but would like to read what the forum thinks.

Thanks,

 

[edmoto]

Forgot to mention, in case this helps with any suggestions:
2008 CR125, TMX carb (stock carb).

 

[PowerKord]

In my experience, changing ratios from 32:1 to 50:1 had a minimal effect on jetting. That being said, you are best off finding a ratio that matches your style of riding and sticking to it. Temperature and humidity both effect the jetting more than oil ratio IMO. Remember 50:1 is 2% oil and 32:1 is 3% oil, so it's not a huge change in the fuel content of the mix.

 

[Idacurt]

When I'm going to change my main I turn fuel off and ride the bike until the fuel bowl is nearly empty so I don't spill excess fuel.I have no scientific proof but my bike tells me temperature affects jetting more than altitude.Some days I ride from 6k-9k and I notice a slight difference but if the temps drop 10-20 degrees overnight I really notice it the next ride.

 

[lankydoug]

It is all about air density altitude which is calculated using temp, humidity and baro pressure. Temperature is usually the factor that changes the most but if you kept a notebook and established a data base you would find that you could nail the jetting every time using air density altitude. Actual altitude can be misleading, I've seen 3500 ft air at 600 ft above sea level and witnessed that same 3500 ft air change 1500 ft in less than an hour when the sun went down and temps cooled off. (when I was drag racing we checked the air before every pass) My advise is to jet your bike to run crisp for the best air you usually have and when the weather changes it might be a shade rich but won't hurt anything. If you are looking for the absolute best performance you will have to keep a notebook and jet according to the air density. The kind of riding you are doing comes in to play for example if you're going to keep the bike pinned all day in a desert race you will want to go up a step or two on the main to save your engine.

As for the 17mm bowl plug I just put a drain pan under the bike and let it spill over the engine and find it's way to the pan. No smoking during this operation