Ok here's my set up...128 main 35 pilot jet a\f screw out about 2 turns or so maybe 2.5 i cant really remember, so I take the backfire screen out
now i am running very rich.....i have lost 9 miles per gallon by my caculations.......

question......should i try leaning it out with the a\f screw or should i drop down to a 125 or even a 120 main jet? my jets are oem, any advise would b great!

 

Something not right here.
Increasing air flow should have caused it to run leaner not richer.
And the backfire screen is, in the scheme of things, a very small change.

Sounds like you've done something inadvertently when doing the screen removal.

 

yeah i see your point neil, it has been runing a little rich for awhile now
not that bad though, just never had the a/f setting dead on, but now its just off the charts!

 

A metal shaving might have gotten lodge in the fuel needle or slide.

Quick question. Is the backfire screen located behind the air filter on the air box? If thats the one, why remove it? Theres not much to restrict air flow.

 

Lear, there's been some flow bench tests done on some airboxes like XR's and some others over the years that shows as much as a 20-30 percent increase in flow at WOT. I don't have access to those results as it's been quite awhile, but if you measure the wire size and calculate how much of the opening is covered by that amount of wire, it's probably more than we think. On my KLR650 it required one full step up on the main jet with just the wire removed. It didn't affect the needle position, pilot jet, or air fuel mixture setting.

 

The a/f screw only affects fuel flow from idle to about 1/4 slide opening.
Have you pulled the spark plug to see if it is covered with soot, an indication of too rich a mixture?
Some winter fuel formulations will give lower mpg.

Ride on
Brewster

 

when you removed the screen did you also clean and (over) oil the filter?

 

I figure other intake items would be more restrictive than the backfire screen. Like the intake tubing, snrokle, air filter, or carb. I just saw the backfire screen on mine when I installed the TwinAir air filter and thought it couldn't be that restrictive. I have seen other backfire screens where its a metal honeycomb mesh stuck inside the intake tubing.

I know the KLX engine sucks a lot of air. You can really feel the air flow with your fingers near the intake.

 

And Lear, please understand that I don't claim to be any kind of engineer on this issue. My limited comprehension of the issue is just that...limited. The full 1 step up on the main jet on my last KLR650 probably convinced me more than anything. I usually do all the possible mods to any bike I've owned all at once, so I don't get the individual impact of each. On that KLR, I had done everything except except the wire because it's a bigger PITA, so I got to see and feel that one element. I had a noticeable detonation after the screen removal when rolling on the throttle requiring the larger main jet.

I think more than the blockage issue with the screen wire or any other obstruction, there is also a "flow" and velocity issue. You see this with intake mods that involve polishing intake and exhaust tracts in a head after the carb or those velocity stack designs that you sometimes see before the carb. Even though volume may not be increased by much, the velocity and flow are improved to affect performance. I think this as much as anything is what occurs with the screen wire. An example of this is in the normal airbox mod that opens the top of the airbox for more flow. If one just measures the carb throat area, you'd think you don't need any bigger an airbox opening than what the carb throat is. But of course, there's a velocity and flow issue that causes that not to be the case. Velocity and flow volume picks up at the carb throat requiring more open volume space to draw air at the airbox opening. And while there may indeed be a 20-30 percent restriction caused by the screen wire, I don't think there's a 20-30 percent flow increase with the wire removed. However, there seems to be a noticeable impediment to the air flow into the carb at some level that is caused by the wire. In the case of my KLR, it was a full jet size. I don't know how that translates into how much percentage of a blockage to flow that screen wire was causing to the fuel mixture.

 

throttle happy perhaps, low tire pressure?