on my 03 trailblazer 400 I removed the stock airbox a while back and have been playing with main and needle jet settings and the mixture screw since. I've gone up 2 main jet sizes and moved the clip one notch down and it seems like it's running pretty good now, but I just want to see if anybody else has done this sort of thing on their 400 2-smoke and where they ended up with their main jet. Factory was 240 and i'm up to 260 now, but I could get it to where it seemed like it was running ok with the factory and about 1 1/2 turns out with the mixture screw. But with each jet size it seems to be getting quicker, so how far have you guys gone up in main sizes?

 

best way to tell if it's right is by checking the color of the spark plug for that nice light tan color for best fuel air mixture. the more air flow the more power, the more gas required for best performance. a pipe would require even more gas(bigger jets) for moving more air and making more power, but the spark plug is still the best way to measure if it's running correctly of s strokes. imo

 

Just make sure you maintain the K&N properly! Keep it cleaned and oiled and have a prefilter over it that can trap a lot of the dirt. I had several engines that went south from people not maintaining this filter! A much better choice is UNI if you run something other than stock.Just my 2 cents worth from what I've encountered! OPT

 

I haven't ran a stock carb on mine since about a week after I bought my Sport 400 back in 1999 so I can't really help you out on the jetting except to say that it's better to run a little rich than a little too lean on these 2 stroke motors.I run K&N filters on all of the ATV's that I own and always run an Outerwear on them and have never had a problem with them.The Outerwear's are really worth the money IMO as far as keeping water, dirt, dust, sand and mud out of the airfilter itself and don't have any negative effects on your performance.I personally wouldn't run an ATV without one regardless of the type of airfilter that I was using be it Uni or K&N.

 

I ran the filter for one trip without the precharger on it and had to wash/re-oil it so I got one for it. As far as the plug, it looks like its running about right. It just doesn't seem to make much more power between 5/8 and wot. It was from about 1/2 to wot with the 250 jet and seems a little better with the 260, I was just curious if someone had done this sort of thing before to get an idea of where they ended up instead of buying one or two jets at a time and end up owning several I don't have any use for. I guess I'll just wait a bit longer and see if someone else has done it and they post.

 

I agree, more airflow = less protection in my book..........Alright for the blacktop IMO, but need to be carefull off-road...........Caper........

 

I've ran k&n's in all my mud trucks and never had any problems besides the dusty part. I feel comfortable with using one on my wheeler with a pre-charger after the first ride with just the filter. I've checked it everytime and the filter itself looks almost brand new, and a quick rinse and dry of the prefilter and back to riding I go. I just want to make sure I have enough fuel going to the motor without the airbox so there's no problems down the road. Like I said, the plug looks pretty much right but there's not much more power after about 5/8 throttle.... It does continue to gain speed. But factory, it made a difference from just shy to wot and everywhere below too. I just wanted to find a less expensive way to go about getting to where I need to be to make good power all the way through and still feeding it the proper amount of fuel.