So we're servicing my daughter's '90 Trailblazer. It was purchased locally brand new, so the jetting should be really close for the altitude, but it has always ran really rich. I put new filters (both the round paper one and the foam pre filter) on it when we bought it, but I'm wondering if this is causing too much airway restriction? Is it really necessary to run both?

 

Welcome to the forums.

You will be ok using both. Some may give the foam cover a light spray of filter oil too.

 

+1 A little oil does help on the foam style prefilters. Think of the prefilter as catching a lot of the fine dust and dirt that the paper filter may not even be able to catch. This dust would then be thrown right into the cylinder and would be between the piston and cylinder grinding away... Clean air filters are the life of two strokes or 4 strokes for that matter.Only other filter other than the stock paper filter with the prefilter would be an Uni foam filter.If it gets clogged up enough the engine won't run or will really run crappy but at least dirt is trapped in the filter and not sucked into the engine. Jets are a 145 main and 40 pilot. You can drop the needle one or two positions and see if that helps. If it doesn't you can drop one main jet size. Altitude and temperature may affect stock jetting. Always check the plug afterwards. Should be a light to medium tan color.http://www.jetsrus.com/a_jet_kit_atv..._1990_2006.htm

 

So if it's not an air obstruction issue, then maybe I need to start looking at jetting. WOT seems fine, but it will sometimes load up and backfire upon let-off, and warm starts require the throttle to be held open. Idle-air mixture perhaps?

 

Air screw normally 2 turns out. This you can experiment with as it's idling.On the back firing make sure you only have 1/8" slack at the thumb throttle.If you have a tad more the etc contacts could bounce off each other when you let go of the throttle and can cause back firing. Also this is where you can drop the carb needle one position at a time and see if that helps. If it still seems rich and boggy at top end,drop a main jet size.

 

Help me refresh my memory. On the mixture screw, out is leaner, in is richer, because we're metering air volume, not gas, right?

 

Correct. More turns out on the air screw the more air that mixes with the fuel from the pilot jet.Turning the screw in cuts down on air. This screw mainly affects idle up to about 1/4 throttle. Main jet and slide needle from there on up.Some older machines may idle better with air screw adjusted a little more out or adjusted in from the average 2 turns out.

 

Exactly where is it running rich...If you haven't done a plug chop then you can waste a bunch of time guessing at jetting. Run it at WOT for a good stretch and kill the motor (don't let off throttle before killing the ignition)...what does the plug look like? If the color of cardboard then DON'T drop the main...if dark then go down 1 size.


If plug chop at WOT shows main is fine, then move on to the needle (if adjustable), drop (move the clip up to blunt end) 1 position at a time until it feels crisp...if it starts running poor at part (1/2) throttle then you've probably gone to lean.


If idle is ugly and off idle throttle (1/4) is sluggish then the pilot may need a cleaning or A/F mixture (if adjustable) may need tuning...as OPT mentioned 2 or even 2.5 turns out from lightly seated is a good starting point. Small (1/4 turn) adjustments should be made when dialing in the A/F mixture...when dialed in there will be no hesitation when throttle is stabbed...