i am a fairly new rider on my khs flite 300. i recently bought a shorter 90mm stem to replace the 100mm that came with it. while installing it i had to remove one space because the newer stem was higher than the old one. took it for a test ride, and the front fork together with the handlebars will shake slightly when i apply the front brake. i traced this to a seemingly loose headset. the weird thing is this has never happened before, only after i changed out the stem. could it have been an installation issue, maybe something i missed on the way?

Welcome,
First, please be sure to correct this before riding again, you can damage your frame. Well let’s get you acquainted to stem and headset adjustments. Not real familiar with you model bike, so if you have a metal steer tube, (the part that you stem clamps around on the fork) first loosen the clamp bolts on the steerer (fork side) then gently tighten the cap bolt till all play is taken up (it is for this reason that the stem must be higher than the steer tube, most recommend 2mm) than align the stem and tighten the clamp bolts to the proper spec (mostly marked on the stem) ensure that when you pull on the front brake and rock the bike there is no movement at the headset.
The only real change if you have a carbon steer tube is the stem clamp bolts may have a reduced torque in that application (check the stem makers web site and the fork makers site to be sure) sometimes 10 to 20% please check.
This should get you back to new feel. Hope this helps --- Bill

yup you're right on. i just checked and it seems the top of the stem is flush or maybe slightly lower than the top of the steering tube. looks like i'll have to get some spacers. i currently have 2 x 10mm on

Great, spound like you have it. 2mm and 3mm spacers are easly avil. You did indicate that you removed a spacer, maybe it was a thicker one? Get that tube 2/3mm down in there and you'll be in good shape, but you don't want it much more than that. I'm glad that my post was easy to understand. All the best -- Bill