air filter

Discussion in 'Performance Mods' started by Crobo, Mar 29, 2010.

  1. Crobo

    Crobo New Member

    If i add a new air filter with higher flow will the engine run leaner? i know more "air" will get in but if i understand it correctly it will be more air/gas mix that gets in not just more air.

  2. G-Superior

    G-Superior Member

    If your airfilter was a really bad flow one and you replace it with a hight flow filter yes it will run lean
    the filter sometimes is like a choke in some engines
    I remenber having a mower that didnt want to run right and swallowed fuel like mad :ack2: when removed the filter to clean it was like brand new so i dicided to try it with no filter and it was better than ever until it siezed!
    So if you going for a high flow filter make sure you set it up propely
    To set it up just get a new plug or clean your old one(SCRAP IT IF IT IS A CHINESE ONE) if it still good start the engine warm it up not reving more than half the way(should only take 2 mins) and then get in a straight line and go as fast as you can, when you get to your maximim speed press the cluch in and stop the engine and let the bike stop slowly. remove the plug when the engine cooled down, you dont want to remove when its warm because it will burn you and you WILL get a stripped plug hole because the aluninium will be really soft when it hot!(dosent happen very often but its better not even take a chance with that cheap quality aluminium)
    If the plug is a nice brown/chocolate black it is just fine, if it is grey/white get your needle on the carb. a bit up(so more fuel can get into the engine), but if it is dark black and oilly it is to rich or your oil/fuel ratio is to high
    After a few hours of riding get the plug out again and :detective:
    I know it sounds boring but it will save you replacing/rebuilding the hole engine because of a stupid mistake
    good luck :D
  3. Dilly Bar Rob

    Dilly Bar Rob Member

    I know that there are lots of relevant threads on here regarding this but seeing this is the newest I'll post here as any posted info may help Crobo as well....

    I also always thought that improving flow on these things (I assume we are talking about a HT 2 stroke) would lean things out, however.... My stock muffler broke off not long ago so I fabbed one (pretty much just a piece of pipe with one of those 2-5HP lawnmower mufflers on the end of it, looks like the ones SBP sell to go on there expansion chambers). At the same time I slapped on a "low profile" air filter from SBP. Also removed one of the head gaskets to raise compression a bit before the other upgrades (I have the PK80 that comes with 2 stock). I figured that this combo MUST lean things out so I richend the mixture by moving the needle one notch up (previously set in the middle notch how it comes factory). The bike 4 stroked like crazy, way more then before the exhaust and filter upgrade, a real machine gun. I went leaner and leaner while checking plug color, finally I am at the leanest needle setting and the bike still 4 strokes quite a bit from part to 3/4 throttle. Plug color seems about right (after both 3/4 throttle and WOT test runs). When at WOT the bike runs perfect, no 4 stroking, lots of power (thanks SBP!), goes way faster then I want. Also no lean bog at all at WOT, not even when going up a steep hill. This leads me to believe that the stock main jet size is pretty much bang on for my altitude + mods and isn't the problem here. Interestingly the bike seems to 4 stroke less if I go up a steep hill with only part throttle compared to cruising on the flat at same throttle (ie it 4 strokes less under high load). All of this isn't really a big problem as the bike runs OK overall, however it does make me wonder why it does this since I increased the volume of air going in & out of the engine and it should be running lean after all of this.

    Could this be because I only weigh around 130# and thus don't really put that much of a load on the engine when just cruising (compared to heavier riders)? Or could it perhaps be a side effect of the "low restriction" muffler? I still have the factory plug in this bike (switched to BOSCH in my other 2 bikes), I wonder if that could have anything to do with it, have to try and swap it just to see....

    Any input on this is welcome.

    Dilly Bar Rob