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Old 07-28-2008, 12:01 AM
cruzmystar's Avatar
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Join Date: Jul 2008
Last Online: 07-28-2008 12:02 AM
Location: Bellingham, WA
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Default 89 YZ250 Jetting baseline

First a little background I guess so you guys know where I'm coming from as I'm new here. Grew up riding and customizing street bikes so this is my first dirbike (yes, let the jokes begin, BG). I'm a master autotech by trade and have recently finished a turbocharged yammi vstar 1100 so I'm not exactly new to tuning. I'm one of the head Mod's on our 1100vstar riders forum. However, I'm a big enough person to admit I know very little about two strokes as this is my first. Spent this evening reading through the KB and it seems general jetting techniques are the same so that should be a piece of cake. Now onto the bike....

Just picked up a gem of an 89' YZ250. Supposed 3rd owner. From the second owner (who knew very little about the bike and very rarely rode it) I got that the bike was originally owned by a buddy of his who desert raced for Yammi "back in the day". He didn't think this was one of the bikes he raced but more of just a play bike. Anyway... I took it out this weekend for a good run. Ran pretty good and felt decently strong. Then 1/2 way through the day I fouled a plug. Ran valvoline 2-stroke at 30:1 (which i now know to ditch and run a better oil). With new plug it ran like a raped ape. wicked midrange and super snappy response then but still a little flat on the top. I'm very open to the fact that the plug fouling was my riding style as we were riding a lot of trails and i was kind-of riding like a sissy trying to get used to this whole "dirt" thing. after the first plug change i stayed up in the topend a bit more but it was still pretty black and wet at the end of the day although to my lack of knowledge at the time i had adjusted the pilot screw in a bit thinking i was leaning it out (hey, my other carbs are like that, lol) so that could have some to do with it. my biggest concern is that the jetting is correct because i have very little info on the bike and i don't know how much aftermarket pipes, etc. have an effect on jetting. someone put some good $ into it at one point (Precision concepts suspension front and rear, Answer Pro pipe, Answer fatty bars, opened up airbox, came with a full container of jets, 5 sprokets, etc.). i'm at sealevel now. popped the carb loose quickly this evening to clean the bowl and found a 380main, and a 56 pilot (if my memory serves me right, might have been a 55). haven't checked the needle yet. it also had a BP9ES plug in it (with a couple spares) so that's what i was running. am i correct in thinking i should drop the main a bit (maybe a 360 or 370), put the correct plug (B8ES) and properly adjust the pilot screw? just want to make sure I'm heading about this correctly. thanks and sorry for the long post.

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