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Giddyd

2 stroke jetting.

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Do we have any 2 stroke jetting wizards here? I got my girl friend a yz 85 and bike is just fussy as hell with altitude. At sea level it starts just okay, has great throttle response and pulls well to Max rpm but stalls very easily. At higher elevations it has to be bumpstarted and borderline unrideable. Cuts out above half throttle like it's running out of fuel. Id really like to dial it in as well as possible for elevation from seal level up to 3000ft but I really don't know where to start with all the different fuel circuits. If someone local (I'm in escondido) was interested in helping me out and showing me the ropes I'd be forever grateful. 

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start with being sure all the mechanics are spot on and are baselined before chasing jetting,  premix ratio to factory spec, spark plug , good gas, timing, fresh top end!!!!, good reeds, no air leaks. no oil seal leaks, PV set up if it has one...etc

 

Get into a Yamaha manual you can find them all over the internet, rocky mtn atv is good spot to access. jet according the manual. from sea level up to 4000 ft you could be as many as 2 mJ sizes smaller. But best to use a manual chart or check the FMF jetting chart (sea level std jetting) The OEM manual will have a pressure altitude chart.

 

For Example my YZ125 was jetted for 4k (McCain, Cahuilla etc) for a dez ride I would go 2 sizes up on the main jet with no other tweaks. I always mixed 32:1 with pump premium and never had any spooge dripping, but my YZ was dialed mechanically, always idled and started easily.

 

BTW what year YZ85?

start with OEM that will be safe at sea level, to go up to 3k probably 1 main jet size smaller. Pilot just keep it the same as OEM, std for air screw is 1.5,  FMF spec may have some tips as well as Pro Circuit spec.

 I have found Pro Circuit tends to be on the leaner side which sometimes worries me, FMF is more conservative on the their chart.

PS it could be a simple as just a jetting adjustment for altitude as simple as a clogged pilot jet or debris in the carb, or as deep as the top end could be done...with associated compression loss.

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Bike is mechanically sounds. Did a new top end. Reeds aren't frayed and look good. Carb is clean be fuel and spark plugs. It's a 2004 yz85. Speaking of mcain valley is it really 4k up? That's where the bike was really having trouble. Are the smaller two strokes more sensitive to elevation. I have an Atc 250r and a lt250r neither seemed to care about elevation as long as I wasnt above 3k. My ktm 525 seems to run the same everywhere but I get that 4 strokes are a different animal entirely? 

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Pull the carb and open her up, remove the jets, and inspect them all. It could be something as easy as a clogged jet. Compare your jet sized and needle position to factory specs. Post back with the details and we'll go from there.

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What oracle said

check float height as well

      FYI                                            OEM is MJ 138   PJ 45   Needle  NBKF-2   AS 2.375

Pro Circuit (leaner spec) set is better for the 4k altitude and for trail riding   MJ 135   PJ  42   Needle same         AS 2

could not look up FMF at this moment, but that PC one set smaller spec sounds good for local mountains at a trail ride pace.

They spec for sea level at 65 degF with premium fuel with a MX ripper  on the MX track = pinned in the power a lot.

 

mix premium at factory spec

PS any questionable issue about jets just buy new ones

 

 

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Trust stock jetting settings. Soaking jets in gun cleaning solvent over night and using a clipped wire from a large wire brush to clear holes works well, so is good gas and proper oil mixture ratios. Air box free of obstructions, air filter clean and serviced. New spark plug and good wiring. 

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I use guitar strings for clearing jets, along with an ultrasonic cleaner for the carburetor body.  Start with a good clean carb and stock settings, then adjust from there.

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Sorry for not getting back, worked through the weekend then went riding in ocotillo on sunday. The bike did better. I found out airbox boot wasnt sealing to the carb so another clamp fixed that which helped a lot. Still didnt feel like it was running 100 percent though. Socalhodaka I may need to take you up on that offer. 

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