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Slightly confused with jetting...

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Finally got around to rejetting my 96 Yz 250 that I just rebuilt... First off let me say that I am quite experienced when it comes to jetting a 2 stroke. I race one and own 4 so Ive been around the block. Ive searched for an answer but the only thing I can find with similar symptoms are with 4 strokes and it seems its usually due to lean conditions durring engine breaking. Never encountered it on a 2 stroke with the clutch engaged that doesnt do it while idling.

Jetting should be spot on according to my plug chops. Only mods made were replacing the reeds in the rad valve, new filter, and lubed/ replaced the throttle cable, my current and best performing jetting specs while under load are 172 main, 45 pilot, N3CD needle in 2nd clip postion, air screw 1.5 turns out float within spec. after installing a new spring running Amsoil Dominator at 40:1 50/50 93 pump and 110 race fuel. Bike pulls super hard throughout the entire throttle range. No bog, no sputter, nothing. When I chop the throttle off and drag the clutch/ rear brake entering a corner the bike surges extremely hard like im chopping the throttle. Im confused as to where to beign this process. Seems like it took me forever to get the jetting dialed in in the first place. Bike also does not surge when idling on the stand or durring warm up if I chop the throttle which doesnt make sense to me.

Anyone have any ideas. Doesnt make sense why it would surge while riding but not on the stand if my pilot was lean and the plug chop read perfect while testing. Could it be the spring that I raplaced?

Edit: Also plan on getting a JD jet kit for the bike but wondered if this would likely fix my issue or if I have another underlying problem.

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Not sure if your '96 is the same as my '98, but the throttle cable seems to be barely long enough. I've tried rerouting it to get every extra inch I could get, but its as good as it gets. Mine runs through the top of the head stay, and its still slightly bent. I've noticed it hangs up the slide on mine slightly, nothing drastic though. Just a thought...

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Thats what I was thinking as well, hence the reason I replaced the cable. My thinking is it would surge while idling on the stand when I chop the throttle the same as if it would when I am riding the bike. Doesnt make a difference whether Im going straight, turning or whatever.

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what are the temps in ohio? pilot sounds lean to me....try a 48 pilot......sounds like a lean surge. i run 100% c12 in my 05 and i have the 45 pilot in mine at 1/2 turn out and it is still lean at 60 degrees FWIW...the race gas allowed me to run 1 jet leaner across the board, with pump i never used less then a 48 pilot. not sure if the 96 is the same jetting though...HTH :smirk:

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Last time I rode the bike last week it was 76 deg. Tried a 48 pilot and experienced a slight hesitation off idle and ended up under 3/4 turn out on the AS for proper throttle response which indicates I needed to go to the next leaner pilot , raised the needle one clip pos. and still had the hesitation. Ended up going back to the 45 and put the clip back to stock 2nd position and it fixed the hesitation and plug chop read perfectly. Also note that this is after extensive jetting testing to get the best throttle response while performing plug chops all along the way... I went through 20+ plugs. This bike was a real PITA to jet because of the jacked up spring for the float.

According to my math which is suspect lol.... I should be around 100 octane give or take a little which should be more than sufficient for a stock motor with port work and squish set.

If thats the jetting in your 05 my moderately modded 08 seems extremely lean and I run 50/50 pump race in that as well. Im at a 175 main, 45 pilot, 3rd clip position 1.75 turns out on the AS

Also the jetting specs are diff for your 05 than my 96

your 05 and my 08 stock specs 178 main, 50 pilot, 2nd clip pos, AS 1 turn out

96 stock specs 175 main, 42 pilot, 3rd clip pos AS 1.5 turns out

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A surge is lean plain and simple. I don't care what anyone says i refuse to try to jet a bike with plug chops. If you do one every 500 rpm it'll work but there are easier methods.

Take note to what throttle positon you are in before it surges. Mark your throttle while riding in 1/16, 1/8, 3/16, 1/4, 1/2, 3/4 and WOT. Hold them steady for 10 (less will work for higher rpm) seconds then chop the throttle and pull in the clutch. When you find the lean spot the idle will hang and surge.

Adjust that rpm area.

Pay attention to the 1/8 to 3/16. That is the needle diameter and what most people miss tuning. Moving the clip will change it but then it'll have a large effect on your 1/4 throttle too. 1/4 throttle is very important to get right becuase it's always when the bike comes on the pipe hard.

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"Tried a 48 pilot and experienced a slight hesitation off idle and ended up under 3/4 turn out on the AS for proper throttle response which indicates I needed to go to the next LEANER pilot" THIS IS WRONG!

if you had the air screw under 3/4 turn out, you need to go richer on the pilot. the more you open the air screw, the leaner it is. so i think you went backwards on the pilot which could cause the symptoms you are having. if you were 2 turns out on the air screw then you would go to a 45 pilot, 3/4 turns you would go to a 50.....

i agree 100% with lilbit also, this works for me not plug chops!

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There is NOTHING wrong with being over/under the majic internet 3/4 mark on the air screw. If it still leaves adjustment then it's fine. If i still have a little adjustment out, say 2 1/8ish out, in my hottest weather i run that pilot. As you richen up the clip position as the weather cools it will richen up the pilot too. You will still have alot of adjustment in on the airscrew(richer) for cold mornings.

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Eddie over in intake fixed my problem for me... had my idle screw up so high it was causing the issues, Im not so sure how I did this but it was cranked up so high that it actually idled normal. My jetting is spot on...

Spinner: you are correct though, I wrote that all wrong I went back to the 45 because I ended up needing to raise the needle clip 1 spot which made the pilot too lean... the only reason I did the plug chops was because I was having trouble getting it dialed in. Now that I reset my idle and have it set low enough that I need to blip the throttle to keep it running and no more surging with no jetting changes what so ever. All is good now!

Edit: I asked Eddie why it would do this even though the jetting should be spot on, his response was it has nothing to do with the jetting itself (in terms of main, pilot, and needle) he said it was due to the height of the slide needed for this particular motor to idle.

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glad you got it figured out! i would have never thought the idle screw was off with those symptoms.....

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Me either... thats why I was so dumb founded. I asked Eddie why it would display near perfect jetting conditions and surge only while riding the bike but not on the stand? Still not sure why the bike wouldnt be able to idle without surging but he's the expert not me... he simply said the slide height was to tall for that particular motor to idle properly causing the hanging rpm. Didnt give an explination but his advice was to turn the idle screw in to the point where the bike wouldnt idle without a blip of the throttle as long as response stayed crisp.:smirk:

Still doesnt make any sense to me lol but it def. worked!

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Sounds like a band aid rather than a cure. A lean spot before shutting down is still my vote. It can be a tiny area of rpm causing the issue.

If your happy then enjoy. I hate a bike that won't idle but many 2t owners insist on it.

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Do you think a number 8 slide would be better?

I turn my idle screw in exactly one turn, start the bike and gear up. Then turn the idle screw back out one turn for the lowest possible idle and ride.

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so people say. I never tried one. But the guys on the number 8 slide thread swear it is the solution to their problem. How old is the top end? If it doesn't idle any more it could be gone.

I put my idle to about 1200 RPM all by ear of course. YZ450 needed 1800. Higher idle makes starting easier too.

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The idle screw is the same knob as the choke... also controls the choke circuit.

Sounds like a band aid rather than a cure. A lean spot before shutting down is still my vote. It can be a tiny area of rpm causing the issue.

If your happy then enjoy. I hate a bike that won't idle but many 2t owners insist on it.

I believe that Eddie was correct considering there is no more surge. Im gonna go with a JD kit soon any ways, so itll work for now. As soon as I have the spare change. Just cant see spending the coin on a kit right now when I have a race and practice bike to keep up and fresh durring the season.

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The idle screw is the same knob as the choke... also controls the choke circuit.

I believe that Eddie was correct considering there is no more surge. Im gonna go with a JD kit soon any ways, so itll work for now. As soon as I have the spare change. Just cant see spending the coin on a kit right now when I have a race and practice bike to keep up and fresh durring the season.

Turning the choke knob has no affect on the slide height! If you look back and read what he said, it was something like "turn the idle adjuster out". You say you did just the opposite and "turned it in". My guess is he was talking about a carb that has a slide stop screw, which will lower the slide when you turn the idle adjuster out. Turning the choke knob on the 96' carb will either raise or lower the plunger. That plunger sits in front of an air passage that bypasses the slide. As the plunger is raised it begins to open that air passage, the more it's raised the more air it allows to pass around the slide. My guess is by turning the knob in you lowered the plunger and reduced the amount of air that gets around the slide to the motor. Less air is similar to more fuel, in that both result in a richer mixture. So the reduction of air enriches the mixture and prevents the lean surge. Again this is just my guess.

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The surge sounds lean.....I mean, on the stand its ok (no load) but riding (with load) it surges...

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Turning the choke knob has no affect on the slide height! If you look back and read what he said, it was something like "turn the idle adjuster out". You say you did just the opposite and "turned it in". My guess is he was talking about a carb that has a slide stop screw, which will lower the slide when you turn the idle adjuster out. Turning the choke knob on the 96' carb will either raise or lower the plunger. That plunger sits in front of an air passage that bypasses the slide. As the plunger is raised it begins to open that air passage, the more it's raised the more air it allows to pass around the slide. My guess is by turning the knob in you lowered the plunger and reduced the amount of air that gets around the slide to the motor. Less air is similar to more fuel, in that both result in a richer mixture. So the reduction of air enriches the mixture and prevents the lean surge. Again this is just my guess.

I do know this, not a total moron, also I did turn the idle out, sorry I must have mis typed what I was trying to say... hence the reason I said I still didnt understand why this works. He said the problem was caused by the slide height. Im just going by what he told me. I dont understand the reasoning behind it, all I know is it worked for the time being.

I followed his direction, heres the link to the thread.

http://www.thumpertalk.com/forum/showthread.php?t=983290&highlight=

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