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OK, I'm confused now...

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I hate to be the guy that asks about decel backfire/popping AGAIN, but I'm a confused now.

I've got the idle running I think pretty good now. It seems to be right on 2 to 2 and 1/4 turns out. I read that at more than 2 turns you should go one jet size smaller.

BUT I'm getting a lot of popping/backfiring on decel. Like almost constant. I've been reading around that that is from it being too lean and I should go UP on my pilot jet...?

Maybe I need to raise the clip one position?

I'm running a JD kit in a '04 YZ250F with starting settings per the little paper that came with the kit.

Altitude is about 2700 feet.

Temp is 60-70 (finally!)

Stock header with Powercore/quietcore insert.

I've also noticed it doesn't pop as much when the choke is on, but that makes the mixture more rich, right? So maybe I need to LOWER the clip one?

Thanks for any help!

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You can richen up the pilot to make it go way but the idle will be messed up, if it will idle, hot starts will probably be impossible.

The clip position has nothing to do with this. If the fuel screw is adjusted for a perfect idle, then it is right.

You have a race bike. Racers go WOT than on the brakes. They do not coast. Race bikes are not meant to be trail bikes or play bikes.

You can put a good muffler on the bike. You can make sure there are no exhaust leaks. You can ride faster.

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OK, so basically it's normal? I just don't want to go off and break something:bonk:

I put the Powercore on it and used the high temp RTV silicone like they said at the interface with the header. I noticed some of it kind of pushed out when I rode it the first time after that. I waited about 48 hours before riding, but maybe I have an exhaust leak... How would I check?

I don't recall the popping being as bad before I installed the JD kit and the Powercore, so I just assumed that I had the jetting wrong. I also heard one guy say that his popping problem occurred after he repacked the exhaust and went away after a little riding. Maybe I just need to break in my exhaust...

Thanks for the help.

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To check for a leak, hold a towel over the muffler end and listen for hissing.

You made your bike louder. Stock muffler actually quenchs some of the pops.

Fuel Screw/Pilot Jet

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

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OK, thanks a bunch. I think I might be a tad lean, I'll dabble with it a little.

I went with the powercore and quietcore insert because people were telling me it was essentially a Q4. I needed to get a spark arrestor anyway. I think it is a little quieter than stock, but maybe it makes the backfires a little worse...

I know a guy that might still have one of those exhaust gas examiner machines that tell you the o2 level from back in his car racing days... I wonder if it still works:thinking:

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OK, I just rode it a little. I ended up at three and a half turns with no backfiring! Maybe a put or two here and there but not a constant loud crackle. I think I'll bump up the jet size by one and repeat. I'm glad I didn't ride it too much with it that lean!

Thanks for all the help William!

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3.5 turns is not good, the fuel screw will fall out. Is the 3.5 turns based on a perfect idle or no decel popping? If it is solely to get rid of decel popping you may find the bike is very hard to hot start.

No damage comes from a 4S being lean.

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I was riding it up and down the street and turned it out until it wouldn't studder at all on accel or pop on decel.

It may be rich, but it was running just like when I had the choke on before (when it was lean) and it was running real well. If I have trouble hot starting, I'll lean it up a little. I also picked up a 45 (I'm pretty sure I have the stock 42 in there right now) pilot jet earlier today so I'll throw that in.

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Just threw in the 45 pilot jet. Dialed it to two and a quarter turns and it's running great! It even hot started after just a few kicks.

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