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YZ250 full throttle popping

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I hit the other local track (sand all the way to your underwear, mostly short straights, sharp turns) for the first time this year. The bike was running sweet, other than sometimes it would pop when accelerating WOT from a turn, in the meat of the powerband. I did not notice popping on smaller throttle openings or under less load (WOT accelerations in the gravel-surfaced pits). Nor have I heard it on the more open tracks. The popping was almost like decel popping on a 4-stroke but not as loud. Couldn't really tell, whether it was coming from the exhaust or intake. I have heard my share of knocking and that is not what it sounded like.

The bike is a 2007 250. Bought it with 70 hours, now it has a hundred hours on the clock. Stock intake parts, I went from a #182 mj to a #185 (running euro pump swill) after a few laps because I felt it would suit the track better, which it indeed did (funny, how the tighter track makes it possible to rev it out more). Same popping still. Pipe is DEP, not a hint of gunk or spooge. The piston is a dual ring Wössner with some 20 hours on it. I am running a 0.5mm paper base gasket and stock head, so squish is somewhere around 2mm. With the stock metal gasket it was 2.2mm IIRC.

So, spark plug, reeds or something else entirely? I did not have another plug to test with, that would be my guess.

Reeds looked impeccable when I changed the piston 20 hours ago. Don't think they have been ever changed, though. And would a leaking reed valve even cause something like that? The VF3 on the 125 would eat the corners of the petals (you could almost fit a pinky through the holes in the end) and because the power loss was so gradual, the only hint you ever got was the bike refusing to idle.


Anyone else had similar symptoms?

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May sound crazy and not be the problem but have seen spark plug cap do that. Has resistor in it I believe. Broken many reeds in my day they make it break up all the time or won't rev. Or start if really bad. 

Edited by Motox367

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It was the spark plug. The gap was a bit on the wide side, too. But I think the main reason for the popping was, that the plug (á la previous owner) was B8EG, not BR8EG. Dunno how I have completely omitted that detail until now:bonk: There most probably is a reason for the manufacturer recommending a plug with a resistor, namely interference created by sparking. Tossed in a new BR8ES and not a hiccup anymore on the same track under similar conditions. Now I'm wondering, if I will see any ill effects from running the cheaper ES instead of the EG. Did not see any when running an EG instead of the EIX the manual calls for on the 125 so I think I should be fine. EG and EIX both have a fine wire electrode, though...

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57 minutes ago, kilikoli said:

It was the spark plug. The gap was a bit on the wide side, too. But I think the main reason for the popping was, that the plug (á la previous owner) was B8EG, not BR8EG. Dunno how I have completely omitted that detail until now:bonk: There most probably is a reason for the manufacturer recommending a plug with a resistor, namely interference created by sparking. Tossed in a new BR8ES and not a hiccup anymore on the same track under similar conditions. Now I'm wondering, if I will see any ill effects from running the cheaper ES instead of the EG. Did not see any when running an EG instead of the EIX the manual calls for on the 125 so I think I should be fine. EG and EIX both have a fine wire electrode, though...

I've ran EG before without any problems.  Little cheaper than EIX. Glad it's going good anyway :thumbsup: I wouldn't think you'd see any difference or problems with the ES. Maybe foul easier but if you're on the gas they don't foul :ride:

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