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Can missing vent lines cause lean condition

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Hello all. Went to the track today and my YZ250 ran lean on the pilot the whole day...as in I pulled my plug towards the begining and it was white. I thought maybe an air leak but it idled great and was never erratic (for the record, I replaced my left crank seal about a year ago with OEM). I'm running a 55 pilot and had to turn my air screw in (was originally 2.5 turns from closed) from one turn out from closed. It was still on the lean side, but it was better. The 55 is the biggest pilot I have, however I feel like there is something else at play here as that's a fairly large jet. 

One thing I did notice as I was loading up, one of my vent tubes popped off the carburator. I read on another forum that that could cause issues with the vaccum inside the carb and could result in a lean condition. Has anyone else experianced a missing vent tube causing problems, or does this sound like FUD? Of course I'm going to put it back on before my next ride so I guess ultimately we will see.  Just wanted to see what you all thought in the meantime. 

Last week was clear in the hi 90s with hi humidity and was running on the rich side. Today was cloudy, mid 70s, humidity felt lower. Both days I'm running 93 octane at 32:1 and 55 pilot. I know that it would naturally run leaner on a day like today versus last week due to the lower temp and humidity, so maybe that's all it was? Still, seems like a lot...

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Well, nevermind about the disconnected vent hose. I just went out and realized one of the hoses was laying ontop of the cases right under the carb, instead of being routed downward. I thought the end I was looking at came off one of the vents. All vents have hoses attached. 

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10 hours ago, Shooter88 said:

Well, nevermind about the disconnected vent hose. I just went out and realized one of the hoses was laying ontop of the cases right under the carb, instead of being routed downward. I thought the end I was looking at came off one of the vents. All vents have hoses attached. 

The temp and humidity drop alone will lean it out especially if you jetted for the hot weather like me. 

Lately we've been riding in 80-90° temps and 90% humidity and yesterday morning when we got to trails it was 65° and barely any humidity. Ours ran very lean til I turned air screw in 1/4 turn

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11 hours ago, Smoking 2's said:

The temp and humidity drop alone will lean it out especially if you jetted for the hot weather like me. 

Lately we've been riding in 80-90° temps and 90% humidity and yesterday morning when we got to trails it was 65° and barely any humidity. Ours ran very lean til I turned air screw in 1/4 turn

Yeah, I figured the temp and humidity change had a play in it. Maybe that's all it was.

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Something seems wrong to me. Your pilot is way big and your bike should not be lean at all on a 55. At 70F your pilot should be 50 max. Even at 30F at sea level your pilot would be around 52 max. What rpm were you lean? Maybe it wasn't your pilot circuit.

What jets/needle are you running?

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based on lhsd's comment, might take the airscrew out and make sure all the parts are there and the oring isn't split or rotten, also that everything is in the right order.

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2 hours ago, LSHD said:

Something seems wrong to me. Your pilot is way big and your bike should not be lean at all on a 55. At 70F your pilot should be 50 max. Even at 30F at sea level your pilot would be around 52 max. What rpm were you lean? Maybe it wasn't your pilot circuit.

What jets/needle are you running?

I tried raising the needle one clip and it got that rich stutter under acceleration. I put it back to the second from the top clip position. I tried bumping the main up as well but to no avail. Currently, my jetting is:

AS: 1 turn out

Pilot: 55

Main: 178

Needle: stock, second clip position (from top).

I agree, 55 is big which is why I think something else is at play...not just the weather temps. I ordered a carb rebuild kit from RMATV and will replace all o rings and seals and see what happens.

 

Edited by Shooter88

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3 minutes ago, LSHD said:

Have checked float level and operation? 

I have checked it in the past and it was fine, however I have not checked since this whole thing happened. When I have it apart to install the rebuild kit, I'll check it again. 

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