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Discussion Starter #1
I have a 2008 yfz450 that I bought from a friend about two months ago it sat for about 4 years. I cleaned it up, carb rebuild, put hmf slip on on and it was running fine, took to on a small dune trip to Bowie az ran great for two days. The friend that sold it to me showed up and took it for a ride and said it's back firing which it did not do when I rode it. So ever since then about three weeks ago it back fires and bogs only when I stand up. If I sit down it runs great but standing immediately causes popping and poor running. I'm at a total loss as to why this is happening but think it's the seat pushing on a wire or switch/relay somewhere when I sit down. I thought it as the relay above the battery which is under the seat but today I started the bike and pulled the relay out and the bike did the same thing, ran fine when sitting but not standing or with the seat off. Messed with the plug that goes into the starter switch under the seat between the two fuses same thing. Does anyone know what this could possibly be? I do have the relay in order but don't know if this would be the issue as I thought it as only for sending power to the starter switch.
 

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Did you check your grounds and the black single wire running back towards the battery?
 

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Discussion Starter #3
I did check the black single wire unplugged it and plugged it back in seemed to make a good connection. As far as other grounds no as I didn't see any others back there near the air box.
 

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Take a video and post it, that would help diagnose. Did you check all of the battery connections for the solenoid and battery under the seat (the bolted ones)? That is a very odd problem to have, I've never seen it happen before. I did have a solenoid come loose once but the seat didn't make a difference that I noticed.
 

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Discussion Starter #5
I'll try to make one tonight and post it. I do get the relay today but I don't think that's the problem at this point but we'll see.
 

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Discussion Starter #6
I've checked the posts to the solenoid and they seem solid I didn't actually put a wrench on them though as they were never removed when cleaning up the bike.
 

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Discussion Starter #8
So I pulled the plastic off the rear end and moved the battery so it's sitting in the air box, stripped the harness back to just before the air box and checked ask the wires found that the small ground that goes to the negative was only holding on by like two copper strands of wire so I fixed that ground but still had the issue. Decided about an hour later to put the bike on a jack and run it in second with the tires off the ground and applying some back brake at times to simulate a load no popping, then rode it down the street and no popping but with the battery just testing in the air box I can't really get on it to see if it's fixed I'll put it all back together today and update everyone. Does anyone know what that small black wire does that's grounded to the negative and runs into the harness? Online someone said it goes directly to the cdi.
 

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Discussion Starter #9
If it does it again I'll make a video this weekend see if anyone can chime in on what they think it is. The fuel doesn't seem like it could be a possibility cause it ran perfect when I'm sitting down it's only when I stand or there is no seat that it runs poorly. I did replace that relay in the back and no change.
 
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