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Discussion Starter #1
Hi I'm new here and need some help with my bike!

I have a '81 Yamaha sr250 that's now a bobber. it has a cone filter, straight piped exhaust and as far as I can tell the rest of the motor is stock. I didn't do any of the work and bought it the way it is now. I got it not running because the clutch pushrod was "broken". Once I got the clutch straightened out I filled it with new oil and went about starting it. It started but only on a jump box, ran great, revved up without missing a beat. But it would only off the jump box and as soon as the jump box was disconnect (with the battery still in) it would die. thinking that the stator was bad I replaced the stator. Now the new stator is installed and it will start up just fine but as soon as I try to open the throttle it runs like crap. It goes from smooth idle to bop, bop, bop type of sound out of the exhaust. It also coughs I guess you could say/backfires slightly out of the carb. leading me to believe there's a intake leak/carb issue. that was removed and cleaned, then reinstalled. it runs a little better but not much and I'm still having the same issue. My mind is boggled!
For your info the bike is a electric start and a CDI electronic ignition all stock. the carb is the stock mikuni diaphragm type carb found on yamahas. Any help is great appreciated!
 

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Discussion Starter #4
What size jets should I got with? And why would jets matter now when it ran like a top off of the jump box?
But this started happening after I put a new battery and stator in. I rode it but it kicks and bucks like a mule with the occasional backfire or pop out of the exhaust. I can get it to go a little with the throttle barely cracked. Anything more and it bop bop bang running like butt.
 

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What size jets should I got with? And why would jets matter now when it ran like a top off of the jump box?
But this started happening after I put a new battery and stator in. I rode it but it kicks and bucks like a mule with the occasional backfire or pop out of the exhaust. I can get it to go a little with the throttle barely cracked. Anything more and it bop bop bang running like butt.
My SR250 seems very voltage finicky. Once I won the carb battle and had a good battery in mine it ran like a top. Bad battery, jumped without battery...crap.

I'd start with a new battery, and definitely clean the carb. You ran nice new gas right over all the crud, it could have clogged any of the myrad of passages in that thing. Took me four times to get mine right.

I went up .02 on my main jet and replaced all three. Any crust on those jets isn't worth the time cleaning for the price of new. You might not believe it but an Iridium plug that REALLY helped.
 

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Retired twice: Navy and as a govt contractor
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Check and make sure all your battery connections are good and are tight
 

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Discussion Starter #7
Deven it has fresh gas and I have cleaned the carb. I put a new battery in it but that was after the new stator. After the new stator is when this issue started which led me to clean the carb and the jets were clean.
From what I'm reading it seems like the sr's are very voltage sensitive in general. Think the stator connections being not that tight or corroded on the new stator aren't allowing enough voltage to flow?
The battery connections are tight and good, I tighten them with a socket every time I put them on.
 

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Discussion Starter #9
Well I found the problem, the carb is leaking fuel internally and when it hits the hot vavle on the intake it backfires. So off to find a new carb lol. Anyone know any replacement carbs because I cant find any new stock carbs
 

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Ghost in the machine
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So really the problem started after you cleaned the carb. I suggest cleaning it again, unless it somehow got damaged. It worked fine before you cleaned it, so most likely you worked a piece of crap loose that is now causing the leak. Where EXACTLY is it leaking internally?
 

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Discussion Starter #11
Me cleaning the carb didn't change the way it ran. It ran fine off of a jump box, sat for a few days while I waited for a new stator then after the new stator was installed in ran like crap off of the new battery. So then I cleaned the carb. I'm not sure where internally exactly but there enough gas to wet my fingers and be visible in the carb on the filter side of the carb.
 

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Me cleaning the carb didn't change the way it ran. It ran fine off of a jump box, sat for a few days while I waited for a new stator then after the new stator was installed in ran like crap off of the new battery. So then I cleaned the carb. I'm not sure where internally exactly but there enough gas to wet my fingers and be visible in the carb on the filter side of the carb.
I had to fight with mine too. For a while I had to cup my hand over the filter side to get it to run, gas pooling in my hand. Was carb. Cleaned 3 times before it was close, 4th time for good measure. Take apart, reassemble. New jets!
 

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Discussion Starter #13
Yeah I'm gonna play with it tomorrow and try to get the needle seat real good. I ordered a mikuni vm32-33 to put in just in case the carb is too far gone. I'll let y'all know what happens!
 
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