carbs backfiring

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xs650dude63

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Hey everyone, I have a 1980 xs650 special, with the BS34 carbs, I readjusted the cam chain, I re-lashed the valves, I totally cleaned the carbs and rebuilt them with a basic rebuild kit it has new spark plugs, but the wires and the caps are not new, but they look in new condition. I set the air-fuel mixture screws 3 turns out, when I start it it idles ok, but it backfires out of the carbs and sometimes stalls.....any ideas? Thanks!:thumbsup:

Also, is it possible that the carbs are jetted wrong? I am running with two small air filters, they look like K&N filters, exhaust is a 2 into 1. If I need different jets, what do you recommend and how can I tell what I have in there now, I don't remember seeing any numbers on my jets. Thanks!
 
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Were you running the same exhaust and filters before you rebuilt the carbs? Did you make sure your float height was correct after you rebuilt them?
 
if you put the needles from the kit in your carbs, odds are they are the wrong ones. clean up the old ones and use them. if you didn't, one up on the pilots will be better. 3 turns out starts to get close to the limit. The spring behind the mix screw keeps tension on the needle so it dosnt move on its own. you want to be 2 to 2- 1/2 out. Please post pic of filters. Those k&n style are known to block the slide lift hole with their larger than needed mount lip. uni pods is what most of us use.
Just as an experiment, take off your filters and run around the block, and see if you have an improvement.
 
Float height is spot-on, checked it several times, same exhaust and filters, I bought this bike from a guy who gave no information on it, so I don't know if the air-fuel mixture screws were original or aftermarket, so I thought it would be best to replace them. he did say the carbs were re-jetted, but who knows??? I will dial down the to 2-2 1/2 out and see if that makes a difference. the filters are the same ones that mikexs sells, they are supposed to not block the holes....so I hope they don't.

When you say one up on the pilots, please be more specific, i'm new to all this, how do I find out what I have in there now? Thanks!
 
Your BS34 carbs are E.P.A. mandated and came set up rather lean from the factory. They usually require 3 to 3.5 turns out on the mix screws to run right and with no popping. Your jets have size numbers stamped on them. The pilot jets are stamped on the sides so will need to be removed to see the number. The mains are stamped on the head and should be able to be read with the float bowls removed. Here's a main in the stock size, 132.5 .....

GenuineMikuniJet.jpg


The pilots are under a rubber plug. Pop the plug out and the jet can be unscrewed and it's number read .....

PilotRubberPlug.jpg


Here's a chart showing the stock sizes .....

CarbSpecsReducedSize.jpg


The problem with your pods is more than just that possible port blocking. The K&N pleated style of filter just doesn't work well on a CV carb. The pleats cause turbulence in the air flow and that messes with slide lift. You may not ever be able to tune the bike perfectly if you use them.
 
Thanks, which pods do you recommend through Mikexs? I will get the jets out later and see which ones I have and you guys can recommend the ones you think I should get. Thanks again!
 
Looks like I got 135 Main Jets and 45 pilot jets, I bought the uni pods, could these jets be causing the carbs to backfire and pop and not idle correctly, I have the air mixture screws out 3 turns, I slowly adjusted them in both directions still backfiring, popping out of carbs. I was told by the previous owner the bike is a 79, the title says its an 1980, it has the black rubber like floats not brass. I have the float height set spot on at 22MM.
 
ok, I tried to sync them, i turned the idle screw in all the way, but the butterfly's barely have an opening, maybe a MM or two, to small to fit a drill bit in the opening, so I used a needle. Does it matter how big the gap is for the butterfly's when syncing, as long as the gap as small as it is, is the same on both sides?

I just removed the main jets and one is a 135 and the other is a 145, could this be causing all these problems?
 
Would the main jets being different sizes cause the idle to be rough, and backfire out the carb, I think the side that had the 145 jet was the one backfiring. Are the 45 pilot jets the right size for my set up? Shoud I go up or down on them?
 
I don't have another 135 main valve, so I have to buy another one. Should I stick with the 45 pilots and the 135 mains on this bike or should I go up one? Thanks!
 
It's hard to say exactly what you'll need. We can suggest a range that has worked for others with similar mods but exact jetting can differ slightly from bike to bike. Only testing a few sizes will tell you for sure. For your mods, 1 to 2 up on the pilots and 1 to 3 or 4 up on the mains should do. The first thing you need to do is get matched sizes in there. That's probably most of the problem. I'd get another 135 and some 137.5s.

One of the first rules of jetting is that when you think you're right, try the size above and below to make sure. Obviously, you're not going to want to go smaller, back to stock, so a few larger sizes are in order. There's no short cuts here. Don't jump around and skip sizes. Increase one size at a time. The 650 usually only needs a few sizes larger than stock for most mods.
 
Thanks, good information. I am going to buy 47.5 and 50 pilot jets and 137.5, 142.5 and 145 main jets. hows that sound, should I get more? Thanks!
 
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You shouldn't need 50 pilots but the rest sound good. Pretty much everybody gets by with 45 or 47.5 pilots in your carb set. Don't forget 140 mains. That just might be the ones you need. Remember, when working in this narrow range of sizes, you don't want to skip any.
 
I put in 47.5 pilots with 135 mains and I got it to start and run alittle, then it stalled and I couldn't get it to start after that, so I put in 137.5 mains and it still won't start, I don't, know what to do, am I right that the float height is supposed to be 22mm, for these carbs, 1980 BS34, black plastic float? The previous owner says its a 1979, does that make a difference, title says 1980. Am I supposed to adjust the needle height on these carbs, or are they non-adjustable?

When it was running with the 47.5 pilots and the 135 mains, I started it with the choke fully pulled out, it ldled at about 1700 RPM, for about 30 seconds then I pushed the choke all the way in and the idle dropped down to about 1100 RPM, at that time the idle screw was turned in all the way, if I was to back it off at all the idle would drop down so low it would stall.
 
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According to 5T's chart...BS34's with plastic floats would be an '81...what year is listed on the frame of the bike? 22mm is correct for floats if that's what you have. Did you reset the air/fuel mix when you re-jetted? Or are you still at 2 1/2 turns out?
 

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