BS34's popping/backfiring thru carb

[Under Dog]

Hey everyone! Just joined up the other day. Glad to finally see a site for the XS400!

I purchased a beater '80 XS400 SG for my son to tinker on. When we picked it up, it didn't run. Well, we've been messing with the carbs lately. They are definitely clean now (all passages clear), but we're still having an issue I'd like to run by you all.

Just this afternoon we got the bike to idle off choke. Seems that while all jets and passages were clear, that our airscrew was pretty much closed off. It was set at 1 1/4 turns from lightly seated, but you could see the screw protruding in to the carb. I'm guessing the o-ring is pretty much shot and that 1 1/4 turns was equivalent to being screwed closed.

Right now I've got the screws at 3 turns out and the bike is finally idling on it's own! However, it's popping/back firing thru the carb pretty regularly. What's that likely indicative of? Still too lean and I should back the air screw out a little more? Or are we ready to move on to timing, valves, and other things?

What do you all think?
Thanks!

 

[bentwrench]

Just want to make sure - the BS34s have an idle screw vertically mounted in the carbs at the front, sometimes under a brass plate (unless a previous owner removed them.)

I didn't think the 80 bike ran on BS34s - I thought it used the BS38s?

 

[Under Dog]

Hmmm...

Well, the airscrew is vertically mounted at the front of the carb. CHECK
It used to be under a brass plate. CHECK
The previous owner removed it (and messed with it too much). CHECK

BS34 or BS38. My Haynes says 34. My carbs have no markings to show what exactly they are. I'm assuming they are 34's...

 

[bcware]

Right now I've got the screws at 3 turns out and the bike is finally idling on it's own! However, it's popping/back firing thru the carb pretty regularly. What's that likely indicative of? Still too lean and I should back the air screw out a little more? Or are we ready to move on to timing, valves, and other things?

What do you all think?
Thanks!

Replace the, likely, disintegrated o-rings. I have also seen leaking carb boots cause this. Check for cracks in the carb boots and that they are clamped to the carbs tight enough. However, there is also the possibility that the boots are not seating against the engine surface well enough. There should also be a gasket between the boots and the engine surface they mount on. Don't over-tighten the carbs boots should you decide to fiddle with them.

 

[Under Dog]

Well, we checked for air leaks around the boots but didn't find anything, so we ended up putting the stock air boxes back on and started focusing our attention elsewhere. Figured we'd get a few other things working on this bike before fine tuning everything.

For grins, when we start the bike up now, we're not noticing ANY popping... ??? I would have assumed the popping was a lean condition and that running with the airboxes back on would have leaned it out even more. ??? Any explanations out there as to what's happening? Or should I just be happy it quit for the time being?

Thanks!

 

[drewpy]

the airbox restrict the air flow to the specification Yamaha designed.

So more air (without the airbox) will lean the mix

 

[Under Dog]

Will these engines pop thru the carb if too rich? Maybe I should back the air screw back towards 2 turns out???