Carb Jetting :: BS34s :: 1981 xs special :: short pipes + UNI foam filters

Rlauchard

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Hi all...been a while for me. Life definitely got in the way and my bike has been sitting with little riding/work for almost 7 years! My son is now 15.5 and we started wrenching just the other day. Basically... the bike sounds like shit as expected. Hard to start, needs full choke to run, pops while running...here's where I am:
- adjusted valves the other night
- changing all oil and oil filters
- definitely dealing with new gas in the tank and carbs
- giving the carbs a good cleaning

I can't find where I posted this question years ago, but I'm wondering if anyone would have suggestions as to the jetting that I would need given the information provided? Just looking for suggestions...I realize it will be a trial and error to get to good.
- short pipes from hugh's handbuilt
- uni foam filters on the carbs

I know I re-jetted at some point, but wanted to start to see if anyone has a suggestion. Thanks!
 
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Not sure why/how I've missed this... there are 2 main jets on the BS34s? There is a main jet on the air filter side of the carb that has a stock 135 and a float bowl main jet that has a stock 132.5!?

So if I originally used a 137.5 main on the float bowl main, but never changed the main on the filter side...then I would/should have changed the float bowl main jet two steps up which would be a 140 main on the filter side.

@jpdevol when you suggested "137.5-140", you meant this? Many I'm going to feel dumb as shit if that's the case.
 
Was the bike running well when life got in the way? If so pour a generous amount of naptha in the tank and take a long easy ride.... Naptha has nearly the same BTU/liter energy as gasoline. It has a smaller molecule so acts as a good solvent for gasoline deposits. It probably has poorer anti-knock properties so I suggested a long "easy" ride...
 
Last edited:
Not sure why/how I've missed this... there are 2 main jets on the BS34s? There is a main jet on the air filter side of the carb that has a stock 135 and a float bowl main jet that has a stock 132.5!?

So if I originally used a 137.5 main on the float bowl main, but never changed the main on the filter side...then I would/should have changed the float bowl main jet two steps up which would be a 140 main on the filter side.

@jpdevol when you suggested "137.5-140", you meant this? Many I'm going to feel dumb as shit if that's the case.
The jet in the intake bell is the main air jet that is a small round type main and feeds the pilot circuit. I usually don't change that jet. The main jet on the BS34 is a large round jet that screws into the bottom of the needle jet - that jet is 132.5 stock and often needs to be ~137.5 with "pipes & pods"
 
I am in the process of cleaning the carbs now @jpdevol ... they were really bad. I was just surprised last night that there are 2 main jets in the diagram and that I missed it. I did change the large round main jet (prior to this) to 137.5 and the pilot jet to 45. I just wanted to make sure I didn't also need to change the small round jet, as I hadn't before.

Sounds like it's fine to keep the stock jet for the small round to the 135 (air filter side).
 
You can use that air jet to fine tune the pilot circuit if need be. Changing it doesn't have as big of an effect on the pilot circuit as changing the pilot jet does. That means you can achieve richness levels in between pilot jet sizes by changing it. I usually reduce it by one or two sizes. The European and "world" models came with a 130 air jet.
 
UPDATE: My 15 year old son learned how to do the oil change, sump filter change, valve adjustments and clean the carbs. Since the jets were shot, I replaced with all new... 135 air jet (stock xs650 special), 137.5 main and the 45 to the pilot. It started perfectly and I actually got a clean idle with no choke.

Ran it up the road and got some popping from the left side...will probably clean the carbs again, and then recheck.
 
The mix screw setting can cause popping if set too lean. The 1.5 turns out setting you may have run across for many of the other carb sets would be too lean on the BS34s. An all stock bike likes 3 to 3.5 turns out. Yours, with the larger pilots, may be OK down to about 2.5 turns out.
 
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