Carbs,are 34,s superior than 38,s?

[XS650D]

I have a 77 with the stock 38,s and they run great BUT after having a 1980G with the 34,s i feel
that the 34,s are better. The fuel flow makes more power down low and in the med range were I feel
the 38,s make better power in the upper mid range to full throttle range.I have my 38,s dialed in pretty
good I feel after experimenting with different jets, needle positions and shimming ect.They just dont
have the same pull on the motor below 4 grand. Off the line in 1st gear it pulls great but if your just
cruising round at 2 or 3 grand and pull the throttle it just doesn't have the same pull .Im not sure if this can be
dailed out.I am running the stock front gear and 1 tooth down on rear gear (33 tooth) I may go back to
the stock gearing to keep the rpms up around 4 grand as your always on the power band with those gears.(The 33 is nicer on the Hwy when just cruising)
I have experimented with another stock 77 that has the stock gearing compared to my geared down 77.
The 34,s may have a lean problem in the mid range based on what I have read especially if you add pods
and pipes but looks like it can be cured with Canadian needles ect.The compression numbers are down on the 76/77 compared to later and earlier years also I noticed, this may have an impact on the way the bike runs also but cant confirm that. I have also experimented with Timing quit a bit and noticed that a retarded setting smooths out the
power band and also lets the motor run cooler BUT at the cost of some slight loss in performance .
Anyone experience this and find a mod to make the 38,s run better in the bottom end.I think that its the typical bigger makes more power but for the type of riding the 650 is designed for the 34,s just work better.
Just my personal thoughts and observations after riding and tuning this bike for 3 years. Still without a doubt the nicest all round bike I've ever had the pleasure of owning!

 

[TwoManyXS1Bs]

Dunno, XS650D. Sometimes they're just mysteriously different.

http://www.xs650.com/threads/top-speed.30903/page-3#post-311967

 

[xjwmx]

There are many who are of the belief that the BS34 is the better carb and that the TCI is the best ignition, incl. aftermarket ignitions, There are even some who think the Special is the best looking of the clan. I count myself among those -- I think the earlier bikes look dainty, But the Special's looks does take a little tweaking, Some radical anti-government types think the '34 was ruined by a government conspiracy and needs lots of modification but will then be the better. Luckily the TCI is too incomprehensible to modify. Some think the Special looks most appropriate posed beside a disco ball and that any kind of backyard hacking is an improvement.

 

[gggGary]

Dunno I have both on bikes that run and pull great from down low. My recent excursion with BS34 jetting had me back at a fairly stock config and now I'm happy. I have one stone stock 79 that is a bit fussier, leaner down low, works fine but a noticeable difference from the restomod that pulls very well from the basement up, near impossible to stall or get a lean hiccup. Sounds like you have it dialed in close, but there is still a detail that's throwing a wrench in the works. Throttle shaft seals done, air leaks hunted down? How's compression?

 

[XS650D]

Yes I have gone thru the typical check for air leaks, changed throttle shaft seals ect Compression 140 both sides.
Plugs look perfect Grey,light brown color.Its not that it runs bad, it runs quite good and clean just a little sleepy between 2000 and 4000 rpm compared
to 4000 rpm and up.I think I will try a stock 34 rear gear when I change my rear tire and do rear pads.I am running one size up on the pilot, have tried 2 sizes up and dropping to 1 cleaned the bottom end up, felt crisp and clean.May try stock pilot but pretty sure its the gearing.

 

[DogBunny]

In my opinion:
The BS38s are way oversized, which makes them a good choice if you are going to put on a free-flowing exhaust and/or put on a big-bore kit, or do other engine performance modifications.
However, the BS38 throttle plates are inherently sloppier. This leads to possible idling issues, particularly when a throttle plate or its bore is worn, and you just can't get the two plates to cooperate with each other at idle speeds. The bs34 throttle plates and bores are very tight and don't have this problem. Yes, they are EPA hindered, but otherwise are a generation of technology improved over the BS38.

 

[XS650D]

Dog bunny, i think u are bang on! I tried dropping the pilot down to stock size and needle down 1 clip and adjusting
air screws for the 100th time and its as good as it gets in performance .I thing the guy I know with better running 77 is just a better motor
and with the freer flowing BUBS exhaust he has, it works well with the design of the 77. That change alone could account
for the better performance but Bugs no longer sells that exact exhaust which he purchased back in 77.It is loud and not as
nice sounding as the stocks pipes or even the Mikes Commandos I have.

 

[XS650D]

Anyone know how much extra HP the old Bugs exhaust gave back in the day?