BS38 Main Jet Question

I have searched high and low and can't find an answer to this one.

I have a 77D , U.S. model apparently that I always thought was pretty much stock however it would appear P.O. swapped out carbs for some reason, and I now have bitsa carbs.

Specs say that I should have BS38 - 584's but after looking at the "Overview of the 38mm" on XS650 and at this chart amended with model numbers added by bluebikerblan posted 08/15/2011,

http://www.xs650.com/attachments/scan_doc0011-pdf.10321/


I now see that I must have 74-75 BS38-447's.

I have one throttle cable ruling out 256, 306, 366, no castings below butterfly's for connector plate, and one "Choke" on left side which rules out 533's and 584/E1E's. P.O has also added the diaphragm cover connecting plate that appeared on the 584's.

When tearing down, inspecting, cleaning , examining this weekend I found the following which also helps to confirm (I think) that I have 447's.

Needle 4N8
Needle Jet Z-6 (don't know if it's a short or long but it is 20mm long)
Pilot Jet BS30/96 45
Float needles 2.0
Butterfly's 120
Main Jet 120 (All I can read on one is 20, due to damage, on the other I can read 120. I have properly moded screwdrivers, P.O didn't)

Sorry for the long preamble but thought I'd better give all the details I could.

Now my question. Specs call for a main jet 127.5, mine is 120. I know that with carburetors a minute change can have have a major effect. Will this different main jet matter or should I order and install 127.5 mains before I button up, re-install and setup?

Thanks in advance.

It does sound like you have '74-'75 carbs. They must have been modified to use only one throttle cable by swapping in later butterfly shafts that mate together. I would go with 127.5 mains. The other carb circuits, especially the midrange, were set up and jetted to work with that size main.

The difference in needle jet length was in the top portion that sticks into the carb. The bottom portion that hangs out into the float bowl was the same on both. This Australian carb chart gives lengths for that upper portion .....

http://www.xs650.org.au/Technical Info/XS-TX650 Carburetor Evolution.htm

The longer needle jet was required on the '73 and older carbs because the shape of the main bore through the carb body differed slightly from the later carbs. The floor under the slide was raised. This made the body in that area between the bore and the float bowl thicker. A longer needle jet was required to reach through .....



Later carbs lowered the floor under the slide and a shorter needle jet was used .....



Needle lengths changed as well. The 4JN19 used in the '73 and older carbs was about 40mm long (top clip slot to tip). When the floor was dropped, longer needles were needed to reach it. The 4N8 and 4M1 needles are 45 to 46mm long.

Thank you for your reply 5twins.
Yes butterfly shafts have been swapped and older carb model diaphragm cover connecting plate was added. I will get the 127.5 mains.
It always wanted to run rich, maybe this was part of the problem. The fact that I was adjusting to 584 specs versus 447's wouldn't have helped with that either. Lol.
Didn't know about the "dead cylinder" method back in the day.

The way you found the mains with numbers barely visible anymore may indicate that these carbs were all gummed up and sat that way for some time. If that is the case, the gas varnish and residue can actually eat away at the metal parts in the carb, both the jets and the passageways. The needle jets could be all eaten away inside as well, making them bigger (richer) than their original Z-6 size. Unfortunately, there are no short Z-6 replacements available. This could explain why the carbs run rich even with smaller than spec mains.

Sometimes a carb set is just plain worn out and needs replacing. If you can't tune out the overly rich running on these with stock jet sizes, that may be the case here. I would replace the pilots as well. They could be all eaten away too.

Yes, you tune these carb sets based on the year of the carb set, not the motor or bike.

Thanks again for the info and advice 5twins.

Actually to my untrained eye the mains and pilots looked remarkably good in terms of their condition. Clean, non varnished.

The illegible condition of the mains was due to improper sized screwdriver camming out and enlarging the slot.

Never suspected that the carbs weren't stock.

Thanks to you and many others here for 'schooling' me. I'm just trying to keep up with my homework!

Can anyone give me specs on jet size, needle adjustment, etc. The p.o. said that he was experimenting and I have no idea what he did. I have an 83 but I believe these are 38s off a 78 or 79

Look at the attachments in post #1. They give the jet sizes and settings.