1974 TX650a Carburetor Question

[modieselguy]

Hello everyone! Long time lurker and searcher but I'm coming up with more questions than answers. My 74 has what seem to be the correct BS38 carburetors. I think I used a MikesXS rebuild kit 10+ years ago and must have swapped our most of the brass unfortunately. I had a situation were the needle must have lifted above the needle valve and was stuck open. I decided to take the carbs apart, to simply take stock of what was inside, especially since the mikesxs kits seem to be notoriously wrong. Here is what I found:

Main Jet: 130
Needle: Y-22 (4JN19) 40mm
Needle Jet: Z-4
Pilot Jet: 42.5

Does anyone recommend a starting point in correcting this? The Z-4 needle jet is throwing me for a loop. Based on the carburetor pictures, is there a recommended combination of jets and needle I should start with? I am running Uni pod filter and different mufflers but they are far from open. Thanks in advance.

 

[toglhot]

I have a 74, but swapped the carbs for VMs some time ago, so can't remember the jetting. There is a plethora of posts in this forum on jetting, everyone of them different. All you can do is try different jetting.

 

[TX650A Van Islander]

The carb guide in the tech section has a listing of proper sized components.

 

[650Skull]

Hello everyone! Long time lurker and searcher but I'm coming up with more questions than answers. My 74 has what seem to be the correct BS38 carburetors. I think I used a MikesXS rebuild kit 10+ years ago and must have swapped our most of the brass unfortunately. I had a situation were the needle must have lifted above the needle valve and was stuck open. I decided to take the carbs apart, to simply take stock of what was inside, especially since the mikesxs kits seem to be notoriously wrong. Here is what I found:

Main Jet: 130
Needle: Y-22 (4JN19) 40mm
Needle Jet: Z-4
Pilot Jet: 42.5

Does anyone recommend a starting point in correcting this? The Z-4 needle jet is throwing me for a loop. Based on the carburetor pictures, is there a recommended combination of jets and needle I should start with? I am running Uni pod filter and different mufflers but they are far from open. Thanks in advance.

No don't try different jetting, bad advice...........Correctly identify your carbs and read the Carb guide, (bottom of post #1, PDF) in the link...... Page 27 in the PDF is a graph showing all the different Jetting for the different carbs
https://www.xs650.com/threads/tentative-carb-id-layout.43392/

 

[5twins]

Not sure where you got Z-4 needle jets? The only bike I know of that had them was the Kawasaki KZ750 twin. You should have slightly richer Z-6 needle jets but unfortunately, you can't get them anywhere, not even repops. There are repop Z-6's sold but they are the long style and only fit/work in '73 and older carbs.

 

[modieselguy]

Not sure where you got Z-4 needle jets? The only bike I know of that had them was the Kawasaki KZ750 twin.

Honestly, I have no idea. The carbs original to the bike were badly corroded, but I had set from another 74. I did not swap the needle jet, those do show age to them, and both carbs have the Z-4 in them. Where should I go from here? Is there a combination of available parts I should try? Thanks everyone for their advice and guidance.

 

[TX650A Van Islander]

My original '74 carbs were somewhat corroded as well and I eventually switched out those carbs for a set of '76-'77 linked carbs which need only one throttle cable. The result was way better. This was documented in my thread on my '74. If you can find a set of those carburetors, that is my recommendation.

 

[modieselguy]

You should have slightly richer Z-6 needle jets but unfortunately, you can't get them anywhere, not even repops. There are repop Z-6's sold but they are the long style and only fit/work in '73 and older carbs.

I wonder if I could shorten the long Z-6 needle jet to match the length needed for the 74-75 carb? Then I could get the correct needle and jets to go with the short z-6.

 

[modieselguy]

I went ahead and ordered all the stock size jets and needles from Yamahaxs650.com. I'm not sure if I can, but it looks like I can shorten the long Z-6 needle jet, so I ordered a couple of them. Fingers crossed it works and hopefully I get the shipment next week.

 

[5twins]

That Y-22 needle @ 40mm long is way too short and probably the root of your problems. Just swapping in the 4N8 needles when you get them may fix things, even with the Z-4 needle jets. Main jet size is tailored to needle jet size. Smaller needle jets can use bigger mains, bigger needle jets need smaller mains. Here's a chart of the stock jetting and you can see how the main jet sizes compare to the needle jet sizes .....

And here's a look at the various needles used in the BS38s. The 4N8 is 5mm longer than the 4JN19 (or the Y-22 copy you have). Running that short needle in your carbs will make them way too rich .....