On the way to happy carbs

[Pat D]

I've been having all sorts of issues with the carbs on my '74. The needle in the right side carb would lift higher than the jet tube and prop the slide full open during hard pulls, the bike doesn't like to idle(I knew I had a buggered up pilot jet on the right side), had some issues with floats sticking and dumping out fuel, other times the bike wouldn't start and regardless of throttle position or enrichener position the plugs would be bone dry. I was planning on waiting for winter to deal with all this but decided to just go for it while there is still some riding weather left.

Tore the carbs down and found a couple interesting issues. First thing was I determined that both float sets had fuel in them, so new floats are on the way. I also ordered rebuild kits to take care of potential float needle leaks and get new jets. The carbs are running 130 mains at the moment, and they look to be in good shape. These are the next step up from the stock 127.5 mains, I believe. The motor is stock other than being bored to 2nd oversize, and the mufflers being somewhat light on fiberglass packing.

The big discovery was that the carbs have 2 different slide needles. The right side, the one that would get stuck open, has a 42.5mm long needle. The left carb needle is 47mm long. D'oh!

Now I'm baffled. The rebuild kit for the '74-'75 BS38's on Mikes site says the needle is 40mm. Now, if I'm having issues with a 42.5mm needle, seems like a 40mm needle would make the issue worse. Am I measuring wrong? I'm measuring the total length of the needle, should I be measuring to one of the slots?

Last check on the plugs showed a pretty lean condition. FWIW, the left side carb, the one with the non-buggered pilot jet and the long needle, seemed pretty happy. It would idle nice and steady on that side, where the right side exhaust note at idle is always erratic, and the right side carb does not respond to blips on the throttle as well as the left side. Being a '74, my carbs have their own cables and are not tied together so I can "blip" either side individually.

I've been through the carb guide quite a bit, I feel like I have a pretty good handle on how these things are supposed to work, but I would love to hear about needle length variables from any folks with more experience, specifically, what is the proper length needle for my application? Seems like the 42.5 is too short, and 40 being even shorter will make the situation worse.

Thanks in advance for any and all insights!

 

[Pat D]

Got my magnifiers out, and found the following data on my needles. The short one is a Y22, the long one is the correct 4N8. The needle jets are both identical, but I can't see any markings on them. Did some more research and found this thread http://www.xs650.com/forum/showthread.php?t=11879
5twins info was very helpful. Hopefully, the needles that come in the rebuild kit will be correct, otherwise I'll be shopping for a 4N8 needle. Sounds like some of the rebuild kits come with the Y22 needle, which is too short for the needle jets in my carbs. Fun, fun, fun!

EDIT- still having issues getting the buggered pilot jet out, heat and an EZ-out have been unsuccesful, I don't want to snap off the EZ-out(seen that movie before), so I may try to drill it out with the drill press and dress the threads up with a tap- anyone been down this road?

 

[jd750ace]

EX500 carbs. Done.

 

[5twins]

$100+ dollars. done. I'm perfectly content with my stock carbs. So much so that I've acquired a couple more sets, sorted and tested them, and placed them on the shelf. If I ever win the lottery, I will scrap my stock carbs and get Delortoes. They are the carb of carbs in my opinion. They are a bit crude but they do meter the fuel well, and that accelerator pump, my God, it really does pump. The Jap ones are a joke in comparison.

But, back to the original post here. What you have to know and take into consideration is that Mikuni needle slots are 1mm apart. That means moving the clip one slot raises or lowers the needle 1mm. This can have quite an effect on how the bike runs. Now think about what that Y-22 needle, being 5 or 6mm shorter than your original is going to do. That's like moving the clip 5 or 6 steps. It won't work, plain and simple. The Y-31 I mentioned in that post you linked to may work. Certainly it would be better than the Y-22, but there's really no way to know if the taper is right. And needle taper is very important. Unfortunately, it's very hard to see and measure.

XSJohn used to grind needles, mostly the right side one, to equalize cylinder temps. I worked with him on some of them. The needles he sent me I could see were ground because of the textured finish on them. But there was little if any difference in the diameters that I could measure (and I did measure). I knew they were "worked" because I could see the texture compared to a smooth stock needle. I don't know, they did work, it was his "magic touch" I guess.