is this a Z-6 Long or a Z-6 Short needle jet?

[5twins]

The test to run now is to grab absolute full throttle at about 3.5K and run up to about 6K. You can do this in one of the middle gears (2nd, 3rd, or 4th). What you're testing here is the midrange to main transition. If the mains are too big or the midrange too rich, you'll get some stumbling or gurgling in the 4 to 5K range. Since you've already leaned the needle, your only option would be to reduce the main. You could lean the needle another step if you feel you absolutely need that main for the upper top end but personally, I don't like running the needle at either extreme (1st or 5th slot). I look for the largest main that will pass this test with the needle leaned one step. On my '76-'77 carb set, that was a 132.5.

Due to the forgiving nature of these CV carbs, it's important to use full throttle for this test. Lesser amounts may not show the glitches.

 

[DogBunny]

5twins, will do, thank you.
BTW, I am very aware of shimming the needle in 1/2 steps, using a 0.5mm washer. For what it's worth, I have opened up at least a couple of unmolested late model BS38s, probably 1979s, and I have found those washers already in there -- on TOP of the e-clip, where they don't do anything, but as if the factory added them for you in case you did want to half-step the clip position at some time in the future.

 

[5twins]

They do act as a .5mm shim if placed on top of the needle clip. They make it sit .5mm lower in the slide. Originally, they were below the clip, probably to protect it from being dislodged by the spring .....

Although maybe what you've found was a factory quick "fix" implemented in '79 to lean this carb set's midrange just a hair more. I found on my '78 set that even with stock jetting, I could coax some minor stumbles out of the upper midrange with this full throttle test. Going from the 122.5 mains of the previous carb set to 135s in '78 is quite a large jump. Maybe dropping the needle jets from Z-8 to Z-2 wasn't quite enough to totally fix the midrange.

 

[DogBunny]

Yes, you are absolutely right, on a 78-79 carb, the shim only has an effect if it is above the e-clip. On a 77 and earlier carb, the shim only has an effect if it is below the e-clip, making the needle sit 0.5mm higher in the slide.

 

[5twins]

Exactly, and the BS34s are the same as the '77 and older 38s - the shim washer must be placed under the clip to have any effect.

 

[DogBunny]

Well, this is timely. I am currently working on my first ever set of BS34s.

The bike has UNI air filters, and what must be the MikesXS 2 into 2 head pipes and Ascot mufflers -- based on the mufflers having glass packs.

My guess is to go up two on the pilots to 47.5, and up 5 on the mains to 145. There is only a single needle clip spot, not much I can do there short of ordering the Canadian needles. I know from reading the Carb Guide that the needle clip position may be okay, as these were lean from the factory. Any comments will be greatly appreciated.

It would also be greatly appreciated if someone would go to the trouble of putting a couple of float valve o-rings in an envelope and mail them to me. I've been caught somewhat flat-footed here -- I never really intended to work on 34s, I only use 38s myself, but here I am working on another person's bike again. PM me if you've got a hoard of those McMaster-Carr o-rings, and thanks.