BS 38 Main Nozzle (emulsion tube) Question

[modieselguy]

Last week I determined the jetting and other brass used in my 1974 carbs were a weird mix of random pieces. I found the needle jets were Z-4 Mikuni instead of the Z-6 it should be. Would leaving the new style, longer Z-6 impede performance? It does put the nozzle tip higher in the throat of the carb. I planned on cutting them to match the older, shorter nozzle height unless someone says leave it. Thanks in advance!

 

[zrx1100]

@5twins

 

[toglhot]

If it ain't broke?

 

[5twins]

Well, I suppose you could try the longer needle jets but they may not flow the same, especially when the throttle is closed. As far as cutting them down, take a look inside the top of your Z-4. See that step about 1/8" down inside, about where the o-ring groove is on the outside? That's the metering orifice .....

Now look down into your new long Z-6. Is the metering orifice down in there deep, about where the o-ring groove is on the outside? If not, if it's near the top, then you'd probably be cutting into it if you tried to shorten them, and that probably wouldn't work.

 

[zrx1100]

I have gotten hard to find carb parts from this guy: Richard Hill <MikuniMan@outlook.com>. he is based out of UK

 

[modieselguy]

I decided to put it back together with Z-4 needle jets, 4N8 needles, 127.5 Main and 45 Pilot. It's runs as it should, strong all the way up to 100mph. No more bog in 5th gear after half throttle. I probably need to replace the diaphrams for a smoother transitions. Thanks everyone for the help!

 

[5twins]

So, it was that too short Y-22 needle causing the issues. That needle can only work in the '70 to '73 carbs, as a replacement for the 4JN19 that was stock in them. If you're wondering why those early carbs used this shorter needle and the longer Z-6 needle jet, it was because the floor of the main bore is raised up .....

Here's what yours looks like .....

As you can see, the floor is much lower. The needle jet needed to be longer on top to reach through that thicker floor in those early carbs. The needle needed to be shorter so it's taper would lift through and work in the higher needle jet's metering orifice.

I've started coating my diaphragms with red rubber grease. I think it helps preserve and soften them. It also helps them make a good seal into that groove in the top of the carb body .....

 

[modieselguy]

@5twins I did the same thing with Super Lube synthetic grease, I use it on orings mainly. The entire kit I used from Mikesxs was wrong but the short needle was not doing me any favors. I think the only thing okay with the kit was the bowl gasket....haha. I'm sure I will need to tinker with it since I'm using that Z-4 jet but I want to get a few miles on it and see how the plugs look. Thanks again for the help!

 

[5twins]

Although B8ES plugs were speced when the bike was new, BP7ES plugs were specced for the later models and it's accepted practice to use them in all models. They are a step hotter and have a projected tip. Both of those attributes should make them burn cleaner and lighter in color. So, if you're running the 8's and they are pretty dark, try the 7's.

 

[modieselguy]

When I first got the bike I bought a Pamco kit, with mechanical advance, coil and plugs. The plugs are NGK iridium BPR7EIX. These only have 91 miles on them.

 

[5twins]

This is the perfect example of why we don't recommend the rebuild kits. Many times the jet sizes are wrong too, lol. The replacement needle and seat assemblies many times leak right out of the box. Like you said, about the only thing you can count on fitting and working is the float bowl gasket, lol.

 

[jetmechmarty]

about the only thing you can count on fitting and working is the float bowl gasket, lol.

Maybe….
Often they’re cut wrong and interfere with the floats.