Carbs - Carburetors

[mustys]

Can anybody explain me what happens when we hook up both vacuum barbs together with a fuel hose?

 

[jay760]

Not sure if it's the wording or a strange question, if you join both the vacuums together nothing happens just acts as a balance pipe similar to how old British bikes used to run, if you join them to a fuel pipe then petrol will just pour straight into the manifolds.

 

[mustys]

yes, i meant to join both vacuum together. I should leave them plugged right?

 

[wilbur60]

I am not sure why you would want to do that? What do you hope to gain from connecting them together?

 

[XSLeo]

Hooking the vaccumm ports together is the very last step in a carb set up. You do all the other tuning, jets , air mix screws, sync. Then hook the barbs together.
This will help balance the cylinders at idle. The difference may not be much.
Do this as a last step, it won't fix other carb problems, just a bit of fine tuning.
Leo

 

[chung]

Hey Guys,
Not new to bikes but I am new to the 4-Stroke Yamaha Twins.
I just drilled out the plug for the Idle-Air Screw and found the screw seated all the way. The Info here said to expect "1-3 turns from seated". Is it Normal to find them seated?

1982 XS650 Special II US Model

PS.... I couldn't find any ID Stamp on these Carbs, I know that's not normal.

 

[chung]

Hey Guys,
Not new to bikes but I am new to the 4-Stroke Yamaha Twins.
I just drilled out the plug for the Idle-Air Screw and found the screw seated all the way. The Info here said to expect "1-3 turns from seated". Is it Normal to find them seated?

1982 XS650 Special II US Model

PS.... I couldn't find any ID Stamp on these Carbs, I know that's not normal.

I drilled the other plug out today and the air screw was @ 2-1/2 turns from seated. I'm thinking this one was probaby not in a "great" state of tune when they shipped it out.
If max adj. was the most they could get then it might have needed a bit more. Plus one at minimum setting and the other at three quarters of the adj range would indicate something was not right on one cyl.
I'll find out slow enough. I'm might actually get it out for a go next week

 

[5twins]

It's possible your drill bit caught the screw head and spun it closed on that one side. The BS34 carbs like yours seem to like about 3 to 3.5 turns out.

 

[chung]

It's possible your drill bit caught the screw head and spun it closed on that one side. The BS34 carbs like yours seem to like about 3 to 3.5 turns out.

Brilliant, I'll look for traces where the bit might have caught my good Watson But I don't think I buried the drill hard enough to do it since the plug came out before I drilled through.
PS... What's a "Euro Twin"? Also is there an XS-TX Mdl History on this site? Guess my lazy-ass can look it up myself, Huh?

I got the second carb assembled today but that L-bracket on the bottom has got to go. What a Boat Anchor! I have an aluminum bar that should work and I have laid out some "Swiss cheese" for the bracket that mounts on the top for the cable. The plating is shot so I'll paint it after drilling.
Seems silly to some, but if you count the grams and ounces, they add up to pounds and free horsepower

 

[BoofMcgee1]

What year carbs will fit a 1971? Would 1978 stock fit?

 

[XSLeo]

The 71 used carb holders that came straight out of the head. This made the carbs point out a bit. They were made this way until 73, in 74 they changed the carb holders. They msade them angled so the carbs were pointed straight back.
The 78 set of carbs are linked and need the later angled holders. They will work fine but not match up to the stock air boxes on the 71. You will need an aftermarket air filter, like the Uni-Filters.
Leo

 

[BoofMcgee1]

So I'll only need the aftermarket air filter to run 'em?

 

[weekendrider]

No, you need the correct intake manifolds and air filters.

 

[XSLeo]

To run the 78 carbs on your 71 you need the carbs, of course, the carb holders for the 78 carbs and aftermarket air filters. Also the 71 carbs used two throttle cables, one for each carb, in 76 they went to the linked carbs that used just one cable. Just remove one of your cables. Keep it for a spare.
Leo

 

[NUTZ]

Just a note if you use this guide http://www.amckayltd.com/VM34-36.pdf
For a VM34 a needle jet series 159 is recommended but this is for a two stroke.It may work but it will not be right.Use a series 176 which is bleed type for a four stroke.Vendors supply new with a 159 so ask for a replacement. All the other recommendations may be OK but the carb vendor should give the correct set up as they sell a full kit.

 

[grizld1]

I've spoken to this once already. Bleed and primary type NJ's flow differently. If you have experience with the 176 NJ, please be so kind as to specify size and full range of jetting including needle. If you have no experience with it, please acquire some before you offer advice.

 

[XSLeo]

On the VM carbs you don't use that carb guide, use www.amckayltd.com/vm34-36.pdf
Leo

 

[Take Warning15]

After looking at the chart, I have one question about he BS38 carbs. If I want to swap a set of 79 carbs onto my 75, can I simply change all the jets and the slide? Will the older style slide not fit in the 79 carb? Would there be any other problems in switching the BS38's from the 79 to 75?

 

[grizld1]

There's no need to change parts. As 5twins and I tried to explain in the Carb Guide, jetting differences from year to year were due to changes in carb body design and changing emissions control regulations, not the demands of the engine, which remained basically the same from 1974 through the end of production. Jet to the specs given for the 79 carb set.

And no, earlier slides won't fit the 1978-79 carb set. If diaphragms are bad, contact JBM Industries.

 

[Take Warning15]

OOhhhh! I understand now. Thanks so much! Out of curiosity though, with the changes made, were any specific years better than others?