Jet size

J

Jtporter9

JT
Messages
58
Reaction score
1
Points
6
Location
Provo, Utah
Hey guys,

So I started up my bike a while ago for the first time in about 15 years, she sounds great, and all. I bought some new carbs for her, because I broke my old ones. And now I feel like the settings are off. My right exhaust wasn't firing very well, and when I checked the spark plugs, the right side one was wet. I learned online that it can mean that my bike is running too rich (to much gas from what I understand.) And I'm supposed to move my jet size down (I think). I am currently running 132.5's. What size should I get, or how far should I go down?? Any advice is good advice. Thanks:D
 
The year is 1980, they obviously are muniki, and to be honest I don't know much else about them, I bought them off of a running xs650 that was bobbed, even had the exact same cones I have. I'll check it out thanks.
 
The seller said they came off an 80, they may even have come off an 80, that doesn't mean they are 80 carbs. The carbs can easily swap year to year. A lot of the Ebay prat sellers are not bike people, they just buy bikes, strip them down and sell the parts, often they don't even know the brand, I have often found parts not Yamaha listed as Yamaha parts.
Now If the are Yamaha BS carbs from an 80 they will have thin chrome steel top covers. This makes then 80 up XS650 BS34's. All the earlier BS38 carbs had thicker cast alloy tops.
The carb guide covers this.
Leo
 
I was 100% sure they were 80 carbs when I bought them, I didn't buy them off ebay I bought them in person, at a local shop, straight of a bobbed 80. I brought my own to compare, I measured in the intake, and the airfilter to be exact, and they are indeed the exact same carbs, even with the chrome finish on top. I'm new but not new enough to screw that one up.
 
The 132.5 main you found would be stock for 1980 carbs. Down sizing jets is not the usual path taken with these carbs. If anything, you usually need bigger ones if you've done mods like pod filters and an exhaust change. Something else not related to jet sizing is probably causing the rich running. The float settings may be off, a float may have gone bad, or maybe the o-ring on the float needle seat is bad. Any of these things could let more gas into the carbs than required and flood things out.
 
Funny that you mention that because last night I took off one of my float and the needle with it, and the float needle has little to no spring at all in it, which from what I understand caused it to not close off right, thus allowing too much gas in. Then again Im not 100 percent sure what they are supposed to feel like. I'm not sure what the O-ring is, but it sounds like it's part of the float needle, I am planning on going today soon and buying new float needles, hopefully that will fix my problem. If someone thinks I should do otherwise please let me know, I'd rather not drop 60 bucks on new float needles. Ps: if someone has some tips to get the pin out of the float tower that would be awesome, it's a pain. Thanks
 
The pin in the float needle is spring-loaded. Sometimes it gets all gummed up with gas residue. You may be able to free it up and get it working with some carb cleaner or maybe even just some penetrating oil. Just keep the carb cleaner away from the rubber tip, it can destroy it. A brief exposure probably won't hurt it but prolonged contact, like if you tried soaking the needle in a cupful, would destroy it.

The needle seat is a push fit into the carb body and held in place by a little screw and clip. It has an o-ring around it to seal it into the carb body. That o-ring can go bad, allowing fuel to flow around the outside of the seat, not just through it as it should.
 
No, I would suggest fixing the stuck float needle pin and maybe replacing the seat o-rings - if they need it. Usually they do need it, they're 30 years old after all. And also check the float setting and condition. If you really do have 1980 carbs, they would have brass floats. They can crack and spring leaks. Then they fill with fuel and become a "sink". They don't float on top of the fuel level as they should because they're full of fuel and too heavy. They let more fuel into the carb than they should.

The size of the float needle seat o-ring is 1.5mm thick x 7.5mm I.D. if you want to try and find some locally.
 
Okay sounds good I will check the floats, the floats look good from what I remember and have any suggestions on getting the needle seat out??
 
The needle seat should just pull out after removing the little screw and clip, but sometimes they're stuck in there pretty good. I gently grasp them with pliers but take care, they're made of brass and easily damaged. You may end up needing new ones if you bugger them up too much.
 
Perfect. Thank you so much. I just bought everything I need, could you or do you know the height in millimeters for what the float should be at? For my 1980 xs
 
The brass floats in the 1980 BS34s are set to 27.3 ± .5mm. Most shoot for about 27.
 
Yes, there is. The plastic floats have a different shape and hence, a different setting. Their spec is 22 ± 1mm. That is measured to the top of the round "bulb", not the flat area up by the pivot.
 
Great thanks a ton for your help. I'll let you all know how it goes once I get everything together. I still need one new needle I ordered in online so I won't know until then if it's a carb problem so..until then.. thanks again. Have any of you heard of coils shorting out when trying to accelerate?
 
The carb guide talks about most of the things you asked questions about. I suggest you read it.
Leo
 

Similar threads

C
Running rich. Main jet size?
Replies
6
Views
180
Melnic
M
Markangels4
What carbs
Replies
8
Views
545
Markangels4
Markangels4
H
Proper Jetting and Fuel/Air Screw Position Mikuni VM-34 on a 1972 Yamaha XS2 (650)
2
Replies
31
Views
952
Hanniusms
H
B
XS650 BS34 CARBS
Replies
2
Views
216
BACK SHIFT
B
L
Any Advice To Get This Engine Started?
Replies
18
Views
1K
jetmechmarty
jetmechmarty
Back
Top