Newbie with jetting question

[JayDee24ca]

Good day. I have a (new to me) 1980 XS with BS34 carbies. The exhaust has been modified as it has a siamese (2 into 1) header pipe and a shortie megaphone. The air intake is modified with twin K&N cones. The bike starts easily and idles fairly well, but it does a lot of backfiring and belching off idle.
I pulled the bowls on one side and found it is running a 42.5 pilot jet and 145 main jet. Is it the correct move to bump up the size of both jets? Should I be looking at increasing only one size or several to start?
Thanks in advance!
JD

 

[XSLeo]

The stock jets for the 80 up BS34 carbs are a 42.5 pilot and a 132.5 main.
You have the stock pilot and your 5 up on the mains.
I would go up one size on the pilot. A 45.
I would go down to a 137.5 on the mains.
It's easy yo get these carbs over jetted. The slides lift by vacuum. If the mains are to big the slide won't open all the way. This can limit performance.
www.amckayltd.com/carbguide.pdf is our carb guide written by very two knowledgeable people on this site.
It has a section on "Tuning For Mods."
If you K&N pods are the short tapered pleated style we have found they don't work so well on vacuum carbs. They cause to much turbulence in the air flow through the carbs, This disrupts the vacuum signal that lifts the slides. Leaving an off idle to midrange flat spot as well as a steady cruise speed surge. A long straight foam pod works much better. just watch out for an internal lip that can block ff air flow to the ports on the carb bell.
These are much easier to tune the carbs with.
From a lot of the reading I have done the two into one exhaust are harder to tune for as well.
Leo

 

[JayDee24ca]

Leo, thanks for your reply. Lots of good stuff there. This is the first have dealt with Japanese carbs; I am used to Dellortos for the past 45 years. I have ordered a selection of mains and pilot jets, but the supplier seems to be out of stock for the 137.5 or 140 mains. I will try with just the larger pilots and see what happens. I will see if I can locate a couple of socks for the intake and jettison the K&Ns. Do you have any recommendations?
Not much I can do with this shorty megaphone to fix the free flow at that end. I was thinking I might be able to find a plate to try and reduce it somewhat.
The link to the carb guide is great, looks like I can get lost there for a while! Thanks!
JD

 

[5twins]

These BS34 carbs use the very common Mikuni N100/604 "large round" main jet. You should be able to find them at lots of places. Yes, downsizing from the 145's to a 140 or 137.5 should work better. And yes, you need to bump the pilots up one. That should take care of all the off idle popping and misfiring. The mix screws usually like to see 3 to 3.5 turns out on a stock machine but with your modded one and the larger pilots, you should be able to back that down to the mid 2 range (2 1/4 to 2 1/2). There is also an air jet associated with the pilot circuit that you can play with to fine tune it. It has less of an effect than changing the pilot jet size so you can use it to achieve richness levels in between pilot jet sizes. Stock size on the U.S. bikes was a #135, the rest of the world got a smaller #130. If you have the Canadian version of these carbs, they may have the 130s already.

Most of us use UNI foam pod filters. For the BS34s, you'll want the UP4229 or UP4229ST (dual layer). These have a 2 1/4" flange I.D. and fit the BS34s nicely. For oiling them, just use a 50-50 mix of gas and motor oil. The special foam filter oils can be too thick and tacky, and clog them. I recently installed some UP1449ST filters on my '83. Normally I wouldn't go with these dual layer ST's but a local guy got them for his bobber and they wouldn't fit, so I got them cheap. Usually I go with the "plain jane" single layer UNI. It's all you need and costs a few dollars less than the ST's .....

For the end of the engine breather hose, many of us install a one-way power brake check valve. They're cheap and fit the breather hose perfectly .....

 

[JayDee24ca]

Thanks again Leo. I've ordered a couple of single layer UP4229s on Amazon. There is a small Honda shop here in town, I will head down there tomorrow to see if he has access to the jets I need. I have no idea if they use the same carbs, but I guess I'll find out. Ordering stuff from out of country is ridiculous these days; I just received a package from Germany for my Nuovo Falcone that was ordered over 8 weeks ago. Thanks again!
JD

 

[5twins]

I think Hondas use mostly Kiehin carbs but it's worth a try I guess. Suzuki, Kawasaki, and Yamaha are the big Mikuni carb users.

 

[JayDee24ca]

Thanks, I found some at Fortnine here in Canada. Not as cheap as from Mikesxs, but shipping from FLA was going to be 48 bucks! I'm still finding my way around, in the part supplier world for Yamaha.
On another matter, how does the vacuum fuel petcock work? I tried simply turning it off at the petcock in order to disconnect the fuel line, but the fuel would not shut off. I have ordered a manual one in the meantime, but I am curious as to how the vacuum operated one is meant to work.

 

[5twins]

I wouldn't recommend the jets from Mike's anyway, they're aftermarket knock-offs. It's best you use genuine Mikuni jets.

The vacuum petcock has a diaphragm with a plunger w/ o-ring attached to the back side. There's a little spring on it that forces the plunger closed until a vacuum signal from the running engine sucks it open. There are 3 positions for the petcock arm - "On", "Res" (Reserve), and "Pri" (Prime). No fuel should flow if the motor is off in the "On" and "Res" positions. The "Pri" (Prime) position bypasses the vacuum operated diaphragm and flows any time just like an old fashioned normal petcock. There is no dedicated "Off" position. There really doesn't need to be because it is essentially off when in the "On" and "Res" positions when the motor isn't running. Well, that's how it's supposed to work anyway.

If yours is flowing fuel in the "On" and "Pri" positions with the motor off, you may just need to replace the o-ring on the plunger. Or sometimes the seat area that the o-ring pushes into gets all corroded up and needs a good cleaning.

If you had the petcock in the "Pri" position when you removed the fuel line, then fuel flow like you got is normal.

 

[JayDee24ca]

Thanks for that! I'll pull the tank and R&R the existing tap then.
JD

 

[Jan_P]

Taking this from memory
There is a hose going between the intake manifold and the petcock
when the engine is running there is a Under pressure that moves a rubber membrane opening for the fuel
without that pressure it is closed or should be.

https://www.z1enterprises.com/petcock-repair-kit-xs650-1978.html

I have had a couple of interesting faults on that . One night I was supposed to go Home.
10 pm ish
Machine started I Got to the city limits ( small town ) machine started misfiring and eventually stopped.
No tools No flashlight. No Knowledge
Pushed it to a transformer station .. Pitch black more or less .. Scratching my head Kicking nothing happened..
A car comes up offering help ..Nice .. but it was one of the Towns leading Roughs .
Street Fighter top 5 .. Which I had seen in action at Bars and Discos many times Picking fights and Hitting people.
As sensation seeking
Dangerous
I suppose you know the type.
with two brothers equally big.(They were not there )
I Thought oh no ..On top of this I am getting my ass Whopped here tonight.

But no problems he was sober .. No Sports activities there in the dark.
He could not much about this ..offered a lift which I declined .. And he left.

After a while gas had leaked past that defect membrane -- Helmet on Thinking now I ride home
believing some electrical fault. Which i Had many of at that time.
2 miles further out in the dark stopped again.
This time i parked and walked back to where I came from was allowed to crash on the sofa
Call the boss next morning I wont be there today in time.
Taking the buss and later return with toolbox That hose had come off that should open for the fuel-- dont ask me How.

Another time the hose was cracked drawing in extra air making it misfire

 

[5twins]

A bad vacuum line or connection won't make the petcock flow all the time. Just the opposite, it will make it stop flowing or not flow at all. Vacuum lines, along with fuel lines, are something I usually replace when I get one of these.

 

[JayDee24ca]

Interesting story, regarding the town toughs. We had the same two three guys in the little town where I grew up in the sixties. Suppose they were related?
The fuel tap issue sounds as though it has been an over-thought addition to the bike. Guzzi had the same over engineered sort of thing for several years with their electric fuel taps, in the 70s. Most folks removed them and went manual. Same as the safety kill switch on the side stand. These things were a great idea, but in practice they often fail, leaving a rider stranded.
JD

 

[JayDee24ca]

Today I finally received the jets I ordered (3 1/2 weeks!), and proceeded to install new 140 mains and 45 idle jets, replacing the 145 mains and 42.5 idle jets the bike had when I bought it. I discovered that the old main jets were the small headed ones. Would that make a difference at all? It appears that the only significant difference between the small and large head jets is just the diameter of the head itself, so I don't see why it should make a difference. Is that right?
Thanks and Merry Christmas!

 

[grizld1]

Right you are, both types of Mikuni main jets flow the same. The large round jets offer more contact area and thus more resistance to loosening and dropping in carbs where the main jet threads into the needle jet. The small round jets are used where the main jet threads into the float bowl.

 

[JayDee24ca]

Thanks for that, griz.