BS 34 carburetors


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Put it out here Apologize if it has already been discussed
I am going down on jets for this season if I recall right I have 2 up on pilot one on main .and dont remember the needle ill take them out soon
I have noticed that the idle adjusting screw slots are not pointing the same
the right is at ca 10:30 to 4:30
the left is at 11:30 to 5:30

I have not adjusted --- that is above my pay grade. I have seen it in some manual Clymer or so that those should not normally be touched But is this something seen before uneven idle screw setting and if so why ?

I have experienced that one cylinder is easier to start than the other left is better.

I have it to rich but is has run without vibrations excellent even .Not perfect on top but in the middle it is perfect.
Always had black plugs mostly because of weak spark those are on the darker side and plugs on the older side
But engine was rebuilt 2 years ago with some adjustment on the crank and new bearing so the vibration problem can have been altered to the better
That is the down jetting reason At the last runs of the season I sometimes had rough running at low rpm in town but I believe I had an air-leak .
But did not put the effort in to fix it since the carburetors was coming out.

Stock intake
Exhaust slightly more open about 10 -15 %

I would appreciate comments on he Idle screw and recommendations on Jetting does Manual stock settings sound good which i am pondering to try.
Or perhaps stock + one up on Pilot .Since I have had good experience with slightly richer.
And know one that went to lean and pretty much scrapped the motor.


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Ignore what you read in Clymer's. It's correct that the version of the BS34 that was fitted to the XS650 was designed to prevent altering the factory mixture screw setting. But the mixture screws must be removed for proper pilot circuit cleaning, and the factory setting was a bit lean at best.

The difference in settings between your carbs is insignificant--less than 1/8 turn. Given that the right cylinder acts leaner, you might open it up a bit.

As to tuning being "above your pay grade," I don't know what to offer you. I hope what 5twins and I wrote in the XS650 Garage USA Carb Guide (click the link in the Tech section) proves more useful to you than Clymer. Pay special attention to what 5twins wrote in "Tuning for Modifications" section.
One more thing: the first rule of carb tuning is "Everything else first." Find out why you have a weak spark and fix that condition before trying to fine tune the carburetors. A defective ignition switch can not only cause incomplete combustion, it can also cause the voltage regulator to read low current and keep the charging rotor magnetized, causing it to overheat and eventually fail. A defective kill switch can also cause weak spark. Use a voltmeter to look for bad switches and connections.
One more thing: the first rule of carb tuning is "Everything else first." Find out why you have a weak spark and fix that condition before trying to fine tune the carburetors. A defective ignition switch can not only cause incomplete combustion, it can also cause the voltage regulator to read low current and keep the charging rotor magnetized, causing it to overheat and eventually fail. A defective kill switch can also cause weak spark. Use a voltmeter to look for bad switches and connections.

Yes Sir Thank you .. i have most of the time for 35 -- 40 years had problems with the weak spark. Mostly because of bad rotors and wiring ( Cheap bikes ) One stator gone bad also But now i think it is fine
With the motor rebuild the connectors and wiring was serviced I don't have a kill switch ...i got stranded one night . And upon finding it I put a small bolt through the washers at the wire bolting them together. No moving parts no more
I had a mechanical regulator intermittently shorting a couple of years ago. Bad problem to find ...only there when warm and at highway little higher rev Making the Boyer Bransden go nuts .
The voltmeter installation showed the overcharging.
I had the ignition directly to the battery with no switches at all while fault finding disconnecting at the battery not to fry the box or coil.

But it is sorted ..And I have a electronic Bosch type regulator And a Voltmeter across the battery. so I have never had the wiring and charging in this good shape
The Boyer Maybe took a hit when the overcharging was there .. but it seems rather robust
That is not meaning good The wiring is substandard as well as the ignition lock. ( Cheap bikes )
I have a separate ground wire and can bypass the ignition lock should I suspect voltage drop. for testing.
but everything can happen of course but these are aspects I know more about. Much more in comparison to carburetors
I know the symptoms mainly starting difficulties on the kick starter and know how to check the spark taking the plug out looking at the spark
.Everything can happen and come spring i look over the connectors with some grease

So the electrics should all in all be fine ..But one can never be certain .i did have the misfiring ..But I am prepared go down a bit on jetting
Since the bike runs perfect on middle range I have waited ..But are now prepared to try a bit leaner.
Air filters is not new but OK 7 - 8 on a scale to ten
Hoping to be able to get it back if it gets bad.
And thank you for the experienced input.

I am leaning towards one step up on Pilot Jet everything else stock and have a check inside idle Someone can have changed there
I have noticed that the idle adjusting screw slots are not pointing the same
the right is at ca 10:30 to 4:30
the left is at 11:30 to 5:30
The idle mix screws are there to fine tune the idle mixture (duh)
Since no two carbs are identical... no two cylinders are identical etc... no two mix screw settings will be identical. They'll be individually tuned to their respective cylinder.
I have my own personal rule of thumb regarding mix screws:
Anywhere between 0 and half a turn from the factory recommended initial setting is fine.... I consider it normal tuning.
A half to a full turn is out of the ordinary. Is it concerning? Depends on what day of the week you ask me. :sneaky: Sometimes I'll re-jet the pilot and sometimes I'll just let it be (read that too lazy to bother).
Anything past a full turn definitely needs looking at.

Again, that's my rule of thumb. Ymmv...
I'm not sure that the screwdriver slots in the mix screws aligning means anything. When the holes for them were tapped, it's possible the tap was started in a different spot in each hole. I suppose it's also possible that the threads on the mix screws were started in different spots as well. A better judgement of the setting would be had by running the screws all the way in until they lightly bottom out then backing them out in 1/4 or 1/2 turns, counting the turns and getting them set the same that way. And once you turn the mix screw in all the way, look inside the carb bore to see if it really is fully seated. If it is, you will see about a MM of the screw's tip sticking out into the bore .....


As far as re-jetting goes, you base that on what was stock in the carbs to begin with. You being in Europe, your stock jetting may have differed slightly from what we got in the U.S. You might also have the adjustable needle. In the U.S., we got 132.5 mains, 42.5 pilots, 135 air jets, and a fixed (non-adjustable) needle. Your carbs may have come stock with 130 mains and 130 air jets. No matter really, because pretty much all '80s carbs on all bikes were set up leaner than in the '70s. For that reason, bumping the mains up one even on a totally stock 650 usually improves running. With your very minor mods, changing the pilot shouldn't be required, especially going up 2 on it. You rarely go up 2 pilot sizes even on a modded bike. If you want to make the idle circuit a bit richer, change the air jet instead. This is another way to tune the idle circuit, and it doesn't have as big an effect as changing the pilot jet size. So, for your set-up, I'd recommend trying one up on the mains, stock pilot, and one or two down on the air jet.

Tuning the mix screw setting isn't as easy on the BS34s as it is on the older BS38s. On the BS38s, get too far past the best setting in either direction and the idle drops off and starts stumbling. The BS34s don't do that. You need to have the mix screw almost closed or really far open (like 4 or 5 turns) before it happens. So, to find the best setting for them, I use small throttle blips. I'll start at somewhere between 3 to 3.5 turns out on each screw, turn them both in 1/4 turn at a time, then blip the throttle to test the setting. I keep going until I get popping or a hanging idle speed, both of which indicate I'm getting too lean. Then I open them back up again just to the point that goes away.
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Thank you Sir 5t -- the trick bottoming out the idle is good ..
I have a feeling the left is set slightly better I then count the left idle screws turns in an then backing out to set back to the current setting as is
And then do the same on right counting see where it is at. setting it as left
Then start the blipping test
The mains are a step up if I recall right already
So the stock on the pilot jets can be a starting point.

The needle is adjustable I remember having done that but don't remember what slot I have used.
I assume the stock setting on that is right

These Carbs was set very lean when I got them and on another motor gave poor running spiking.
I got the recommendation of some shop go up on Pilot s also to get rid of it Eliminating fueling as the problem I went for two steps.
Since back then I did not know if it was another problem on the bike . Giving it plenty of fuel to rule that out
Not really bothering change back since it ran smooth. Without vibrations and no spiking.
And myself not good at carburetors .
The crank was adjusted at the rebuild so there is perhaps less vibration problems Now
I don't think so. As 5T says the carbs are individually different and it could be because of the thread starting position or just one carb needs to have just that much more fuel than the other.............basically running 2 systems, one for each cylinder...............coupld also be because one of the rubbers is slightly squashed or a damaged washer.

My SG, one of the washers was actualy bent in half and squashed flat and put back in the hole under the pilot screw........?????..........really couldn't make out how that happened or why someone would reinstall it like that. Real odd.
Jan, you can have weak spark due to a weak battery caused by a weak charging system, and you can have a good charging system and a weak spark caused by ignition system defects (faulty kill switch, weak coil, bad plug wires and/or caps, bad connections). I don't know why it's so tempting to try to tune out ignition and engine defects through the carburetors, but it's a trap. Anybody who's done much wrenching and says he's never fallen into that pit is either selectively forgetful or a bald faced liar.
Opened the carburetor and looked inside
I remembered wrong there was no oversized pilot I have taken them out forgetting about it
Most likely at the rebuild
Main 135
Pilot 42.5
Air 130
Needle in the third slot from the non pointed end.

5 T -s
So, for you set-up, I'd recommend trying one up on the mains, stock pilot, and one or two down on the air jet.

Perhaps making this an electrical problem after all .. I did not believe that but the facts are there.
The reason for people ( not me ) adjust the carburetor can be that it is more hardware oriented you take out something and replace
Whereas electrical can be there and then go away More abstract.

I do know the ignition lock can be a candidate for replacement
I will bypass it for testing ( Again ) come spring and check ignition.
And the first connector downstream from the fuse box.
Charging is fine. But something can have happened a wire or connector.
Need to think and do the reading in tech section
A quick look in Clymer mention 140 Main anyone have that ..???
Is third slot on needle wrong Should it be 4 th or even 5 th instead leaning a bit midrange ..???

Although there is indications now on electrical things to service I have had rich in the midrange and not the real OOomph at top
I felt rich was OK running in ... and not getting to lean.
Sorry about this old mans bad memory I did it at nights then . Not writing it up. And at least one person was surprised I got around the block with it after the rebuild. Unable to hide it.
Weak spark can masquerade as rich carburetion. Don't bypass switches to test, use a decent volt-ohm meter. Open the headlight shell and find the brown wire out of the main switch. Turn the main switch on. Check battery voltage. Then check voltage on the brown wire. If voltage on the brown wire is more than 1/2V lower than battery voltage, service or replace the main switch. Next, check voltage on the R/W wire from the kill switch. If it reads more than 1/4V lower than voltage from the main switch, service or replace the kill switch.
Truth be told, I've never seen a listing for the stock clip setting on the Canadian or "world" needle. Now, usually the stock setting on most carbs is the middle #3 slot (counted down from the top), but that's not always true .....


For instance, the early BS38s used on the first few 650 models came with their needles set stock in the #4 slot (from the top).

The jetting you've found is exactly the same as what I was running when I first got my '83. It was all stock except for having the baffles drilled on the stock mufflers. The only difference is your adjustable needle and that may be responsible for your richer running. The "fixed" needle we got in the U.S. was a 5HX12 and measured 50mm from the clip down to the tip. Your adjustable needle is most likely a 5IX11 and measures 50mm from the top clip position down to the tip. That means that running it in the middle #3 slot would make it 48mm long, or 2mm shorter (richer). The needle setting has a big influence on plug color so you might try leaning yours a step to see if it improves yours. That's what I'd try first. I wouldn't mess with the air jets as they're already quite small (rich). If changing the needle setting doesn't help enough, you might try one smaller on the mains. That would still leave you one size richer than what I'm pretty sure was your stock size (130).
Thank you gentlemen
I am going to respectfully offer a perspective that can be of value for the " Low Budget " Riders as Myself for the future enthusiasts

The bypassing has its usefulness not as a permanent fix but as a Fault finding procedure
Say you buy a $ 800 bike with some problems It is not uncommon that the problem is not there all the time
only when hot or wet.
take fex the mechanical regulator that only malfunctioned warm days at highway speed
You cant measure on a warm engine out on the highway And when the bike is cool one cannot find the fault simply because it is not there
anymore the thermal expansion is not there
if it is possible fex bypassing the ignition switch with another switch for a couple of months. Not only can you ride the season
The fault region is narrowed down
Otherwise people after messed with the Carburetor .Scratch their head and starts replacing parts
Alternator $ 200 --$ 300 Harness $ 300 $ ignition lock 20 ignition $ 250
Not counting the man Hours that if a shop shall do it makes it a no starter .

At some bikes ( and cars ) the problematic circuit can be deep in under fairings whereas the bypassing can be done somewhere accessible.
Again not as a permanent fix but for fault finding

So the needle needs to go to slot 2 on BS 34 to lean out This is something I probably will try.
It is possible to change back later with carbs in the bike.
And lave the Main as is for now.
Thank you
I'm also going to add that if your carbs have never had a complete rebuild, they probably could use one. By complete, that means replacing the butterfly shaft seals and the little o-rings on the mix screws.

The P.O. of my '83 said he always had carb issues. When I took them apart to clean and rebuild them, I think I discovered why. He didn't do a complete, 100% job. He didn't replace the mix screw o-rings and they were pretty bad. They were hard, mashed nearly flat, and no longer had a round profile. I could see that he attempted to change the butterfly shaft seals but buggered up a couple of the plate screws so never got them out. I changed out all those things and the carbs work quite well now.
The only time I've bypassed the main switch was to show an unbelieving owner that his rich symptoms were caused by weak ignition caused by the main switch. If you'd rather bypass the switch than buy a good meter and find the problem, that's fine. I won't presume to advise anyone who knows better. Done here.
I'm gonna add a question that i have mentioned before but no one has ever addressed it. ...........

It relates to the BS34's with the US non adjustable needles 5HX12, these are mated with the needle jet, (YO), with 14 bleed holes. 2 banks of 4 and 2 banks of 3 and the World adjustable needles 5lX11that are mated with the Needle Jet, (336YO), with 18 bleed holes with 2 banks of 5 and 2 banks of 4.

Excuse the bad pic
Photo130 shpn vertically text 4 crop copyright.jpg

if the 2 different needles, (US fixed and world adjustable), sitting the same height/position in the needle jet, is one and the other going to be running either leaner of richer due to the difference in bleed holes in the 2 different needle jets

This is how i read it

The, 336YO, with their 18 bleed holes would be running leaner because it lets in more air to the Needle jet above the main jet because of the extra 4 bleed holes and this is why the needle is set at the 4th from top, clip position to enrich-en it slightly more
1 d5 Adjustment fuel level XS650SJ Service manual copyright.jpg

Here is the 5lX11 needle clip position i used on my SG.
P1000735 crop copyright.jpg

It was like this when i dismantled the carbs and the bike was running well with good milage and didn't seem to be running adverse in any way so i kept using the same position when i put it back together

I guess the question is..........does the extra air bleed holes in the needle jet 336YO lead to a different outcome when recomending jet size and clip position

I added a 5Xl11 needle to one of 5twins needle comparison pics to look for any difference between the 5lX11 and 5HX12 needle taper that goes into the needle jet.
Clipboard copy.jpg

pic is a different angle but for me i do see a longer taper on the adjustable 5lX11 compared to a shorter taper on the 5HX12.

I haven't looked to see if there is a difference between the needle jet bores for the adjustable and non adjustable needles, to allow for any difference there might be in the needle design if any
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Do you remember what jetting for that needle position
Is it a US Version or European / Oceania

Thinking out loud about Needles and Markets I got the impression that the version being non adjustable being emission laws
regulated and leaner. there is a lot of variables
US bikes were setup leaner and the law prohibited changes.
So if I get it right the designer are designing 2 different objects with different internals
Object A and Object B He has the task to get it working and running on respective markets
Normally he has little interest in matching these between markets
Because very few are going to drill out and adjust needle as a customer in the US
by warranty and legal reasons back then on a new bike.
Not that common either ship bikes between countries then.
I think the designers order was do the setup and test it manufacture and ship.
Perhaps taking the parts from another bike setup
My guess is that the US Bike runs leaner than the other for the same jet.

More Air ( Extra holes ) is no problem if you can adjust Needle and other
But it is dangerous if you cannot adjust
I think I have made mention of the 336YO needle jet having more air bleed holes in the past, and hence, being leaner. But I think the 5IX11 needle is so much richer that the midrange overall ends up richer than the U.S. spec 5HX12 needle and Y0 needle jet combo.
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