Jetting question

funky

funky

XS650 Junkie
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Hi All hope you are well.
I rebuilt my top end and re jetted my BS34s to stock jetting.
The book says for my year and and carbs that the mixture screw should be 3 1/2 turns out .
I started to get my jetting done based on what the plugs are indicating bearing in mind I do not want to run the bike lean , that why i returned to stock jetting.
My issue is with the pilot circuit I have a 85 air pilot and went to a 37.5 Pilot jet ..... Stock should be 47.5 pilot jet Plugs were black so i started on the the mixture screws to lean off the mix. 3 1/2 turns out is far to much no matter what jet i use.
I slow the engine down once nice and warm, screw to mixture screw in to weaken the mix looking for a slight increase in RPM (The Sweet Spot ) and the same thing happens. The sweet spot is always about 1/4 turn out from bottoming out on either carb. I have gone from a 47.5 to a 30 pilot jets same outcome.
Now as far as i am aware my float levels are perfect 21 mm to 22 mm checked and double checked. no air leaks checked. diaphrams are ok also .Carbs are balanced
main jet is a 140 as when it was stock the engine ran like shit more than quater throttle.
needle jets set on number three notch . as per manual. im stupped
Remember every things good after quater throttle its just when idleing. Any ideas I am sick of pulling those carbs apart.
All the best Funky
 
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funky every set of 34 carbs I've opened have a stock fixed needle. Some of the other countries are rumored to have adjustable's can you confirm yours are stock?
Are your choke valves closing and sealing?
 
The one thing i have not checked choke valves,,,, my needles are ajustable I have a second set of BS34 from a british bike and they are the same. when you refer to Choke valves do you mean the brass bar that pulls to inguage the choke ?????
 
I think your #85 pilot air jet is too small. In the States, our BS34s came with a #135. Also, 42.5 pilots were stock (not 47.5) as were 132.5 mains (not 140). But, it's possible your adjustable needle equipped carbs were jetted differently stock than our E.P.A. mandated versions. The only other jet listing I've ever found is in a Japanese parts manual for their domestic 1980 model. It had the adjustable needle stock as well. They use a 130 air jet, 130 main, but the same 42.5 pilot.

When you say "the book says ....", what book are you referring to?
 
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Hi thanks for reply 5Twins
I went back to stock settings and my plugs came out BLACK and SOOTY. engine ran like a pig... Checked choke plungers plus everything else.
I have always had this issue. which belive contributed to me having to rebuild the top end.
Book says 137.5 main ,,,,85 air pilot,,,, 47.5 pilot,,,, needle 5hx13 set on 3rd notch,,,, 3.5 turns out on the mixture screw ,,, float hight 25mm +-1mm
The book is Haynes manual.
my engine is 3L1 533 con rods 707cc big bore .... Carbs BS34 CV
I went back to my last setting bar the main which i put a 137.5 I agree 140 was too much
Carb setup now is 137.5 main,,,,85 air pilot,,, 30 pilot,,, needle as above. mixture screws 1/4 turn out from bottoming out....
What i am wondering i can't go lower on the pilot jet , can i increase to air pilot will that weaken the mixture and give me more flexability at the mixture screw.
But if i increase the air i increase the fuel do I not ??????
 
No, you do not. Like I said, I think your problem is the pilot air jet. It supplies the air that mixes with the fuel from the pilot jet. Yours is very small compared to the ones I mentioned. Personally, I wouldn't trust the specs from your Haynes manual. I've never seen listings like that for any 650 BS34 carb. I've never seen a 5HX13 needle listed. Is that what you have? The adjustable Canadian needle is a 5IX11.

That small #85 air jet is probably what's making your idle circuit so rich. The black plugs and 1/4 turn mix screw setting are telling you it's too rich. I would try a 130 air jet, I'll bet that solves your problems. Or you may need to experiment with air jets in the 130 to 135 range, but bottom line - I really think that #85 is way too small.
 
Ok 5Twins I have those jets on the shelf and will try what you said .
many thanks.
I will double check the needles also and post back .Thanks
 
I will also add that a 25mm float height is something else I've never seen for the BS34 carbs. The usual settings are about 27mm for the brass float, 22mm for the plastic one.
 
RSCN2101.JPG Hi 5twins I just checked the needles they are 5IX11 not what i said i was wrong.does this change matters I have fitted a 130 Air jet also.
 
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No, I don't think that would change things too much. I've been doing a bit more research and I have found that 5HX13 needle mentioned. Maybe it was stock in Europe and England on some years? Keyster rebuild kits sold in your country have a generic replacement needle and say the stock was this 5HX13. It would be interesting to get a look at one of those and compare it to the 11 and our fixed 5HX12, but I doubt I'll ever see one in this country.

I also found a page of carb specs from the Haynes manual and it seems to confuse matters more. Yes, it lists the jets you say but for the XS650SE (UK) model. That would be a 1978 Special. As far as I know, BS34s didn't come along until 1980.
 
The VM carbs are a high performance type. If you can't manage the stockers, I don't think a VM carb set will be any easier. There's like a bazillion different jets, needles, and slides available for them. You could spend the rest of your life trying to sort them, lol.
 
lol bloody hell .....My model is a 1981 SH must be a canadian one imported i guess engine numberDSCN2107.JPGcould be wrong ,,,, will stick with the BS34 at least i know a little about them and how to strip and clean them with your support i may get there .
 
Hi All ... I fiited the 130 air jet as advised by %5Twins and warmed the bike for 10 minutes checked the plugs and they came out virtually clean,,, may be too lean, I think my options are to increase the pilot jet 1 or 2 sizes but its not that far off i feel. I dailed in the mixture screw from 3 1/2 turns out after slowing the idle down to barely ticking over and there was no pic up in idle speed the mixture screw turned all the way to the bottom out position ,,,which leads me to beleive its too lean ...But not sure if my thinkings right. need further advise from 5twins I think,,,
 
Well, I think you're on the right track here, at least with the air jet. For the other jets, I would follow a known and established baseline. That would be the U.S. specs of 42.5 pilot and 132.5 main stock. Start there but if you have mods you will probably need bigger sizes. The norm is 1 to 3 bigger on the main, 1 or 2 on the pilot. Start with the mains, then do pilots if needed. The mains can effect everything so it's best you sort them 1st, or at least get them close. Larger mains often require a leaner needle setting and that can cause the need for a larger pilot. I would leave the air jet alone for now and only return to it if main and pilot swapping won't get things right.

If your float levels are off, the mix screw can have very little effect. I assume with '81 carbs, you have the plastic floats. Don't go by that Haynes manual spec of 24 or 25mm, the fuel level will be too low and lean. The spec for the plastic floats is 22mm ± 1mm. Probably the best way to check fuel level would be with the clear tube held up alongside the bowl. You want the level in the tube to be right around the bowl/body seam, not too much above or below it.

Also, the pollution controlling stepped mix screws on the BS34s don't show big variations in idle speed or slow speed running like the older screws on the BS38s. It's not as easy to tune them by ear or with the dead cylinder method. Instead, I do it using small throttle blips. I'm watching for the RPMs to drop quickly back to idle and no popping out the exhausts. You have to do both screws at the same time. Start them both around 3 turns out. Do some throttle blips. Turn them in 1/4 turn each and do more blips. Keep turning them in and testing until you start getting hanging RPMs and/or lots of popping out the exhausts. That's too lean. Working/testing them in the other direction, turning them out, you won't get quite the same symptoms. You won't get the sharp popping out the exhausts or the hanging RPMs, the throttle response will just get worse as you get too rich. The motor won't want to cleanly take and react to the throttle blips.

What you should be aiming for is to get the low speed circuit to run well with the mix screws set somewhere between 2 to 3.5 turns out. The U.S. spec bikes usually like 3 to 3.5 but they use a 135 air jet. Your 130 jet may give smaller screw settings, maybe somewhere in the 2 to 3 turns range.
 
Ok thanks 5Twins Just for referance i have brass floats. i have standard air box and baffled exhaust... my main is a 132.5 will change out the pilot to 42.5 and leave the air jet at 130 will check in again soon . Thanks for all your help.
 
Ok good news fitted a 37.5 pilot i know you said 42.5 but thought id try that first I set the floats at 27mm:bike::heart::bike: its runs like a swiss watch,:rock: went for a blast and it did just that brilliant :cheers: checked the plugs and they looked good also.The bike pulls great through all the gears no popping or back fire .
Going to take it for a good ride tomorrow. Really 5Twins thanks ever so much I am very greatful for your help will post further tomorrow and post pictures :D Thanks again Funky;)
 
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Glad to hear it's working out. I dug my Haynes manual out and looked over the carb specs. I saw a couple errors. I really think the #85 air jet listing is one. That's 18 to 20 sizes smaller than the 130 or 135 you usually see associated with the BS34s. That just plain couldn't work.

It is rather odd to have to go smaller than spec on a pilot but every bike is a bit different and it wants what it wants. But, here's a few more tests you can run on the pilot circuit ..... find a parking lot or deserted back road where you can cruise along in 1st or 2nd with the throttle barely cracked open, at like 2K RPMs. If you can do this and it runs smoothly, your pilot size is probably good, if it surges you're lean, if it bucks and stumbles you're rich. Then, from that 2K or so RPMs, grab a big handful of throttle. If the bike falls flat on it's face and you encounter a giant flat spot, you're lean. If it stumbles and breaks up, you're rich.
 

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