Troubles tuning BS38's for open intake/exhaust and 750 kit

Rexxis

Rexxis

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I'm trying to jet my carbs for open intake/open exhaust plus a 750 kit and not having much success.

Early on I had no clue when it came to tuning carbs so all my jetting changes were going off of what I read on this forum. I had no baseline to start with bc my bike never ran before all the modifications were made. Carbs have been completely rebuilt using www.amckayltd.com/carbguide.pdf and are super clean inside. Everything has been given the once over, even needle jet o-rings and butterfly shaft seals replaced(OEM shaft seals)

When I first pulled my carbs apart, they had 135 mains and 27.5 pilots in them so I assumed they were the '78-'79 bs38's. To deal with my mods, I started bumping up on the pilots and mains and got it running decent with some bogging coming off the pilots through the early midrange but then it cleaned up and ran good the more I twisted. I didn't run it all the way to the top so I don't know if it ran strong at WOT or not. At that point I was using #30 pilots, with a #140 main.. I was 1 up on the pilots and 2 up on the mains, needles in slot 3, right in the middle. I don't remember where I had my mix screws, probably at factory for late 38's though, 2 1/4 out I'd bet.

But then 5twins tells me that my carbs are a set from '76-'77 and the man knows what he's talkin' abt so I start over, this time I went with original pilots (25) and two up on the mains(127.5) and leaned my needle a step(slot 2) with my mix screws 1 1/2 turns out. Now I can barely even get her to start, have to kick her 10-15 times.

5twins said:
That's not a '78-'79 carb set. From the looks of it, it's a '76-'77 set.
Click to expand...
Here's the thread: http://www.xs650.com/forum/showthread.php?t=21117

I'm not running a tach and I pulled the internals early in my build, wish I hadn't done that. Hindsight is 20/20 and now I know to keep the tach useable till she's running strong, then you can take it off if you want.

Also, my carb gaskets are glued to the head but aren't glued to the manifolds(JBM's) b/c I have to take the manifolds off to get my carbs out. This could be an air leak but I haven't checked yet bc the f*@%er currently won't start. Is glueing the gasket to the manifolds a necessity or can I get by until everything is running strong? I don't know what else to do bc I am having to repeatedly take my carbs on and off and I can't afford new gaskets every time and I definitely can't scrape gaskets at every change and keep my sanity.

Looks like I might go back to the 30/140 set-up and try it with the clip in slot two this time, maybe that will help my previous midrange problem. I don't know though bc my plugs were black as night when looking at them while trying this setup, albeit my clip was still in slot three then. Anybody have any insight they can share?

I have been trying to get this thing tuned for two weeks now. This isn't even fun any more.

I'm wondering how does one tell the difference between '77 and '78 carbs just by looking?

Has anybody ran open intake and exhaust w the 750 kit? If so, what's your jetting???

Thanks guys.
 
Rexxis said:
I'm trying to jet my carbs for open intake/open exhaust plus a 750 kit and not having much success.

Early on I had no clue when it came to tuning carbs so all my jetting changes were going off of what I read on this forum. I had no baseline to start with bc my bike never ran before all the modifications were made. Carbs have been completely rebuilt using www.amckayltd.com/carbguide.pdf and are super clean inside. Everything has been given the once over, even needle jet o-rings and butterfly shaft seals replaced(OEM shaft seals)

When I first pulled my carbs apart, they had 135 mains and 27.5 pilots in them so I assumed they were the '78-'79 bs38's. To deal with my mods, I started bumping up on the pilots and mains and got it running decent with some bogging coming off the pilots through the early midrange but then it cleaned up and ran good the more I twisted. I didn't run it all the way to the top so I don't know if it ran strong at WOT or not. At that point I was using #30 pilots, with a #140 main.. I was 1 up on the pilots and 2 up on the mains, needles in slot 3, right in the middle. I don't remember where I had my mix screws, probably at factory for late 38's though, 2 1/4 out I'd bet.

But then 5twins tells me that my carbs are a set from '76-'77 and the man knows what he's talkin' abt so I start over, this time I went with original pilots (25) and two up on the mains(127.5) and leaned my needle a step(slot 2) with my mix screws 1 1/2 turns out. Now I can barely even get her to start, have to kick her 10-15 times.

Here's the thread: http://www.xs650.com/forum/showthread.php?t=21117

I'm not running a tach and I pulled the internals early in my build, wish I hadn't done that. Hindsight is 20/20 and now I know to keep the tach useable till she's running strong, then you can take it off if you want.

Also, my carb gaskets are glued to the head but aren't glued to the manifolds(JBM's) b/c I have to take the manifolds off to get my carbs out. This could be an air leak but I haven't checked yet bc the f*@%er currently won't start. Is glueing the gasket to the manifolds a necessity or can I get by until everything is running strong? I don't know what else to do bc I am having to repeatedly take my carbs on and off and I can't afford new gaskets every time and I definitely can't scrape gaskets at every change and keep my sanity.

Looks like I might go back to the 30/140 set-up and try it with the clip in slot two this time, maybe that will help my previous midrange problem. I don't know though bc my plugs were black as night when looking at them while trying this setup, albeit my clip was still in slot three then. Anybody have any insight they can share?

I have been trying to get this thing tuned for two weeks now. This isn't even fun any more.

I'm wondering how does one tell the difference between '77 and '78 carbs just by looking?

Has anybody ran open intake and exhaust w the 750 kit? If so, what's your jetting???

Thanks guys.
Click to expand...

You should not have to take off the manifolds to remove the carbs. On my 78SE, my carbs come right out no problems at all. I remove the camchain tensioner nut, the oil dipstick
and the throttle cable guide bracket that mounts on the left carb. Carbs come out on the right hand side.
 
The usual scenario for re-jetting these carbs is one or two up on the pilots, 1 to 3 up on the mains, then some needle tuning (leaning usually) if need be. However, your '76-'77 set is a bit different. It will require the pilot jet increase like all the rest but may need a bit more on the mains because they were so small when stock (122.5). Those small stock mains quickly run out of steam when you start modding. The hard starting is probably because you haven't increased the pilots. I would go back to what you found in there, the 27.5 pilots and 135 mains.

What you have to take into account with this carb set is the very large Z-8 needle jet it has, the largest ever installed in a 650 carb set. That's the reason for the tiny 122.5 stock mains, the smallest ever installed in a 650 carb set. That needle jet is going to be the limiting factor on how big you can go with the mains. While many of the other 650 carb sets may be able to run mains up into the mid 140s, you probably won't be able to get much bigger than the upper 130s.

So, if I was trying to set these up, I would get 27.5 and 30 pilots, 132.5, 135, 137.5, and maybe even some 140 mains. I would start with the 27.5 pilots and 135 mains, needles leaned a step, and mix screws at the factory 1.5 turns out. I would fine tune the mix screws then start testing the mains and needle setting. If I could coax no break-up out of her through the upper midrange under heavy or full throttle applications, I would start increasing the mains until I could. When you start getting that upper midrange break-up, that's a sign your mains are getting too big or your needle needs more leaning. However, I'm not a fan of leaning the needles more than one step. Since that was done already, when I started getting the break-up, I'd take that as the indicator that my mains were as big as they could go and in fact probably needed to go back down one size.

There's another factor at play here - the pods you've chosen. If you have K&Ns or that style of pleated element, you will encounter this upper midrange break-up sooner and maybe even before you've reached the ideal main jet size. If you're going to run pods, the UNI foam style work best and are easiest to tune with.
 
You're the man 5twins, bigtime thanks. I think I ready have all those jets so I will try to play with her today and update what I find.
 
I'm only replying to this, so I can reference it later... I have a set of BS38's that i plan to run on the exact same setup... Thanks 5twins!
 
Remember, I'm referring specifically to the '76-'77 carb set here. As far as the 38s go, it's considered one of the best sets, probably because of that large needle jet which gives great midrange power. And even though you won't get mains in it as big as some of the other sets, the top end doesn't suffer much, if any, from this. The reason is your mains flow through the needle jet. That large Z-8 needle jet is going to let more fuel flow through compared to the Z-6 and Z-2 used in the other 38s. Even though your main may be slightly smaller, you're going to get pretty much the same performance as the other 38s running larger mains through smaller needle jets.

As I've mentioned before, I feel the '78-'79 BS38s operate the smoothest through the low end and midrange probably due to their spring-loaded needle. Their midrange "umph" isn't as strong though due to their smaller Z-2 needle jets. An experiment I'd like to try some day is putting slightly larger needle jets (and slightly smaller mains) into a '78-'79 carb set. If it works, I might get the benefits of the later carb's smoother running and the earlier carb's stronger midrange.
 
As far as the manifold gaskets go, I seal them to the manifold, not the engine. I do this so they come off with the manifold each time it's removed and not really for sealing purposes. If you've ever seen an original manifold gasket, you'll notice that they're very thick, 1/8" or maybe a bit more. They seal fine usually without any sealer applied. I just use some for convenience sake, so I don't have to scrape them off the motor. That's a bitch of a job as access to that area is limited.

I'm not sure what sort of gaskets the JBM manifolds come with, if any. I know the replacement manifolds I got from Mike's several years back came with just thin paper gaskets, which I refused to use. Instead, I used the thick ones that come in the Athena complete engine gasket set as they more closely replicate the thick originals. Since then, I did score some O.E.M. originals off eBay which I will use in the future.
 
The JBM's aren't supplied w/ any gasket. I bought the Athena set last year when I rebuilt my top end so that's what I'm using now. Good info to have abt the gasket thickness.
 
That thick gasket may be put there by design. Besides sealing, it may also be that thick to help isolate the manifolds and carbs from engine heat. If you look at the intakes on various different bikes, you'll notice that many do use a very thick gasket in this location. I would never use a thin gasket here unless that particular bike came from the factory equipped with one.
 
27.5 on the pilots and 135 on the mains. Needle clip is up one slot to slot 2. Still hard starting, can't quite get her to fire up. She almost did twice after abt 25 kicks.

So what do you think, step it up to a #30 pilot? Remember, I'm running the 750 kit.
 
Was just thinking, decided to touch the battery charger to ma capacitor terminals to make sure it had a charge after kicking the bike so many times and not getting it to start. Usually it sparks just a lil but not this time, no spark, nothing. Put my meter to it while set on ohms and got nothing, infinite resistance, OL. Could my cap be bad? I know this belongs in a different thread.
 
Yes, maybe you have some electrical problems. Have you checked for spark? Really, a 750 kit doesn't change the jetting required much, if at all. If you look back at the jetting specs for the old TX750 twin, you'd see that they were the same as the 650 of that period.
 
Well just to throw in my $.02, when I put the 750 kit on, I thought it would need rejetting so I spent about two weeks trying different combos of jets. It ran the best with the same setup as it did with as a 650.
Leo
 
Really!? Good to know.

No idea where to start w my electrical issues. I'll update again on jetting once I get that figured out.
 
Not wanting to high Jack this post .but 5 twins seems to be very bloody knowlagable. I'm having issues to ..iv now fitted 78/79 bs38's..its only starting with a squirt easy start. And when choke is on its running faster. Iv not had it on the road yet with the twin carbs (had a single carb b4)..so im guessing a bigger pilot jet is needed..im running all standard jets ..so I'm about to order new ones .what size should i order as I dontbreally want to pay shit loads of pounds and only use2 but realise I have to buy ect for trial and error. Any help would be greatfull oh yea and when tbey arrive il be using open (gauze)bellmouths and its got short pipes ..thanks all
 
Maybe it's just me but I don't see CV carbs ever working "well" with no air box. Short pipes aren't so great for good running motors either, guess if looking good while parked is your goal.........
 
It ain't just you, Gary. But WTF do we know? We obviously don't know what's cool, we just put 'em together to ride.
 
obviously,
DSCN9816.JPG
DSCN9815.JPG


:laughing: :smoke:
 
I guess these would be classed as short pipes
 

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