Help! My carbys are a mash up - what to do?

Vish79

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Hi guys,
So I have a 74tx650a , that is running (never tested out how well) and hasnt been registered for a while (I know it was registered by the 3rd owner before me). So I thought I would give the carbys a birthday (float gaskets were leaking).

So far they havent been too bad but as its got after market exhaust and pods, I thought I would get a keyster kit for it (ordered a 76-77 one as it was all that was available - didnt realise the jetting was so different at the time or I didnt have original carbs))

1) I have a 78-79 BS38 bodies based on US specs (im in Australia)

2)needle is a 4N8 clip 3 being a 74-75

3) needle jet is Z8 being 76-77

4) Main 135 and pilot 27.5 are 78-79

So what should I do? Can I rejet it with the kit I have effectively? Should I rejet it as original (78-79) ? Is there certain models that perform better then others?

Cheers
Shane
 
Welcome, Shane! The best advice I can give you is to use genuine Mikuni brass and original 78-79 jetting as your baseline if possible. Keyster kits contain questionable stuff, sometimes of dubious quality. There's an inverse relationship between NJ and MJ sizes (the bigger the NJs, the smaller the mains), so if you can't locate the Z-2 NJ you might get away with using the Z8 and reducing MJ size; even with relieved exhaust and intake, I very much doubt you'd be running lean with your present combination . Don't know if the 4N8 needles will play well with 78-79 slides, but you might as well try moving them a notch either way and see what happens.
 
You'll need to determine what slides you have as the slide type dictates the needle needed. That 4N8 might be OK if you have '74 slides but it's much shorter than the 5 series needle (5O2) used in the '78 slide. If you have '78 slides, the 4N8 needle would be very rich because it's so much shorter .....

syuKACC.jpg
 
Thanks 5twins. You dont happen to have a reference for the slides? I just went and had a look at one and couldnt see any identifying marks. I had checked the plugs and they were showing it was running rich....
Cheers
Shane
 
The slides are easy to identify by the way they mount the needle. The '77 and older slides are rather simple. The needle drops in the slide and is held in place by the slide return spring pushing down on a large plastic disc .....

4SOvlZG.jpg


The '78-'79 slide is more complex. The needle has a small spring on it and is held in the slide with a snap ring. There's a funky looking top hat or golf tee shaped plastic retainer on top of the needle that the snap ring holds in place .....

JBHxvr2.jpg


There's also a thick plastic spacer ring on the needle. It's location (above the e-clip) is important. Place it under the e-clip and the needle will sit too high, and be too rich .....

TJkQOok.jpg


This complicated mounting set-up makes the needle sit up higher in the slide. That's why a longer 5 series needle was required, to get the same amount of needle hanging out the bottom of the slide as the earlier slides had with their shorter 4 series needles.
 
Thanks again. Well I definitely have the pre-77 slides, just to keep things uniform :banghead:

Might be cheaper, easier and quicker to bow to peer pressure and get a pair of new mikunis....

Now Im starting to get really curious as to what lead someone to make such a frankenstein set of carbs... Ill probably try and make it original but for the mean time I might have to sweet talk a mate who has a couple of projects that havent moved since the dinosaurs.....
 
Next model carbs; the BS34's, reverse the plastic donut, e clip, and washer order.:confused:
 
Although I've never tried it, technically the early slide should work in the later carb body as long as you tailor the jetting around the needle jet and needle combo you have. Even though your 4N8 needle and your Z-8 needle jet are a mismatch, they may work fine together. If you look at the needles pic I posted above, you can see the tapers are very similar between your 4N8 and the correct 4M1, the 4N8 is just a little longer. You could easily compensate for that with the clip setting. Here's a stock jetting chart. I would tailor your jetting around the '76-'77 spec and that Z-8 needle jet .....

7wbHkpb.jpg


This may be your only option because the matching Z-6 needle jet for your 4N8 needle is not available anywhere as far as I know. You will run across a Z-6 needle jet but it is the longer version and only fits (works) in the '70-'73 carb bodies.
 
im curious where guys are buying there rebuld kits, the ones on ebay 2 for 18 bucks seem like crap.. ive messured the mains that say 135 , and there never the same ,some smaller some larger than a mikuni 135, usualy the only part I reall use is the float valve and gasket, sometimes the orings, also does the later 78 79 air mix screw soposesd to have a little washer and oring on the end or just the oring under the head.. at that same point do the early air mix screws supposed to have any o ring? ive never seen a washer or oring on any xs 650 bs38 carbs
 
Only the BS34 carbs have the washer and o-ring at the bottom like in your pic. '78-'79 carbs just have the o-ring near the top, no washer. All earlier BS38s have no o-ring, just the screw and it's little pre-load spring.
 
I also meant to ask you - how exactly did you determine that your carb bodies are '78-'79?

I actually used one of your documents to do so - Single pull, linked carbies, dual choke, 5 hole intake horn. So thank you again

As for the carby kits, I do know some of the quality of them arnt the best, but have found they come with the gaskets, springs, orings, some of which is impossible (or so I have found) to find The jets and needle are there if needed and atleast can be used short term before swapping them out for quality components. If it was the jets, I would just get them from MikuniOz, A mate highly recommeneded them and my 2 experiences have been nothing different.

I do have a copy of the Suduco mikuni manual so I was going to go through it a bit more throughly. Initial thoughts were that the 4N8 and 5O2 were a similar enough curve that the adjustment in the needle might be enough to compensate it
 
Well just an update.
Well I did a couple of laps to warm the bike up to do an oil change and the regulator decided to reject a capacitor and blow it out the back of its body. In doing this, I later found out that it had blown a resistor in the ignition module (Mikes xs/pamco) so with the help of the forums, I managed to get it sorted. I got an upgraded quality reg as well and rebuilt the carbs to 76-77 spec. The butterblies were also way out. Ive matched them but havent balanced them properly yet,

2 issues along the way :
Stripped thread in the carb body so a nyloc nut was installed
Mixture screws are different so I left the 78-79 in there with it set to its specs

Fired right up and sounded better then before. Still not dialed in but I havent taken it for a proper spin either but I think its the right direction
 
Yes, you'll need to use the mix screws that match your carb body and use their recommended setting spec (2.25 turns out) as a starting point. In your case, the '78-'79 mix screws are quite different from any of the earlier BS38 mix screws and I doubt any of those older ones would even fit .....

96i404f.jpg


pt1rxO2.jpg


hSRhiiN.jpg


As far as why your '78-'79 carbs ended up with '77 or older slide/diaphragm assemblies in them, that may be because replacement '78-'79 assemblies are not available as far as I know.
 
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