Best stock carbs?

[acebars]

Just stripped down my TX650 carbs, they need new float bowls for sure corroded and full of fuel, diaphragms look ok just a little stiff.

Before I go ahead buying parts for the carb I have I was wondering what are the best stock carbs for an XS650?

I want to stick with the CV carbs and I understand later ones had linked throttles and such which is a nice thing to have?

 

[5twins]

Yes, '76 or later BS38s or the '80-on BS34s. As a general rule of thumb, the newer the carb set, the more refined it is. The '78-'79 BS38s are a bit smoother operating than the '76-'77 set. The BS34s are a bit more refined and modern than the 38s.

 

[acebars]

Thanks, I'm looking at buying a set of what I can only tell to be '76-79 BS38s.

My next question is whether or not the jetting would mess up my TX650 engine and whether or not it would need to be adjusted accordingly or is it a bolt on swap carb?

I note there are differences in main jet size at 122.5 or 135 on the '76-79 ones rather than the 130 on the TX650 and pilot jet size with the TX650 being 45 and the '76-79 ones being 25-27.5

 

[jd750ace]

They all feed fundamentally the same engine through different orifices and passages, but essentially they all provide the same mixture at a given RPM. You tune to the year of the carb, so if you got a 78 or 79 set of 38's, then the stock jetting for those years would be the basis of your tuning. Bolt them on, tune starting from stock for the year of the carbs.

 

[5twins]

You jet these carb sets by the year of the carb set, not the motor year. The motors are all basically the same. So, whatever year carb set you get, use that jetting - don't convert it to what you have in your current carbs. If you do mods and need to re-jet, base that on what came stock in the carb set you're using.

The main jet sizes are different because the needle jet sizes were different. The two work together and one influences the other. Larger mains are run with smaller needle jets, smaller mains with larger needle jets. You can see it all here, how the two component sizes relate to one another, in this chart I made up .....

 

[acebars]

Thank you 5twins that was actually the diagram I was looking at when I wrote my post, thanks for making it.

So any '76-79 stock carbs will do and then I just jet from there? These also have a single throttle cable which is a nice feature.

I've read the BS34s are inferior, which would you say perform the best, would it be the '78-'79?

 

[TwoJugs]

I had good luck with both the 34s and 38s. I think many like the 38s with the premise that they have bigger openings and will make more power? Some think certain models tune easier than others. I like the later single cable, racked carbs and had both designs work fine as long as they are clean, tight and dialed in right. I am not a fan of the ones that use a remote choke pull nob and cable setup. The lever or pull nob is easier for me.

 

[5twins]

The BS34s used to have a bad rep because they were so pollution mandated and had fixed (non-adjustable) needles. That's pretty much all changed now. You can get adjustable needles for them now. They also have other benefits. They are much lighter than the 38s and a more modern design. They have a removable air jet for the pilot system which can be tinkered with for fine, fine tuning. I really feel that in the future, they will become the most desirable stock carb set. But, any of the three sets mentioned are good. I would go for whatever you can find in the best shape and for the best price.

Personally, I've run both the '76-'77 set and the '78-'79 set on my '78. The '76-'77 set has very good midrange power due to the large Z-8 needle jet. Even though it's main is very small, it doesn't seem to suffer much on top because of that. It does run a bit rougher in the idle and lower midrange than the '78-'79 set, probably due to the old slide design. In '78, the slide was redesigned and the needle was spring loaded. This really smoothed things out.

 

[acebars]

Well I was looking at a later set of carbs on ebay, I missed the bidding for them and they went for much more than I expected.

That made me rethink, is it worth upgrading to later '76-79 carbs or should I just sort out the '73 carbs I already have (with the separate throttle cables)? Really only need a set of new floats for replacement?

Or like you say maybe the BS34s?

 

[nj1639]

Well I was looking at a later set of carbs on ebay, I missed the bidding for them and they went for much more than I expected.

That made me rethink, is it worth upgrading to later '76-79 carbs or should I just sort out the '73 carbs I already have (with the separate throttle cables)? Really only need a set of new floats for replacement?

Or like you say maybe the BS34s?

Check out post #2... http://www.xs650.com/forum/showthread.php?t=31396 .....might be something to consider. Not sure of the availability in 'ol Blighty though.
BUT, I'm all for the 34's.

 

[acebars]

Thank nj1639, those EX-500/GPZ 500 carbs are definitely cheap over here, about the same price for a good set and BS34 boots as a set of floats for my '73 carbs, so it's certainly tempting. From all the reading I've done so far I understand they offer the best performance for a CV carb on an XS650?

Only thing I'd rather not be doing is messing around with the jetting etc. at this point in time because it's a new (old) bike and I need to make sure everything is running correctly first, that and the GPZ carbs have that ugly black plastic top.

I've just noted another slight advantage with the BS34 carbs their boots have a valves for carb balancing which is a nice feature.

My reason for sticking with CV carbs is fuel economy, can the BS34s be made to perform as well/powerful as the earlier BS38s while keeping the same fuel economy?

 

[5twins]

The BS34s usually give a few MPG better fuel economy over the 38s. Yes, they can be made to perform well. My buddy has two '80-on 650s with BS34s. I've ridden them both and honestly, don't feel they're down on power at all compared to my BS38 equipped '78.

The Ninja carbs are nice, no doubt about it, but they're not "plug and play". Jetting needs sorting and a throttle cable needs to be rigged up, probably a choke cable too.

 

[acebars]

My buddy has two '80-on 650s with BS34s. I've ridden them both and honestly, don't feel they're down on power at all compared to my BS38 equipped '78.

Wondering if there is anyway to know for sure, although I'm not looking to make bhp gains by carb swapping I'd rather not power down by going to BS34s.

 

[jd750ace]

I doubt you'll find anything quantifiable as far as a side by side comparison on the same bike, but, In my experience, smaller bores fill more efficiently in the middle RPM, where your motor spends most of it's life. The extra diameter would only help if the engine could efficiently use the maximum volume that carb could flow at the max potential RPM of the motor. If the 34's will effectively feed the bike to it's maximum power point ( I believe they have no problem doing that on these mild tuned low compression motors) Then given the choice I'd go with the slightly more modern, lighter weight carb. Bigger carbs don't often mean bigger power, unless it's a high compression, high RPM motor, and then, not always.
$0.02

 

[weekendrider]

The BS34s usually give a few MPG better fuel economy over the 38s. Yes, they can be made to perform well. My buddy has two '80-on 650s with BS34s. I've ridden them both and honestly, don't feel they're down on power at all compared to my BS38 equipped '78.

ditto this experience.
Although with the right jetting you can drop that mileage down to low 40's
And with the XS Performance carbs you can get it down to the mid 30's

 

[acebars]

Just one last question you guys have been most helpful, is there a way to distinguish the '76-77 carbs externally from '78-79 ones?

 

[acebars]

Ah I found the answer. The '78-79 carbs have similar ports in the inlet side as the BS34s do, whereas those older don't.

Of the '76-77 to '78-79 I understand the later ones are much better, I read somewhere on this site that the '76-77 range are great in the mid range but struggle in the upper ranges?

 

[5twins]

No, they don't struggle in the upper range. Actually, they perform quite well considering the mains are so much smaller than in the '78-'79 set. As I mentioned a few posts up, they're just a bit rougher running through the idle circuit and into the lower midrange. It's something you'd probably never even notice unless you did a back to back comparison to a '78-'79 set like I did.

 

[acebars]

I've decided to stick with the BS38s for the time being but will be thinking of trying out the BS34s in future.

So would you say that for all around use the '78-79s ones are a little better than the '76-77 ones? I'm trying to decide which extra carbs to purchase you see.

 

[scabber]

The best stock carb if one can see past the black plastic tops is the PVK stock to the EX 500 and GPZ. I liked the BS 34 carbs and they worked really well once I got them sorted not an easy task. Factory float levels settings wouldn't work correctly trial and error after that. Yamaha changed the carb design, needles jets several times trying to get them to work better on the 650 motor. The PVK carbs are a better design. The light weight flat-slide is vacuum actuated and gives you instant throttle response I have found. The carbs have vacuum ports for tuning. The jetting and needle selection given by mrriggs in the thread works well with 650 and 750CC modified and stock motors. mrriggs is getting 54 miles per gallon not bad I would say and better performance to go along with it. Buying another set of old 30 something year old with old rubber parts and wear when newer unused carbs are available makes no sense to me.