'76-77 bs38 carbs on a 1980 bike? what main jet to start?

[strang]

My bike came with some aftermarket carbs that well.... just look for one of the largest carb threads recently and you'll get the picture.
I bought a set of BS38s and taking them apart my pilot jets have "25" on them and my needles are "M41" so going via the carbguide.pdf - the chart says these are stock on '76 XS650C and '77 XS650D models.
However my bike is a 1980.
My question is this:
Do you treat the carb as it is or do you take into account the newer bike engine it is mating with? ie. the chart says that '76 XS650C and '77 XS650D carbs had a 122.5 main jet (can't find any identifying numbers on main jets of carb I bought) whereas the carbs for the later bikes (ie. my bike engine) had larger main jets. Do I have to take this into account when jetting? These carbs will have ramair filters as my bike doesn't have an airbox, so just wanted to know where to start.
If I just follow the suggestions for this particular carb I will buy the next two sizes up from 122.5 - is this the right ballpark starting point for my situation?

 

[Classic]

The carburetor should be completely stock other than the needle raised one notch to allow a richer mid and high range to account for the pods. The idle adjusters might need an eighth to quarter adjustment out from the factory setting. This might not be perfect due to your weather or elevation and other variables but I'm inclined to believe that it's the best starting point for the 38's.

 

[nj1639]

I'll follow this one too....

 

[weekendrider]

Strang jet and adjust to the carb set you are working with.
The year of the motor isn't a factor.
Pods, short pipes, larger displacement all play a factor.
But you still use the jet (range) and mix screw setting for that set of carbs.
5twins, grizld1 and others could tell you why. I can't.

 

[5twins]

Yes, base your jetting on the year of the carb set, not the motor. These motors changed very little over the years but the carb sets did. The main jet size must be chosen to work with the needle jet size. If you study that stock jetting chart some more, you'll notice larger mains are run with smaller needle jets, smaller mains with larger needle jets. The '76-'77 carb set used a Z-8 needle jet, the largest ever put in any 650 carb set. That required the small 122.5 mains which were the smallest ever put in a 650. In fact, some overseas markets even got this carb set with 117.5 mains.

That being said, the small main in this carb set quickly runs out of steam when you start modding things. Yes, I would recommend trying some larger mains. If the pods are your only mod, the next 2 sizes up would be a good place to start. If you change the exhaust too, you may need even larger. Since the midrange is so rich, you will probably need to lean your needle setting as you increase the mains.

I have a set of these carbs that I dialed in for my bike. I have what I consider to be minor mods (pods and pipes). I went up one on the pilots (25 to 27.5), four on the mains (122.5 to 132.5), and leaned the needles one step to position #2.

With this carb set, you won't get much above the low to mid 130s on mains. That large Z-8 needle jet won't allow it. The 140 to 145 main jet recommendations from the MikesXS site and the Minton Mods article just won't work here. That's fine, they work well enough with mains in the low 130s and have a wonderful strong midrange. Better in fact than the midrange on my stock '78-'79 set. I don't feel they lose any on top either even though the mains are smaller.

All in all, I consider the '76-'77 carb set to be one of the best of the 38s. The '78-'79 set is a bit more refined with it's spring loaded slide needle but it's really a toss up between the two. I could be happy with either.

 

[strang]

great info for my newbie head - thanks people
My main jets don't have any markings on them that I can see - is this the norm? mine are pretty hacked up though.

 

[5twins]

No, that's not the norm. The jet head should be stamped with it's number (size) and would also have the Mikuni emblem if genuine. Yours are probably aftermarket replacements. If they have no markings then there's really no way to determine what size they are. It's best that you replace them with a known item. I recommend genuine Mikuni jets only, not the knock-offs. They cost pretty much the same so there's no reason to use the copies.

This is the large round style used in the BS34s but the small round type you use will be similarly marked .....

 

[leggers]

great info for my newbie head - thanks people
My main jets don't have any markings on them that I can see - is this the norm? mine are pretty hacked up though.

I thought mine didn't have markings either, but I just couldn't see them. Someone suggested taking some 220 sand paper and lightly sanding the surface of the jet. This worked and revealed the jet size number.