Jetting advice when switching to pods?

[Square Circle]

Okay, im going to ask this same old question for you expert carb guys.. Seems as if a alot of things contribute to the way a carb is jetted by what is modified with engine..
I have a 79 with bs38 carbs, I am looking to use the K&N pod filter " knockoffs from mikesxs." I already have them.. My carbs are totally cleaned out and floats set correctly at 24mm each, and are stock jetted at the moment..
After briefly talking with MMM with 650 central about this, he mentioned that a typical jet set up is 1 up on the pilot (27.5 to 30) and 3 up on the main(135-142.5?)...The 3 up on the main sounds like a lot to me, as I am still running stock exhaust . I will be adjusting my cam chain and valves shortly , and checking timing soon when i switch to a pamco ignition with the high output coil and iridium plugs.
In your experience guys, what is the most usable and functional jet setup for what i have?

 

[XSLeo]

MMM has a long time working on these bikes, raced them for many years. I think his advice is a good starting point.
I might get main jets from 137.5 up to the 142.5 and the a 30 and maybe a 32.5 pilot. start at 1 up on both jets. Adjust the air idle mix screws as needed. Test them out, the carb guide tells how to. Try the next larger pilot, test. Better? worse?
Next start one more up on the mains, test, better?
Move up the range of mains till you reach the best perfomance.
Leo

 

[scabber]

I would put them on and see how it runs. Have a 79 that had a 140 main jet stock mufflers and no air cleaners it was running ritch. The stock set up might work fine.

 

[5twins]

You're going to need to lean the needle setting a step if you increase the main jet size at all.

 

[Square Circle]

You're going to need to lean the needle setting a step if you increase the main jet size at all.

Even 1 up? sounds like you can chase this rich or lean deal back and forth.
I dont have a carb manual for the 38s and I was eyeballing the inside of the slides for needle adjustment.. it looks like there is a large snap ring maybe about 3/4in. dia or so.. kinda coupled with a teflon washer ring of some sort at basckside of the needle . I would assume this clip comes out and then another micro snap ring around the needle?? whats the technique to make proper needle adjustments on this carb without screwing (breaking) it up? I've been looking around for an extra long set of snap ring pliers, but no avail.. i did find a set at Sears that has different pins.. i could possibly retrofit a longer hex key into the pliers for an extra reach. , Unless i just have know idea at all.. (most likely the case here)
Changing the jets and cleaning the crap out of the carbs was pretty easy.. i wasn't about to screw with the butterfly screws. Now changing the needle height might be a challenge for me. As i am a total beginner.
Thanks.

 

[jchrisk1]

There is a CV carb guide in the "Tech" section at the top of the page. You can buy some 90* needle nose pliers from mikesxs for getting those snap rings. I just bought the smallest needle nose I could find at Lowes and ground the tips to fit in the holes.

 

[hotrdd]

I have the 76 BS38 carbs with a single K&N filter and I've been chasing the jetting all year. I made it all the way up to 137.5 but thought I was running to rich. It's hard to tell between rich and lean conditions. I'm in the same boat as you and will probably be starting from scratch again. I believe the factory main jet was 122.5

 

[5twins]

The '76-'77 carb set and the '78-'79 carb set are as different as night and day. '78 was a "big change" year for the BS38s. They got that new style slide that retains the needle with a snap ring and all different jetting. Overflow tube in the float bowl was eliminated along with it's outlet nipple on the bowl bottom. External vents for the float bowl area were moved to inside around the bell mouth.

Yes, get those 90° snap ring pliers from Mikes. Even though they're sold as a master cylinder rebuilding tool, they work like they were made for this job. Here's the slide break down .....

Make sure you don't mix up the positions of the thin washer and thick plastic spacer washer, that'll throw the needle height way off .....

With the '78-'79 carb set, Yamaha attempted to give the 650 a top end performance boost. They did so by increasing the main jet from the 122.5 used in the previous carb set to a #135. That's a big jump. To make it work, they needed to lean the midrange out. They did so by decreasing the needle jet size from the previous Z-8 to a Z-2. Even so, the upper midrange is still right on the verge of being too rich in the '78-'79 carb set. That's why you need to lean your needle with even one step up on the mains. On some of the other 650 carb sets, you may get away with it but you won't on this set.

 

[Square Circle]

My haynes manual lists the xs650C carbs as having a stock 122.5 main jet.. My 1979 is the f or sf type xs650, i guess??(haynes manual for 650 twins goes to 1970-1983 but doesnt specify any F or SF type 650s) I had a main jet of 135 already in my carbs and the replacements in the carb kit from Mikes xs were also 135s.. i guess my year did in fact come with the 135s. I put the carbs back on bike to finish with other tuning stuff ... i'll no doubt be pulling them back off shortly.. as of now I'm going through stuff like valve adjustments and and timing.
One more saturday or so and i might get this machine on the road.

 

[5twins]

Here's a chart I made up of the U.S. model carb specs. Study it and you can learn a lot about the 650 carb's "evolution". Many think the '76-'77 set was the best of the 38s but I like the '78-'79 set a bit better. That new slide design with it's spring loaded needle gives nicer low end and midrange response in my opinion. The '76-'77 set does have wonderful midrange power with that large Z-8 needle jet, it's just delivered in a little "rougher" manner. And yes, I've tried both sets.

 

[Square Circle]

Thanks for the info and diagram 5twins. So are you saying i need to adjust needle height with my existing needle? or change needles and heights when going up on mains with the 1979 bs38s?
When you go down on size with needles , the needle dimentions/diameters are actually getting bigger around i guess?

 

[5twins]

There are really no needle alternatives available for these 38s so you're kinda stuck with what you have. That's pretty much OK because you can usually tune them well enough just by adjusting the clip position (needle height). So, you need to adjust your current needle, not change it. Like I said, there's nothing really to change to. Many of us shim them by adding a half MM washer similar to the one you'll find below the e-clip. This allows you to set the needle in "half" step positions, like 2 1/2 or 3 1/2. Moving the e-clip changes the needle height by 1mm. With the shim washer, you can set it in .5mm increments. Sometimes that half step is just what you need, a full 1mm step being too much.

I have found one needle alternative for the '78-'79 carbs, the 5Z1 needle from an XS400. That's actually it in my needle pic above. You'll notice it tapers to a nice sharp point. When you get a look at your stock 5O2, you'll see it doesn't, it's much thicker at the tip. This 5Z1 needle works quite well when combined with larger mains. I don't recommend it for a stock bike because it is thicker (leaner) on the upper portion and that upper portion is where you run most of the time at lower speeds. You need the larger mains to add richness to the midrange (via bleed-over or overlap) to compensate for this leanness.

 

[Square Circle]

So would the 5z1 needle be a good set up on the 38s with big mains and if you were to say equip the bike with a performance 2 into 1 exhaust? I'd guess you have to fly around town in low gears and in higher rpms... make it feel sorta 2 strokey? Would this be a total mid range loss?

 

[Square Circle]

Where is the best place to pick up some of the micro shim needle washers?

 

[5twins]

No, in fact it makes the bike much snappier off the line. Yes, they would probably work well in the set-up you're contemplating. The problem will be finding a set. I don't think Yamaha sells them any more. I pulled mine out of a 400 in my dealer's bone yard.

 

[5twins]

P.M. me your address and I'll send you some washers. I bought a bag of 100, more than I'll ever need or use, lol.

 

[Square Circle]

P.M. me your address and I'll send you some washers. I bought a bag of 100, more than I'll ever need or use, lol.

Done,, thanks alot 5twins!!

 

[Square Circle]

No, in fact it makes the bike much snappier off the line. Yes, they would probably work well in the set-up you're contemplating. The problem will be finding a set. I don't think Yamaha sells them any more. I pulled mine out of a 400 in my dealer's bone yard.

Well this sounds fun. Im in full trust here that the 5z1 needle works.. so i just ordered a used set from a boneyard on ebay... 20bucks a set.

 

[5twins]

Oh, they work. They are the same length as your original 5O2, just tapered differently. Here's a comparison shot also including the BS34 needle. You'll notice the 5Z1's taper more closely resembles that .....

Here's a close-up of the tips. The 5Z1 and the BS34 tips taper to a sharper point than the 5O2 .....

Sustained slide lift on these CV carbs really doesn't start happening until about 4K or so. However, if you grab a big handful of throttle at low RPMs, the slide will jump up momentarily and then drop back down. I think that sharper point lets in a richer shot of fuel during that "jump", hence the snappier off-the-line performance.

 

[Square Circle]

5twins, Where would you suggest needle position of the 5z1? I just got mine in the mail today.. "5z1 " even sounds performance enhanced ..lol