Which jets and where to buy?

Norton7d

Norton7d

XS650 Junkie
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Here is what I have;

9k mile 76, original BS38's with original jetting as seen by the chart below.
MAC 2 into 2 pipes
Will run foam filers once the bike is back together in its final configuration.
Looking at the old plugs that were in the bike when purchased, they looked like they were run on a "lean" motor although not to a critical degree, but the bike was unused for approximately 3 years so tough to tell with certainty.

Working on vacuum leaks currently, seals and gaskets ordered.

Based on this info, do I increase;
mains only, 1 or 2 sizes?
needles and needle jet as well?
currently on middle clip, stay there until final tinkering is performed?

From what I read, Niche cycle is a good purveyor to deal with.

Thanks.

CURRENT CARB JETTING.JPG
 
If you study that chart, you'll notice that your carbs have the smallest mains fitted to any of the 650 carb sets. To do that, they had the largest needle jets of any 650 carb set. Normally, on the other carb sets, you bump the mains up 1 to maybe about 3 sizes for the basic "pods and pipes" mod. Your carb set however, since the mains are so small to begin with, may require a bit more of an increase, maybe in the 3 to 4 sizes range. Those small stock mains work fine on a stock bike but are quickly outpaced when you start modding.

The rest of the re-jetting routine follows the standard recipe used on the other sets - bump the pilots up one and lean the needles a step. It's very rare on these to just be able to bump the mains. It usually takes tinkering with all 3 of the carb circuits. Here's the usual routine - you increase the mains one step at a time until you incur break-up in the upper midrange under heavy throttle applications. That's telling you you're getting too rich. You lean the needles a step to fix that but that also leans the upper part of the idle circuit so a larger pilot is often needed.

So, for your carbs, I'd order one size larger pilots (27.5) and a couple larger mains a few steps above what you have stock (130, 132.5). I set up a pair of '76-'77 carbs for use on my '78. My mods were just like you plan, exhaust change and pods. My final settings were one up on the pilots, 132.5 mains, and the needles leaned one step. The 132.5 mains were the biggest it would take.

In reading up about re-jetting the carbs on these, you may see many recommend mains in the low to mid 140's. That can work on some of the other carb sets but with your large Z-8 needle jets, you'll never get that big on your set. Low to mid 130's is about the most you can put in them.
 
5twins said:
The rest of the re-jetting routine follows the standard recipe used on the other sets - bump the pilots up one and lean the needles a step. It's very rare on these to just be able to bump the mains. It usually takes tinkering with all 3 of the carb circuits. Here's the usual routine - you increase the mains one step at a time until you incur break-up in the upper midrange under heavy throttle applications. That's telling you you're getting too rich. You lean the needles a step to fix that but that also leans the upper part of the idle circuit so a larger pilot is often needed.

So, for your carbs, I'd order one size larger pilots (27.5) and a couple larger mains a few steps above what you have stock (130, 132.5). I set up a pair of '76-'77 carbs for use on my '78. My mods were just like you plan, exhaust change and pods. My final settings were one up on the pilots, 132.5 mains, and the needles leaned one step. The 132.5 mains were the biggest it would take.
.
Click to expand...

Twins, regarding the following quotes from your larger quote.
"you lean the needles a step to fix that"
"and the needles leaned one step"

does that mean different needles are needed, or just move the clip up one notch to move the needle downward to a more lean position?
 
Just move the clip up one step and lower the existing needle you have. It should be a 4M1 with the clip in the middle (#3 from the top) slot. You move it up one step to the #2 slot.

uVUwOei.jpg
 
Twins, I called Niche cycle to get some jets, Kevin said because the series of the jets are not known, he doesn't know which ones to sell me, even if I provide the size. He asked if I had a picture of mine, that would solve the issue but mine are in my carb.

Does anybody have a pic of the pilot and or main of a BS38 so I can provide to Niche?

Thanks.
 
5twins said:
If you study that chart, you'll notice that your carbs have the smallest mains fitted to any of the 650 carb sets. To do that, they had the largest needle jets of any 650 carb set. Normally, on the other carb sets, you bump the mains up 1 to maybe about 3 sizes for the basic "pods and pipes" mod. Your carb set however, since the mains are so small to begin with, may require a bit more of an increase, maybe in the 3 to 4 sizes range. Those small stock mains work fine on a stock bike but are quickly outpaced when you start modding.

The rest of the re-jetting routine follows the standard recipe used on the other sets - bump the pilots up one and lean the needles a step. It's very rare on these to just be able to bump the mains. It usually takes tinkering with all 3 of the carb circuits. Here's the usual routine - you increase the mains one step at a time until you incur break-up in the upper midrange under heavy throttle applications. That's telling you you're getting too rich. You lean the needles a step to fix that but that also leans the upper part of the idle circuit so a larger pilot is often needed.

So, for your carbs, I'd order one size larger pilots (27.5) and a couple larger mains a few steps above what you have stock (130, 132.5). I set up a pair of '76-'77 carbs for use on my '78. My mods were just like you plan, exhaust change and pods. My final settings were one up on the pilots, 132.5 mains, and the needles leaned one step. The 132.5 mains were the biggest it would take.

In reading up about re-jetting the carbs on these, you may see many recommend mains in the low to mid 140's. That can work on some of the other carb sets but with your large Z-8 needle jets, you'll never get that big on your set. Low to mid 130's is about the most you can put in them.
Click to expand...

Mr Twins, I haven't done this jetting yet, working on other cleaning first.

Question, you want me to go up on the pilot and Main Jet and then lean down the needle one notch, why the up on the jets and then down on the needle? why not just go up on the Jets but not as high, that requires the needle to come down?

Thanks as always!
 
Your carbs consist of several different circuits serving different RPM ranges. You're changing several different jets and adjusting the needle to cater to all these different circuits. But, what you have to understand is that these circuits aren't isolated from one another, they overlap the one or ones next to them a little. That means changes to one circuit will influence the adjoining circuit to a certain extent as well .....

cC7PbUq.jpg


So, as you increase the main jet size, eventually, after several steps up, the bleed-over into the upper midrange will make that area too rich, and that's why you lean the needle. But I guess you're asking why not just step the main back down, or don't go that big on it to begin with. Well, then it may not be big enough. You're seeking the largest main jet you can use that will run cleanly at high RPMs but also play nice with the lower circuits. As I outlined a few posts up, the usual routine is to increase the mains one size at a time until you start incurring upper midrange break-up under heavy throttle applications. This is an indication that the bleed-over from the larger main is making the upper midrange too rich. So, you lean the needle a step to fix that. This is pretty much the cut-off point or indicator that you've gone as big as you can on the mains. If you step up another size, it will bring that upper midrange break-up right back.

If you didn't have to lean the needle then you might not need a bigger pilot jet. But leaning the needle often creates a flat spot or bog just off idle, so a larger pilot is needed. So, it's all sort of a trickle down effect. Larger mains eventually require leaning the needle. Leaning the needle often then requires a larger pilot.
 
Ok, it makes sense, as much as it can to a guy that hasnt ridden as XS in 30 years.....I like the way you put this part:

"You're seeking the largest main jet you can use that will run cleanly at high RPMs but also play nice with the lower circuits."

Thats what provided the most help to my understanding.
Thank you.
 
This is pretty much basic carb jetting 101. It would apply to most any carb on any bike. The CV type carbs we have can be a little trickier because they're very forgiving. They work on engine demand and that makes it easy to over-jet them. When testing the jetting, you have to push them hard. Normally, you won't ride the bike like that but for jet testing, you have to work it hard. Testing the main and looking for that telltale break-up in the upper midrange will require full or near full throttle from about 3.5 to 4K on up through to 5 or 5.5K RPMs. If you don't work the carbs hard like this, you may not uncover that stumbling in the upper midrange. Testing the idle circuit and idle to midrange transition for flat spots or break-up will also require some hard running, but not as much as the higher circuit test. Just a big handful of throttle applied just off idle is usually enough. If you're too lean, the bike will fall flat on it's face and hit a big flat spot. Too rich and you'll get stumbles and break-up.
 
First rule of carb tuning: check everything else first. Second rule: everything affects everything else. Example: the needle and needle jet are main circuit components. Even at idle, they feed fuel to the engine through the slide cutaway. Changing needle position by just one step will alway force a compensating change in fuel screw adjustment and will often force a pilot jet change. An increase in MJ size may force you to drop the needle, dropping the needle may force you to increase PJ size. Work in cycles of change and compensation.
 
Yes, carb re-jetting for mods is all about experimenting. We can suggest jetting that usually works and has worked for others, but it may not be exactly what your particular bike and mod set-up needs. I'm pretty sure you'll need the larger pilot and the needle clip position change but the exact main jet size may not be what my bike needed. It should be close, just maybe not exactly like mine. I guess a lot will depend on just how free flowing your new MAC exhaust is. If it is a brand new MAC pipe set, I see they are now making many of them with larger diameter headpipes than they used to (1 3/4"). That's getting kind of large for this engine. It will produce better high RPM power but at the expense of losing some low and midrange power. You may need headpipe inserts at the motor to correct that .....

X6lOanj.jpg


The 650 motor likes a smaller diameter pipe up at the head for at least the first few inches. These inserts fool the motor into thinking it has them.
 
Re. pipes, increasing diameter moves the torque peak higher up the rpm range. This can be corrected by reducing diameter with an insert like Michael Morse's "torque peak optimizer" at the outlet end. See 650 Central for this. I use Michael's exhaust port optimizers and torque peak optimizers in 1.75" pipes with good results. Why the oversized pipes? Because nobody offers 1.5" pipes in stainless, and I don't weld. Michael prescribes welding the outlet reducers in place, but securing them with set screws has worked just fine for years.
 

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