JNaws Carburetor Adventures

[5twins]

No, I don't have any way to measure the air jet size, but from looking over the service data sheets from the various years, they appear the same from '73 up through '75. For '76 -'77, it appears the pilot air jet gets a little bigger. Unfortunately there is no air jet size listed on the '78 and '79 service data sheets but I kinda think it's the same as the '76-'77 carbs. The difference, I would think, is between the carbs that use the BS30/96 pilot jets ('75 and older) and the ones that use the VM22/210 type ('76 and newer) because of what they flow and their size differences..

 

[JNaw]

No, I don't have any way to measure the air jet size, but from looking over the service data sheets from the various years, they appear the same from '73 up through '75. For '76 -'77, it appears the pilot air jet gets a little bigger. Unfortunately there is no air jet size listed on the '78 and '79 service data sheets but I kinda think it's the same as the '76-'77 carbs. The difference, I would think, is between the carbs that use the BS30/96 pilot jets ('75 and older) and the ones that use the VM22/210 type ('76 and newer) because of what they flow and their size differences..

Right, but aslong as I'm using the correct pilot type that shouldn't be a concern I'd think.

Okay thanks for the information, I shall continue to remove and reinstall the carburetors to the bikes content till I can get this figured out lol.

 

[5twins]

You can change main jets through that bottom drain plug, carbs still in place, using a stubby screwdriver - as long as you didn't install them super tight with a full size screwdriver when the carbs were off, lol. Pilots can be changed by just removing the bowls, carbs still in place. You can remove the carb tops and slide/diaphragm assemblies to get at the needles with the carbs still in place as well.

Since these are metric bikes, the screwdriver slots in the jets are metric as well. This makes it difficult to find a normal American screwdriver that fits them well, so I've taken to custom grinding 1/4" insert bit to fit perfectly. For installing jets, I only use a stubby screwdriver or a knurled thumb wheel bit holder. These allow me to get the jets tight enough but not too tight. For removal of ones that are really tight, I use a T-handled bit holder ......

For the small round mains, I found these Bosch #8-10 slotted power bits work perfectly .....

They need to be narrowed and shortened a little to get a perfect fit .....

If you don't narrow them down so they match the width of the jet, they won't fit in the bottom of the bowl, they hang up on the drain plug threads. Here's one that isn't narrowed on the right (won't go in) and one that is on the left .....

For the pilots, I found a 4mm slotted power bit works well. It almost fits perfectly, you just need to shorten it a little .....

Needless to say, I grind and fit these to brand new jets, not old ones with hogged out screwdriver slots, lol.

 

[JNaw]

You can change main jets through that bottom drain plug, carbs still in place, using a stubby screwdriver - as long as you didn't install them super tight with a full size screwdriver when the carbs were off, lol. Pilots can be changed by just removing the bowls, carbs still in place. You can remove the carb tops and slide/diaphragm assemblies to get at the needles with the carbs still in place as well.

Since these are metric bikes, the screwdriver slots in the jets are metric as well. This makes it difficult to find a normal American screwdriver that fits them well, so I've taken to custom grinding 1/4" insert bit to fit perfectly. For installing jets, I only use a stubby screwdriver or a knurled thumb wheel bit holder. These allow me to get the jets tight enough but not too tight. For removal of ones that are really tight, I use a T-handled bit holder ......

View attachment 330202

View attachment 330203

For the small round mains, I found these Bosch #8-10 slotted power bits work perfectly .....

View attachment 330207

They need to be narrowed and shortened a little to get a perfect fit .....

View attachment 330208

If you don't narrow them down so they match the width of the jet, they won't fit in the bottom of the bowl, they hang up on the drain plug threads. Here's one that isn't narrowed on the right (won't go in) and one that is on the left .....

View attachment 330209

For the pilots, I found a 4mm slotted power bit works well. It almost fits perfectly, you just need to shorten it a little .....

View attachment 330212

View attachment 330213

Needless to say, I grind and fit these to brand new jets, not old ones with hogged out screwdriver slots, lol.

Yup, got a whole assortment of screwdriver bits. Main jets are easy to change like you said. I'm abit weary of removing the bowl on the bike as two of the screws threads were stripped out by the previous owner. Jbweld fix only works once, but found the ol' copper wire trick works a treat.

Maybe someday I'll find some helicoils small enough, when I have the $60cad to drop on a set lol. That or go up one size on the screw and rethread along with grinding down the diameter of the head to fit in the tight spot.

 

[5twins]

For screw lengths, you have to be careful with the one under the choke housing. If it's too long, it will hit the housing before the bowl is tight. Continue to turn it and you strip the threads. Here's a write-up I did on replacing the carb screws with stainless Allens. It contains lengths and sizes for both the BS38's and BS34's .....

https://www.xs650.com/threads/carb-allen-screws.62686/

..... and here's one on the jet bits ......

https://www.xs650.com/threads/jet-bits.59779/

 

[JNaw]

Swapped 27.5 pilots in and 125 main

2-1/2 turns out on both carbs

Also adjusted timing as it was slightly too retarded

Sweet spot is definitely around 127.5-130 for the mains. Still breaking up at 5k and zooms up afterwards, albeit less powerfully with the 125 (as I'd expect for properly drilled mains and current setup)

I'll try lowering the needle one more step which will be max, and if that doesn't solve it then looking at machining the 502 needles to adapt on a z2 needle jet with old style carbs.

That or just go vm34 lol

 

[JNaw]

Turns out midrange was lean not rich.

Slot #4 fixed 99% of the stumbling. When cold it breaks up abit from 5k to redline.

Cold breakup that is cured when hot is lean condition correct?

Either going to try raising needle to 5th slot or try a 132.5

 

[5twins]

Yes, that would be a lean condition. Yes, start stepping the mains back up. The set-up I ended up with on both '76-'77 carb sets I've tried was one up on the pilots (25 stock to 27.5), four up on the mains (122.5 stock to 132.5), and the needles leaned one step to the #2 slot. On most of the 650 carb sets, for the typical "pods and pipes" mod, you usually only need one or two sizes up on the mains. The '76-'77 carb set is the exception because the stock mains were so small to begin with. You usually need 3 to 5 sizes bigger.

 

[JNaw]

Yes, that would be a lean condition. Yes, start stepping the mains back up. The set-up I ended up with on both '76-'77 carb sets I've tried was one up on the pilots (25 stock to 27.5), four up on the mains (122.5 stock to 132.5), and the needles leaned one step to the #2 slot. On most of the 650 carb sets, for the typical "pods and pipes" mod, you usually only need one or two sizes up on the mains. The '76-'77 carb set is the exception because the stock mains were so small to begin with. You usually need 3 to 5 sizes bigger.

See I was tryna find information on guys who tuned the different year carbs to see what ballpark I need to be in. Thanks for that! I'll try the 132.5 tomorrow.

Interesting that my midrange is wanted richer than what you experienced.

 

[JNaw]

So I've exhausted most of my options now.

4m1 with a 132.5 still breaks/cuts out at 5k, clip at 5th mitigates it the most, but brings really rich low end. Leaning out needle worsens with each step

4n8 with z8 jet at 5th clip is rich
4th notch with 130 is lean
4th notch with 132.5 is better but cuts out at both 5 and 6k.

I may try a 135 and see if that does much but that's going to kill my top end (bike seems to like the 130-132.5 the most at/near redline)

I wonder why the transition point is so lean? It pulls fine up to 4, noticable lack of power, sputters and breaks up at 5, then pulls fine afterwards. (1st gear pull will run up to 5, hiccup once or twice then continue pulling)

 

[JNaw]

Thought I'd post an update.

Ended up getting a pair of vm34's from tcbros.

Bike runs miles better. I think there was just something peculiar about the setup and bike that the CV's simply didn't like.

Anyways, working on main jetting this weekend, but as it stands:
27.5 pilots (25 would be ideal but it's all I got)
2 turns out air screw
175 main (will try one up and down to confirm)
Main air jet removed, may try with it in and see how that performs.
Standard xs650 specs otherwise

Originally when I put them on It was late and I swapped slides between the two, long story short, don't do that it will run way too rich lol. (Assuming you've got left and right pairs)

 

[Kevin Werner]

Thought I'd post an update.

Ended up getting a pair of vm34's from tcbros.

Bike runs miles better. I think there was just something peculiar about the setup and bike that the CV's simply didn't like.

Anyways, working on main jetting this weekend, but as it stands:
27.5 pilots (25 would be ideal but it's all I got)
2 turns out air screw
175 main (will try one up and down to confirm)
Main air jet removed, may try with it in and see how that performs.
Standard xs650 specs otherwise

Originally when I put them on It was late and I swapped slides between the two, long story short, don't do that it will run way too rich lol. (Assuming you've got left and right pairs)

I have VMs on a pods and 1.5 headpipe, Commando muffs stock motor. 180 mains and 25 pilots. I had stumbling at part throttle 3500-4000 . Disconcerting in 35-45 mph sweepers. By steps I went down to 15 pilots where it runs really nice. Remember The VMs are working where YOU set the throttle slides and not where the BS series decide where the slides should be. Cheers

 

[JNaw]

I have VMs on a pods and 1.5 headpipe, Commando muffs stock motor. 180 mains and 25 pilots. I had stumbling at part throttle 3500-4000 . Disconcerting in 35-45 mph sweepers. By steps I went down to 15 pilots where it runs really nice. Remember The VMs are working where YOU set the throttle slides and not where the BS series decide where the slides should be. CheersView attachment 330894

Correct, interesting you got 15's to run. I couldn't get the thing the idle properly till I through 17.5 in but was way too lean still.

Very nice bike btw

 

[Kevin Werner]

Correct, interesting you got 15's to run. I couldn't get the thing the idle properly till I through 17.5 in but was way too lean still.

Very nice bike btw

Thanks. The commando muffs are a little restrictive. Air jet 1/2 turn out to richen the idle mix. Carbs were a PO purchase I assume from Mikes XS. Idle is steady at 800, 900, 1000 if I choose when it is warm. I get barking when I roll off the throttle during a shift but the on throttle, steady state throttle and trailing throttle behavior is very good. Once you ditch the air boxes and change the exhaust, each bike becomes different. I attacked this bike with 195 mains early on and have steadily arrived at 180/15. Others say 180 is usually a good number on the main (for an essentially stock motor). YRMV
Cheers, keep posting as you go.

 

[JNaw]

Well was in the middle of a plug chop and ignition advance blew itself up. I replaced with a new unit from xs650direct but not sure if it was build quality or lock nut not tightened enough. Looks like I'm replacing again lol.

Dropped a piece in the cylinder I think, so gotta find a magnet small enough to fit in there before I get it going again.

Thank god I didn't loose the locating pin though as that's hard to get.

Edit: hotglued a magnet to a piece of copper wire and fished around Inside the cylinder. Got nothing so we should be good to go. Plug chop reading wasn't long enough so nothing showed on the plug unfortunately. I think I'm still gonna richen it up as most people are running 180's anyways.

 

[JNaw]

Hey guys, follow up here.

I bought a new advance system off xs650direct again to replace the one that broke...

Well let's just say after a quick rev to 4k it exploded on me again.

I've asked for a refund but doubt I'll get a reply.

Does anyone have one/know where I can source one? It's for a 1979.