another carb jetting thread

[sid693]

ya i'm gonna be that guy that starts another carb jetting thread so tie me up and throw me to the wolves. so here it is...
i changed my exhaust out and every thing went to sh!t. when i am running mid to full throttle i have a real bad hesitation with spurts and back fire. so i went to the carb guide and starting researching to get all my ducks in a row. here is what i have: 78 650, mikes xs pods, ascot type 2 pipes, bs 38, float bowl within specs, no visible rips or tears in diaphragm, vm 22/210 (not sure whats the difference between the 22/210 or if that is the nomenclature) 135 main which i have a 140 and 145 on hand, 27.5 pilot, and around 2.25 on the mix screw, and i am on the 4th out of 5 position on the needle. after running the bike my plugs seem to give me a rich indication (super black). i know i have to go up on the pilot and throw in the 140 mains for starters. from what i have found the 4th position is around where i want to be. my question for the xs650 gods is which leans and richens on both the needle and mix screw. my second question is how many turns out or in on the mix screw til i go up or down a size on the pilot. thanks in advance for the shared knowledge everyone.

kudos to whoever (5twins and a few others) made that carb guide. i have been reading and rereading that thing and i wouldn't say i know it by the back of my hand but a few more times and i will be there.

 

[angus67]

In on the mix screw is lean, 5 is rich ,1 is lean on the needles from the top

 

[sid693]

Thanks for the quick reply Angus

 

[5twins]

Knowing a little of the 650 carb history will help you understand the changes needed for your particular carb set and why. Your '78-'79 carb set has pretty big mains (135), the largest ever put in a 650 carb set. The previous set ('76-'77) had very small mains by comparison (122.5). The 3 circuits in your carb overlap a bit and effect the one(s) next to them. Changing one often makes a change in the one next to it necessary. In order for Yamaha to use the much larger 135 mains, they had to lean the midrange. Even so, the midrange settings in your carb set are still right on the verge of being too rich. What this means when jetting for mods is that any main jet increase will make the upper midrange too rich and require leaning the needle. On some of the earlier sets, you might get away with increasing the main a size or two before having to lean the needle, but not on your '78-'79 set. Here's the stock jetting chart from the Carb Guide. Note how larger mains pair with smaller needle jets .....

The usual routine for re-jetting these is to start at the top with the mains and work your way down. You do this because the larger mains often require needle setting changes, and those often make a larger pilot necessary. Do the pilots and midrange first and you will most likely need to change them again after doing the mains. So, I would recommend you pop in the 140 mains, lean your needles a step from stock (to slot #2 from the top), and get some #30 pilots. The leaner needle setting will make the larger pilot necessary due to that circuit overlap I mentioned.

The VM22/210 is the type or style of pilot jet used in your carb set. Some of the other sets use the BS30/96 type. Physically, these two types look the same from the outside and will actually swap, but they flow differently. You need to use the type your float bowl was designed for, and in your case that is the VM22/210.

 

[grizld1]

FYI, sid, 5twins and I wrote the Carb Guide; there were no "others." With all due respect, every one of your questions was answered there.

 

[sid693]

FYI, sid, 5twins and I wrote the Carb Guide; there were no "others." With all due respect, every one of your questions was answered there.

As I said it was another post. I did not see my questions answered or I would not have wasted anyone's time. I have been a member here for a bit and don't just post for entertainment. As I said I reviewed the carb guide multiple times and even performed other research and could not find the answers to my questions. Aside from that thanks for writing the carb guide. It probably saves lots of people from asking redundant questions.

 

[sid693]

Knowing a little of the 650 carb history will help you understand the changes needed for your particular carb set and why. Your '78-'79 carb set has pretty big mains (135), the largest ever put in a 650 carb set. The previous set ('76-'77) had very small mains by comparison (122.5). The 3 circuits in your carb overlap a bit and effect the one(s) next to them. Changing one often makes a change in the one next to it necessary. In order for Yamaha to use the much larger 135 mains, they had to lean the midrange. Even so, the midrange settings in your carb set are still right on the verge of being too rich. What this means when jetting for mods is that any main jet increase will make the upper midrange too rich and require leaning the needle. On some of the earlier sets, you might get away with increasing the main a size or two before having to lean the needle, but not on your '78-'79 set. Here's the stock jetting chart from the Carb Guide. Note how larger mains pair with smaller needle jets .....

The usual routine for re-jetting these is to start at the top with the mains and work your way down. You do this because the larger mains often require needle setting changes, and those often make a larger pilot necessary. Do the pilots and midrange first and you will most likely need to change them again after doing the mains. So, I would recommend you pop in the 140 mains, lean your needles a step from stock (to slot #2 from the top), and get some #30 pilots. The leaner needle setting will make the larger pilot necessary due to that circuit overlap I mentioned.

The VM22/210 is the type or style of pilot jet used in your carb set. Some of the other sets use the BS30/96 type. Physically, these two types look the same from the outside and will actually swap, but they flow differently. You need to use the type your float bowl was designed for, and in your case that is the VM22/210.

Sounds good 5twins. My pilot jets do not have ports on the side. Not sure if that matters or not

 

[5twins]

The air bleed holes on the sides of the pilot jets don't matter. Originals don't have them, any replacements you get probably will. Either type works fine. The way the pilots are mounted and operate in the '78-'79 float bowls, air feed holes on their sides weren't required. I'm thinking Yamaha contracted to have "no hole" versions made special and got them cheaper because less machining was involved in making them. If you do want larger "no hole" #30's, you can scrounge them from late model XS500 BS38s.

 

[sid693]

The air bleed holes on the sides of the pilot jets don't matter. Originals don't have them, any replacements you get probably will. Either type works fine. The way the pilots are mounted and operate in the '78-'79 float bowls, air feed holes on their sides weren't required. I'm thinking Yamaha contracted to have "no hole" versions made special and got them cheaper because less machining was involved in making them. If you do want larger "no hole" #30's, you can scrounge them from late model XS500 BS38s.

I ordered the 2 hole type. I can see that penny pinching happening with yamahas mikuni contract. I am going to get everything set up minus the pilot jets set up and reinstall my carbs I already replaced the Phillip head fasteners for button head screws for easy dropping of the bowl on the bike. I usually do that with all my bikes due to the Philips screws constantly stripping out. Thanks again for all the shared information. I really appreciate the history of the 650 mikuni mains and pilots that was actually interesting to look at after you mentioned it.

 

[peanut]

excellent comprehensive reply from 5twins .
It should be made a sticky for owners that fit pod or foam type filters and open type exhausts.
it saves folk wading through the excellent carb guide trying to find the bits that are pertinant to their problem.