I know I will probaby get a lot of grief for posting this but... What jetting works well with open intake(velocity stacks w/screens) and open exhaust(stock head pipes w/mufflers removed)? I know there must be tons of people running this combo in the bobber community and I'm sure someone out there has the jetting figured out. Its a 79 BTW.
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What jetting for Open exhaust and Stacks?
- Thread starter gixxsta04
- Start date
I'm just guessing it's a XS650
and I'm just guessing you have found a way to mount a Rochester Quadra-Jet
I'm thinkin you're using 1 5/8" primary tubing about 18" long to cut down on back pressure
now with the exhaust open and no air filter with stock pistons ...
'bout 1/8" on the primary and 1/4" on the secondary with AX metering rods .
Now you can do that or give us a little more to work with
Year
compression
camshaft
header length and id
carb throat and type
approximate weight of the thing
gearing
intended use
mean altitude where it will be used
and what color underwear she is wearing !
Damn people get a little bit of a grip , this is science but not rocket science .....
~kop
and I'm just guessing you have found a way to mount a Rochester Quadra-Jet
I'm thinkin you're using 1 5/8" primary tubing about 18" long to cut down on back pressure
now with the exhaust open and no air filter with stock pistons ...
'bout 1/8" on the primary and 1/4" on the secondary with AX metering rods .
Now you can do that or give us a little more to work with
Year
compression
camshaft
header length and id
carb throat and type
approximate weight of the thing
gearing
intended use
mean altitude where it will be used
and what color underwear she is wearing !
Damn people get a little bit of a grip , this is science but not rocket science .....
~kop
Year - 79 as stated in original post
Compression - Stock
Camshaft - Stock
Header Length - Stock
Carb - Stock
Weight - I dunno, its a barebones hardtail bobber
Gearing - Stock
Intended Use - Plan on Riding it to work 25 miles on the highway
altitude - St. Louis, Mo. Probably somewhere around sea level.
Compression - Stock
Camshaft - Stock
Header Length - Stock
Carb - Stock
Weight - I dunno, its a barebones hardtail bobber
Gearing - Stock
Intended Use - Plan on Riding it to work 25 miles on the highway
altitude - St. Louis, Mo. Probably somewhere around sea level.
BigJimmyW13
XS650 Guru
Off topic -
In STL or near STL? If near where abouts?
In STL or near STL? If near where abouts?
BigJimmyW13
XS650 Guru
Ah, gotcha. I grew up in Grafton. I still go up to the area. I have a friend that just picked up a XS in Quincy.
No I'm not a complete azzhat I just impersonate one on the forum .
For the BS38's we have an issue that needs to be addressed . The inlet that operates the slide needs a somewhat straightened and reliable column of air even in an individual air filter . In a pinch ($$-) I have gone down to the local muffler mangler and had him drive adapters (the kind you find at the auto parts store to go from one size to the other ) onto a cone so that you have a low budget velocity stack you can fit to the inlet of the carb . In better days I actually bought velocity stacks and inserted them in the K&N's then tightened the whole mess down at the carb so that the bell mouthed entry was placed about half way into the element or less so the inlet wasn't choked by the back of the filter .
In later oval section K&N's for the 38 you will find a molded bell extending into the filter . Seems extensive lobbying by the community paid off in a better part .
Okay , now to the meat of this long winded yet informative verbal diarrhea...
verify that the carbs are indeed 38's and that the jetting is somewhere near stock #135 main and 27.5 pilot . Remember that these recommendations are for some provision of straightening out airflow to the carb inlet .
I remember having to massage #30 pilots because the 32.5 were marginally too large . Ideally you should see lean missfire within 1 1/4 turns from seat and rich missfire within 3 1/4 from seat . Outside this range the carb meters unpredictably .
Usually the 140 will get the job done on the main with one caution . if the needle jet discharge to the throat is mangled or worn oval (likewise for needle wear) this may be where it goes rich on top . If however the needle and needle jet look to be in good shape the 142.5 can be used but you will find the needle dropped all the way down (clip in top groove) .
Rich mixtures like this can use a bit of ignition advance but again if you encounter used parts you may have to compromise between sufficient advance at idle and nothing over 38-9 degrees at full advance . If using stock coils get as much dwell into the system as possible while still hitting your timing marks . I start arbitrarily with .014" on the left and measure dwell and make it the same on the right then move on to timing . Synchronizing becomes a bit difficult due to reversion at idle and you may have to resort to the time honored method of hose to inlet to ear .
Last comment . The one thing that will make all this possible is a careful and accurate valve adjust a bit on the loose side (only by a thou or so ) to cut down on the amount of overlap .
Hope it helps
~kop
For the BS38's we have an issue that needs to be addressed . The inlet that operates the slide needs a somewhat straightened and reliable column of air even in an individual air filter . In a pinch ($$-) I have gone down to the local muffler mangler and had him drive adapters (the kind you find at the auto parts store to go from one size to the other ) onto a cone so that you have a low budget velocity stack you can fit to the inlet of the carb . In better days I actually bought velocity stacks and inserted them in the K&N's then tightened the whole mess down at the carb so that the bell mouthed entry was placed about half way into the element or less so the inlet wasn't choked by the back of the filter .
In later oval section K&N's for the 38 you will find a molded bell extending into the filter . Seems extensive lobbying by the community paid off in a better part .
Okay , now to the meat of this long winded yet informative verbal diarrhea...
verify that the carbs are indeed 38's and that the jetting is somewhere near stock #135 main and 27.5 pilot . Remember that these recommendations are for some provision of straightening out airflow to the carb inlet .
I remember having to massage #30 pilots because the 32.5 were marginally too large . Ideally you should see lean missfire within 1 1/4 turns from seat and rich missfire within 3 1/4 from seat . Outside this range the carb meters unpredictably .
Usually the 140 will get the job done on the main with one caution . if the needle jet discharge to the throat is mangled or worn oval (likewise for needle wear) this may be where it goes rich on top . If however the needle and needle jet look to be in good shape the 142.5 can be used but you will find the needle dropped all the way down (clip in top groove) .
Rich mixtures like this can use a bit of ignition advance but again if you encounter used parts you may have to compromise between sufficient advance at idle and nothing over 38-9 degrees at full advance . If using stock coils get as much dwell into the system as possible while still hitting your timing marks . I start arbitrarily with .014" on the left and measure dwell and make it the same on the right then move on to timing . Synchronizing becomes a bit difficult due to reversion at idle and you may have to resort to the time honored method of hose to inlet to ear .
Last comment . The one thing that will make all this possible is a careful and accurate valve adjust a bit on the loose side (only by a thou or so ) to cut down on the amount of overlap .
Hope it helps
~kop
littlebill31
Smells of Raw Fuel
I run one up on the pilot, one to three up on the main, and the jet needle clip in position #2. It's a good place to start, but all bikes are different and the bike must be adjusted correctly first to make sure you are not chasing your tail or compinsating for something else. Cam chain tensioner, valve lash, timing, advance, before the carbs. And in that order.
The air jet in your '79 carbs is fixed, can't be changed and therefore, doesn't enter into the tuning equation. If you go up 2 sizes to the 140 mains then, yes, you probably will need to lower (lean) your needles a step.
littlebill31
Smells of Raw Fuel
If you have the jets, then it is a good place to start. Again, no 2 carbs are alike so there will be testing and adjustment. Make sure the carbs are in sync, mix screws set correctly using the "dead cylinder" method. By air screw do you mean the mix screw? If so it is on the left side of the carb, above the throttle. There are 2 screws above it. It's the one on top oif the throat.
A good place to look at this stuff, and a MUST read, is the carb guide. www.amckayltd.com/carbguide.pdf and also in the "tech" section, carburettors, the first thread.
A good place to look at this stuff, and a MUST read, is the carb guide. www.amckayltd.com/carbguide.pdf and also in the "tech" section, carburettors, the first thread.
manofsteele
XS650 Addict
I have a 79 also and I'm running the same set up, I went 1 up on the pilots and 2 up on the mains and moved the needle to the second position from the bottom. Runs great but I wanna replace the exhaust, really loud and kinda crappy sounding.
manofsteele
XS650 Addict
If you haven't yet, I highly recommend upgrading the ignition to a Pamco. I also put some iridium plugs in it.
Just bought a pamco and pma conversion as well. This is going to be my daily driver for the summer and I want it to be super reliable. Bike only has 13000 miles on it and the rest of the engine seems to be very solid.
Last edited:
77ChopOn
XS650 Member
Hi there
I have a 71’ Xs650 motor with 81’ carbs running with 137.5 main and 42.5 pilots.
Bike is running open exhaust maybe 5’ in length. See pic
Should I increase the pilot jets? If so, What size
Should I change the needles? If so, What size
Are the Main jets good at 137.5?
...heating up a #2 phillips
