My continuing saga! '76 BS38 carbs!

jgimar

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First off, thanks to those that have been helping me. Now, my continuing saga. I have a '77 XS that I've been slapping back together. Stuck a Hugh's PMA on it with a Pamco ignition. It has 1 1/2 inch open pipes(with torque cones) and no air boxes, just pods.
Started out trying to pull off a 2 into 1 set-up but some of you wisely told me to forget it and come back down to earth. So, I have come up with a set of 76 BS38's, needed some cleaning but they were complete..
Got them looking good, followed some of the forum talk and asked some questions and here I am. For jetting, I have installed 145 main jets and dropped the needles all the way. needle and needle jets remained stock. the pilot jet's, they originally had 22.5's non bleed type in them(contrary to 45's as my manual specified). I bumped them up to 30's. okay, ready to go. Crank it up, sitting at dead still idle we got it to run pretty smooth and quiet but the minute ya bump the throttle it would back fire badly.
I'm thinking I haven' even got to the point of addressing main jet problems so I'm still looking at the pilots still being to small. Am I going down the right path? Can anybody guide me on how much larger I should go if this is my problem? Again, any help greatly appreciated.
 
...and dropped the needles all the way.
Depending on how hard you are "bumping the throttle," that could be contributing. Dropping the needles leans it out just off idle... up to just past mid range where the mains will start to have more impact. Try setting the needles back to middle range or slightly higher.
 
the pilot jet's, they originally had 22.5's non bleed type in them(contrary to 45's as my manual specified). I bumped them up to 30's.
If your carbs are 77's a 25 pilot was stock. 30's sounds about right to start with for your setup.

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145 mains are probably too big for that carb set. Mains in the mid 130's are probably the biggest you'll be able to use. This is because of the large Z-8 needle jets. But, be sure you do indeed have '76 carbs. And yes, raise the needle back up, I'd say one step, so it's set in the 2nd slot from the top.

145 mains can work in some of the other carb sets, ones that had larger mains and smaller needle jets to begin with.
 
Also make sure you're using the correct pilots. If your carbs are off a 76-77, they should use the VM22's. Use the orifice location and not the bleed holes to ID 'em.

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I still need to finish my rollers. Rollers so I can go thru gears with AFR gauges hooked up. While running at operating temp. This is the only full proof, exact way to know where you're at. 5Twins and Jim know their shit and those charts are right. Start there. To fine tune....AFR gauges. My way is what I've seen my dad do for drag racing.
 
Depending on how hard you are "bumping the throttle," that could be contributing. Dropping the needles leans it out just off idle... up to just past mid range where the mains will start to have more impact. Try setting the needles back to middle range or slightly higher.
Thank you for your advice. To define “bump”, anywhere from just a tap to a turn of about 1 1/2 inches. Sounds like ‘ll leave the pilot as is and make suggested adjustments to the needle and main.
 
First off, thanks to those that have been helping me. Now, my continuing saga. I have a '77 XS that I've been slapping back together. Stuck a Hugh's PMA on it with a Pamco ignition. It has 1 1/2 inch open pipes(with torque cones) and no air boxes, just pods.
Started out trying to pull off a 2 into 1 set-up but some of you wisely told me to forget it and come back down to earth. So, I have come up with a set of 76 BS38's, needed some cleaning but they were complete..
Got them looking good, followed some of the forum talk and asked some questions and here I am. For jetting, I have installed 145 main jets and dropped the needles all the way. needle and needle jets remained stock. the pilot jet's, they originally had 22.5's non bleed type in them(contrary to 45's as my manual specified). I bumped them up to 30's. okay, ready to go. Crank it up, sitting at dead still idle we got it to run pretty smooth and quiet but the minute ya bump the throttle it would back fire badly.
I'm thinking I haven' even got to the point of addressing main jet problems so I'm still looking at the pilots still being to small. Am I going down the right path? Can anybody guide me on how much larger I should go if this is my problem? Again, any help greatly appreciated.


Don't know if this will show up, but now the continuing-continuing saga. I would like to thank all that provided input to my problems and I thought you'd like to know; SUCCESS! First off, no one said to mess with the pilots so I laid off of them. I reduced the mains from 145 to 140 since I didn't have a set of 135's and hoped for the best. I bumped the needle up one notch as suggested, and voila, back firing disappeared, smoothed out and finally chugged on down the road without a whimper. in fact I think it ran smoother now than before I had to tear it down. In between shifts, it's slightly slow coming down from acceleration but not a severe problem, more of a picky, annoying issue. If anyone has a thought on that , appreciate the input, but otherwise, thank you, thank you, thank, you!
 
Just so you know, main jet sizes change in 2.5 number increments, so you're jumping around here 2 sizes at a time. Since we only change a few sizes from stock, every size can make a difference. It's best you don't skip around like you've done. Eventually, you should try 137.5's. If they're good, or better, then try 135's. One of the first rules of jetting is that when you think you're good, try the size above and below to make sure.

At a #30, you're also 2 sizes up on the pilots. Eventually, I'd try one up, a 27.5. It may be all you need. Your slow to return to idle issue could be related to jet sizes being slightly off but it could also be the carb sync being off. Also, make sure the mix screws are set for best running. The required setting may change (often does) when you change the pilot jet size. With larger than stock pilots, the mix screw setting needed is often slightly less (open) than before. Setting spec for your carb set stock is 1½ ± ½ turns. With larger pilots, you may find somewhere closer to 1 turn works best.
 
Just so you know, main jet sizes change in 2.5 number increments, so you're jumping around here 2 sizes at a time. Since we only change a few sizes from stock, every size can make a difference. It's best you don't skip around like you've done. Eventually, you should try 137.5's. If they're good, or better, then try 135's. One of the first rules of jetting is that when you think you're good, try the size above and below to make sure.

At a #30, you're also 2 sizes up on the pilots. Eventually, I'd try one up, a 27.5. It may be all you need. Your slow to return to idle issue could be related to jet sizes being slightly off but it could also be the carb sync being off. Also, make sure the mix screws are set for best running. The required setting may change (often does) when you change the pilot jet size. With larger than stock pilots, the mix screw setting needed is often slightly less (open) than before. Setting spec for your carb set stock is 1½ ± ½ turns. With larger pilots, you may find somewhere closer to 1 turn works best.
Just so you know, main jet sizes change in 2.5 number increments, so you're jumping around here 2 sizes at a time. Since we only change a few sizes from stock, every size can make a difference. It's best you don't skip around like you've done. Eventually, you should try 137.5's. If they're good, or better, then try 135's. One of the first rules of jetting is that when you think you're good, try the size above and below to make sure.

At a #30, you're also 2 sizes up on the pilots. Eventually, I'd try one up, a 27.5. It may be all you need. Your slow to return to idle issue could be related to jet sizes being slightly off but it could also be the carb sync being off. Also, make sure the mix screws are set for best running. The required setting may change (often does) when you change the pilot jet size. With larger than stock pilots, the mix screw setting needed is often slightly less (open) than before. Setting spec for your carb set stock is 1½ ± ½ turns. With larger pilots, you may find somewhere closer to 1 turn works best.
5twin, thank you for getting back to me. I will definitely make a copy of your response and follow all suggestions in regard to jetting. It’s running good enough now, i can get it home and continue to work on it with your instructions. As far as the screws go, they are both about 1 1/2 give or take a hair. Got ‘em close as I could sound wise . For sync, I used a feeler gauge and feel I’m in pretty good shape on them. May be a bit fora return report. Got cold out so it has me slowed down a bit. Keep ya posted. Thank you again.
 
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