22's 1980 xs650 build thread

I may be way off here but when my pilots were too small I was getting an awefull lot of black on my plugs. My theory is that I was setting the butterflies (idle speed) open enough to draw fuel (too much) through the needles so my low speed running into my neighborhood with a little throttle was blacking my plugs. Since I upped my pilots the issue has all but disappeared. I said, I may be way off here.
 
Okay so I went back to slot 3 on the needle and upped the main to 137.5 and the stumble remains around 4k+ at WOT, it really feels the same as it did with the 135, maybe I should try going back to the stock 132.5? Maybe what I'm feeling isn't a flat spot....
 
Okay so I went back to slot 3 on the needle and upped the main to 137.5 and the stumble remains around 4k+ at WOT, it really feels the same as it did with the 135, maybe I should try going back to the stock 132.5? Maybe what I'm feeling isn't a flat spot....
I hope someone corrects me. At 4000 your slides are half open or less(?). Your needle is still a restriction in your needle/mainjet flow(?). Are your plugs white or tan/brown. When my mains were extra rich I do not remember a stumble in roll on at 4000.
 
Just to throw this in for thought. IIRC The non-US BS34's had factory main jets of #130, along with a 336-Y-0 NJ and a 51X11 needle. If true, that would mean you're currently "up Three" on the main. If the needle jet has been replaced at some previous point, that's another unknown in the mix.
 
I hope someone corrects me. At 4000 your slides are half open or less(?). Your needle is still a restriction in your needle/mainjet flow(?). Are your plugs white or tan/brown. When my mains were extra rich I do not remember a stumble in roll on at 4000.
My plugs were quite lean looking if you ask me, mind you they were new plugs on a short test where I was WOT as much as possible.

Just to throw this in for thought. IIRC The non-US BS34's had factory main jets of #130, along with a 336-Y-0 NJ and a 51X11 needle. If true, that would mean you're currently "up Three" on the main. If the needle jet has been replaced at some previous point, that's another unknown in the mix.
I can't say for sure what the stock main jets were, when I got it, it had 132.5 mains, stock everything aside from more free flowing mufflers, headers were stock, I didn't really ride it in that state. I added the commando mufflers and 1.5" headers and upped the mains to 135 then 137.5.

So today it was good news/bad news.... good news I changed the main jets with the carb on the bike for the first time, a much more enjoyable experience, bad news it still stumbles with the 132.5 but I think it was better. I'll have to do a longer test before tinkering any further but I guess my next step would be to lean the needle a step?

The picture of the plugs is from after the test with the 137.5, new plugs at WOT for as much as possible.

20220810_140857.jpg
 
WOT and high revs are 2 different things with vacuum slide carbs. Is that WOT at 4000-5500 rpm or is 5500-7000 rpm? Did you consider lifting or dropping the needle a notch? You can do that with the carbs still on the bike too.
 
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WOT and high revs are 2 different things with vacuum slide carbs. Is that WOT at 4000-5500 rpm or is 5500-7000 rpm? Did you consider lifting or dropping the needle a notch? You can do that with the carbs still on the bike too.
It happens at WOT and when the revs get to about 4,000 or just over 4,000. I richened the needle a step to step #4 when I had the 135 jet in, it didn't make much of a difference so I moved it back to slot #3 and increased the main to 137.5, that didn't fix it so now I'm at slot #3 and a 132.5 main jet and it seems maybe a bit better so I think my next step is to keep going leaner and try moving the needle to slot #2.
This does seem the opposite of what I should be doing with a more free flowing exhaust....
 
It happens at WOT and when the revs get to about 4,000 or just over 4,000. I richened the needle a step to step #4 when I had the 135 jet in, it didn't make much of a difference so I moved it back to slot #3 and increased the main to 137.5, that didn't fix it so now I'm at slot #3 and a 132.5 main jet and it seems maybe a bit better so I think my next step is to keep going leaner and try moving the needle to slot #2.
This does seem the opposite of what I should be doing with a more free flowing exhaust....
Going back to @5twins or @grizld comments earlier, go up a step at a time until you get some break up , then lean the needle 1 notch.
 
Are you second guessing yourself? While the throttle is WOT, 4000 rpm is midrange. The slide is probably nowhere near wide open and the needle is doing alot of the fuel flow regulation/metering. For and experiment, ride your bike for several miles never exceeding 3000 or 3300 rpm. Then pull the plugs. Are they white?
 
Are you second guessing yourself? While the throttle is WOT, 4000 rpm is midrange. The slide is probably nowhere near wide open and the needle is doing alot of the fuel flow regulation/metering. For and experiment, ride your bike for several miles never exceeding 3000 or 3300 rpm. Then pull the plugs. Are they white?
Oh sorry, so maybe that's where my thinking is wrong, I assumed I was fully on the main jet at WOT and 4000rpm.... but you're saying I'm actually still on the needle. When can I assume I'm fully on the main?

I'll do that test but now I feel like I need to come up with a starting point again.... maybe 135 main jet and slot 4 on the needle?
 
By definition all the way twisted. In practice, perhaps 7/8's.

I'm curious what needle jet and needle are in there - given the difficulty. Do you remember? (sorry didn't read all way through )
 
The diaphragm controls the slide. More seasoned contributors could tell you when the slide is completely open, I do not know. But remember that the pilot, needle and main overlap eachother with regards to metering fuel. My under 3000 to 3500 riding was suggesting look at operation when you likely have very little mainjet metering and mostly pilot and needle. Roll on gently and enjoy the essence of the motor, is it smooth, does it pop or spit on decel? Then look at the plugs.
 
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The diaphragm controls the slide. More seasoned contributors could tell you when the slide is completely open, I do not know. But remember that the pilot, needle and main overlap eachother with regards to metering fuel. My under 3000 to 3500 riding was suggesting look at operation when you likely have very little mainjet metering and mostly pilot and needle. Roll on gently and enjoy the essence of the motor, is it smooth, does it pop or spit on decel? Then look at the plugs.
This was to verify pilot and needle are working well...
 
By definition all the way twisted. In practice, perhaps 7/8's.

I'm curious what needle jet and needle are in there - given the difficulty. Do you remember? (sorry didn't read all way through )
So if I'm WOT am I on the main jet and not the needle irregardless of where the RPMs are? This is where I'm getting confused cause it only stumbles @ WOT combined with 4000 rpm, would this signify a main jet issue a needle jet issue, or could it still be either?
The diaphragm controls the slide. More seasoned contributors could tell you when the slide is completely open, I do not know. But remember that the pilot, needle and main overlap eachother with regards to metering fuel. My under 3000 to 3500 riding was suggesting look at operation when you likely have very little mainjet metering and mostly pilot and needle. Roll on gently and enjoy the essence of the motor, is it smooth, does it pop or spit on decel? Then look at the plugs.
I can do this test for sure but it idles great at 1100rpm and rolls through the throttle fine.
 
Until actual experts show up, I'll try not to muddle this too much. The mechanical throttle plates open relative to the throttle grip, the slide and needle respond to the pressure difference created through the venturi. There is some lag and some difference between the two, if all is right (no leaks) and given time, the slide should follow approximately. I've watched 'em in the shop, but not running WFO. You can blow air across the oval port in the carb mouth and watch the slide move.

Then you have the overlap in carb circuits that dictate primary fuel mix, but they're all affecting mix to a degree.

Given some load and enough time, fully rotated on the throttle plate should equal full up on the slide and redline on rpm. Shouldn't be 4000rpm @ WOT for very long. If I understood the question???
 
So if I'm WOT am I on the main jet and not the needle irregardless of where the RPMs are? This is where I'm getting confused cause it only stumbles @ WOT combined with 4000 rpm, would this signify a main jet issue a needle jet issue, or could it still be either?

Either - it depends on a variety of factors
 
Until actual experts show up, I'll try not to muddle this too much. The mechanical throttle plates open relative to the throttle grip, the slide and needle respond to the pressure difference created through the venturi. There is some lag and some difference between the two, if all is right (no leaks) and given time, the slide should follow approximately. I've watched 'em in the shop, but not running WFO. You can blow air across the oval port in the carb mouth and watch the slide move.

Then you have the overlap in carb circuits that dictate primary fuel mix, but they're all affecting mix to a degree.

Given some load and enough time, fully rotated on the throttle plate should equal full up on the slide and redline on rpm. Shouldn't be 4000rpm @ WOT for very long. If I understood the question???
I think ill keep trying different combinations of main jet and needle and see what happens, I have the 51X11 needle and I assume the correct needle jet as it probably came stock, it's a Canadian bike.
My understanding is that with a free flowing exhaust I should be at least a 135 on the main jet, I'm gonna try 137.5 again with slot 4 on the needle to make it richer and go from there.
 
The only fly in that ointment is if a previous owner ordered a jet kit and put a US needle jet in there. That'll cause issues.:shrug:
 
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