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Needle position question 78 XS400

Discussion in 'The Garage' started by sandmanred, Jul 22, 2020.

  1. sandmanred

    sandmanred XS400 Addict

    I'm re-jetting due to and exhaust change, still running stock air box and filters. I've gone up 2 sizes on both pilot and main and so far dropped the needle one groove. It runs good across all throttle positions and through all rpms except in the 5000-7000 rpm range at WOT where it falters a bit until it hits about 7000 rpm and then it's strong all the way through red line. Most recently I increased the baffling in my exhaust and this falter got worse as I increased the restriction so I believe the falter is due to being rich. Which to me means I need to go one more groove leaner on the needle. Just wondering if this conclusion makes sense to those who have more experience re-jetting and if it's normal to have to go leaner on needle when you go with bigger pilots and mains?
  2. amc49

    amc49 XS400 Enthusiast

    5-7000 at WIDE OPEN is main ONLY and not needle at all, it will be pulled out fully at that unless your slides have lagged opening fully due to the extra rich down low. They then put you rich for a bit until they open further.

    You have to be careful on CV carbs adding jet, CVs like the mixture slightly lean to open fully. Richen them up and the slides proceed to lower and open fully slower. You should realize you really have no control over CV slides and leave the needle position alone. You may have dropped needle over overly rich coming off idle and what that too big (to me) pilot move did. On CVs the preceeding running condition if used for a bit can affect how the slides dump open and engine responds later as the engine loads up with fuel and has to purge it.

    Confusing, but what I am getting at is that with CV carbs you think you know where the slide will be but you really don't, and that can really mess with your tuning.
  3. xschris

    xschris a lifestyle not a trend Top Contributor

    Stock is third clip. Raise the needle to make it richer. 5-7K is your needle jet and emulation tube. All the circuits do overlap a bit and have an effect at all rpm ranges but adjusting the needle will affect your dead spot most. Read your plugs. Those will tell you the most. Look up plug chops also.
  4. amc49

    amc49 XS400 Enthusiast

    I'd be lifting that needle back up too, leaning it was likely wrong. You don't richen everywhere else and then put a fuel delivery hole right in the middle of the rest.
  5. sandmanred

    sandmanred XS400 Addict

    Are you suggesting I raise the needle or just stating the direction the change provides?
  6. xschris

    xschris a lifestyle not a trend Top Contributor

    If dropping didn't help, yes I would raise it. Over time the needles do wear and will cause them to run richer. On a stock bike lowering the needle would help with that. But if your bike is no longer stock you may need more fuel.
  7. sandmanred

    sandmanred XS400 Addict

    So I dropped the needle to the lowest point. It largely takes care of the falter from 5000-7000 rpm at WOT and didn't cause any other new issues. Plugs are a nice brown/tan color. Idles OK. In summary I've got, stock air boxes and filters, +2 on sizes on pilots and mains, 4 turns out on fuel screw and top notch on needle and custom exhaust with baffles. Additionally, my mounting boots for the carbs appear to be in good shape and I have new vacuum caps on the vacuum ports.

    I'm still a little concerned something else is amiss. Being 4 turns out on the fuel screw says could go another size up on the pilots. After all the changes above I tried going to 3 turns on the fuel screw but then the idle hangs unless I force it down by engaging the clutch so I think the 4 turns for my current set up is optimal. But having the needle as lean as it can go mean I'll likely have trouble if I go up another size pilots.

    Any thoughts? Anything else I should be checking or just be happy it's running good?
  8. xschris

    xschris a lifestyle not a trend Top Contributor

    So why are you going leaner on the needles? I would be raising them to make it richer. That will also account for you having to use such large jets.
  9. amc49

    amc49 XS400 Enthusiast

    When you 'have to' drop a needle that much something is seriously wrong somewhere else that has been overlooked. Once the engine carbon coated to settle in to the bigger mains the needle change may be screaming the main jets are too much and rich enough to mess up the area just underneath. The slides quit lifting and engine went rich. I've found that on CV type carbs no way does the full effect of jetting show up instantly, the finer effects can take up to two weeks of driving to show up fully and even more so with upper rpm issues. The way the slides bed in over time to new changes will have you swearing the last jetting change was great that DAY and later it settles in and things start going wrong.

    If it was me I'd be starting all over to go back to OEM setup and then make slow measured changes. If the needle is left very low like that at some point the engine will likely melt if there is any long term cruising going on in bike behavior pattern.

    OP is way too rich on both sides, did not recognize it and then made up for it by dropping needles when slides quit responding, possibly complicated by exhaust modding that went the other way from desired.

    In short, lost.
  10. amc49

    amc49 XS400 Enthusiast

    '...+2 on sizes on pilots and mains, 4 turns out on fuel screw..'

    '...Being 4 turns out on the fuel screw says could go another size up on the pilots.'

    So now you want to go up a 3RD pilot size??? (shaking head sadly)

    Look here..............

    '...custom exhaust with baffles.'

    A likely starting point that went awry, throwing everything off.

    When one totally ignores the stock OEM fuel curve to change EVERYTHING instead of making a minute small change here or there, then that person is lost.
    CVs by their nature self adjust jetting and not recognizing that will get you into a world of hurt. When you kill slide lift you introduce fake issues in there to make you go nuts thinking you are 'tuning it'. ALL CV slides lift better and faster slightly lean, it is built into the physics of the design to do so.
  11. sandmanred

    sandmanred XS400 Addict

    Here are the plugs with the current settings, left and right cylinders. Left has always seemed to run a bit hotter than the right.

    FWIW it starts good hot or cold, runs good from idle to red line across the entire throttle range.
  12. xschris

    xschris a lifestyle not a trend Top Contributor

    They still look on the rich side. If the bike runs fine just leave it. I think because of the very rich pilot and main settings of the carbs you had to go very lean on the needles to even it out.
  13. sandmanred

    sandmanred XS400 Addict

    I'm okay with a little rich, I think it's better than a little lean? I also have been close to your recommended 285 degrees F measured near the plug after a ride, the left side 5-10 degrees warmer than the right. Thanks for taking the time to comment.
  14. amc49

    amc49 XS400 Enthusiast

    Proper plug coloring does not happen instantly, it can take a week to color them and often perfect is almost bone white except for way up inside the steel where the porcelain goes into the steel. If you have colored that fast they may be too rich as said. The one on the right where the wire electrode is pointing at the shell end and black mark there says rich.

    Your problem is that you are mostly past pilot jets at cruise and on needle restriction close to only. Full throttle plug checks won't show that at all and how you can melt the motor with a lot of low load cruising.

    I'd be looking at the exhaust mods you did, likely they are less than optimum. I've seen them do close to what you have here before. Messed up engine scavenging problem, it's not timed right. Late negative pressure pulse and it then sucks out too much fuel at overlap to be lean even with more jetting. One can get that to running 'well' but still throwing away power. How does one determine which engine is running faster, a 33 hp. one or a 36 hp. one with both running well? You can't, a dyno is needed there. In fact, if the 33 hp. one runs slightly cleaner due to more accurate carb tuning then most people would pick it as faster.

    You most certainly have something wrong around idle, going up two pilot sizes, I for one have never heard of it as needed. Tuners recommend them all the time but I've never done it hardly ever in countless hop up setups, the engine simply needs no major changes that low unless it has been radically hopped up and then maybe only one. Idle being the slowest speed is where you have enough physical time to mess up the overlap like I pointed out.

    You may want to yank carbs and check where your normal idle location has the butterfly edge sitting at, it should be just covering the first fuel transfer hole, or just beginning to expose it. If the butterfly has it fully exposed the carb is too far open at normal idle and then the transfer hole is already 'sucked dry' of fuel to be not able to give its' shot when it would normally be getting exposed with more throttle tip-in. That timing can be critical to the idle fuel needs, ergo, jetting. Don't confuse that hole with the normal curb idle hole the mixture needle feeds which is further forward and the butterfly cannot cover it at 100% closed.
    Last edited: Jul 26, 2020
  15. sandmanred

    sandmanred XS400 Addict

    The plugs were darker brown before I dropped the needle.

    The headers are stock. The cones are hollow with a screw in baffle that can be removed for modifying as needed.

    The baffles right now have no slots like you see in the photo, the only escape for exhaust is the gap between the muffler body and the baffle provided by the U nuts.

    How would I look at exhaust changes?

  16. xschris

    xschris a lifestyle not a trend Top Contributor

    I would have used a MAC style baffle with one pipe seated into one end of the cone and the other bolted at the end. Still not as good as the stock setup but something.
  17. amc49

    amc49 XS400 Enthusiast

    '...the only escape for exhaust is the gap between the muffler body and the baffle provided by the U nuts.'

    Clarify that...................does the small baffle held by hand in the last pic have a plug welded inside to not flow through the center tube? And does the bigger muffler body have plates inside it to support the front of the baffle tube?The slots cut in the pic seem to hint at it. I see welding cleanup like a plug is in the middle too.

    If your only gas escape is that small clearance no way will it run right, the restriction long term is begging for burned valves. If a 400 at 200 cc. each side you need a minimum of 1 inch hole area to flow right at max rpm. I'd prefer 1 1/4". And that's in a straight direction, the holesize must go up if there is a severe turn at the exit.

    S turns are fine for noise control but when I go for power I always use a completely straight exhaust path, that MAC baffle is why. Throwing away power for nothing.
  18. sandmanred

    sandmanred XS400 Addict

    Clarification. Baffle is capped and sealed except for the gap provided by the U clips.
  19. xschris

    xschris a lifestyle not a trend Top Contributor

    That is not very good for flow. I would try another setup.
  20. amc49

    amc49 XS400 Enthusiast

    Agreed. The baffle is no longer a baffle. it is a plug.

    You can drill the center 'cap' using succeeding bigger drill bits until you find a happy medium of noise vs. performance, and I also sized the bits to ones need for pipe taps so that if I went too big I could tap the hole (it WILL tap even though only maybe 1-1 1/2 threads) and use pipe plugs to plug the holes if too loud. On a single bigger muffler assembly for a CB550F, I used 4 holes at 3/8" plug each. You could change the sound in less than a minute doing it. That muffler used internal design very similar to the MAC baffle picced above. TOO quiet and sounded like a typical muffled header when I was done but not loud enough to call the cops.

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