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Shorty Exhaust Re-Jet Question

Discussion in 'The Garage' started by Peter Ronney, Apr 1, 2021.

  1. Peter Ronney

    Peter Ronney XS400 Enthusiast

    Hi everyone! I just got a pair of eBay shorty mufflers (emco knockoffs) on my 1978 XS400 and I’m having issues getting the mixture in tune. I found a thread here about which way to turn the mixture screws and it is helping so I think I have that right. When I had stock mufflers I had the mixture just about dialed in but now with the screws turned out an entire 5 full turns the mixture still seems lean according to the plugs. I pull them just about every ride to see if I’ve got the mix right. It isnt running too hot but I haven’t ridden it that far, only a handful of miles between adjustments. My question is if some of you with shorty mufflers have rejetted, what size do you use and where could I find them? eBay just has generic parts and I don’t know if they’ll fit my carbs. I’ll probably buy a few different sizes but I’d like a starting point.
     

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  2. spectra

    spectra XS400 Junkie Top Contributor

    Jets R us is the only place to get them from.........genuine mikuni......... not sure what you have in there now but maybe go up one size........ a little rich is better than lean.........
     
  3. amc49

    amc49 XS400 Junkie

    OP get this and cast in concrete.

    You do NOT use mixture screws for anything other than IDLE ALONE, you are completely bullsh-tting yourself if thinking the plug coloring comes from that. Unless you are driving around at an 1100 rpm limit and pretty sure you aren't doing that.

    The mixture screws cease having any effect as soon as you open the throttle beyond the slightest bit.

    There is also absolutely no way to tell which jet or how far to go as every engine combination is different and meaning they all jet differently. You could take 5 different shorty mufflers that look the same and every one flow differently to require different jetting based on how they are baffled inside, some will even make you jet lower down instead of up higher.

    The normal expectation is one or two sizes bigger on the main but that relies on the correct ones already being there, often they are not as so many screw with carbs without a clue, it is a male rite of passage. Maybe a slide needle change needed too. The plugs need to be the correct ones to be accurate.
     
  4. sandmanred

    sandmanred XS400 Addict

    If it was running good stock I'd find out what jets were in there and start by going up 1 size. There's some pretty good writeups where you can buy the jets. https://www.jetsrus.com/FAQs/FAQ_rejetting_101_how_to_rejet.htm http://www.iwt.com.au/mikunicarb.htm It helps a lot to mark your throttle at at least 4 positions, idle 1/4, 1/2 and full open. You try to run the full range of rpm at each throttle position and take lots of notes on the response for every set up change you make.

    I've used both generic and brand name jets and gotten to a decent set up eventually, I just wouldn't mix them as reportedly the calibration on the no name brands is not the same as name brand. I'd lean towards name brand as they are not that much more expensive and you'll be putting a lot of time iterating to get it right and mixing in no name jets could make it more confusing.

    Jetting is pain for me because I have go on at least a 30 minute ride to get to where I can make all that noise. You'll be tempted to try to short cut and make a bunch of changes at once but that can make it hard to sort out what really affected your performance. In theory each jet has a role in a certain throttle range but in reality there's quite a bit of overlap
     
  5. amc49

    amc49 XS400 Junkie

    X2, a lot of sense in that post.

    'In theory each jet has a role in a certain throttle range but in reality there's quite a bit of overlap'

    Yes..........and no.

    While true in and of itself, it helps to think of each jets' ultimate endpoint fuel dropping off location and how the vacuum changes at each one. Take the main jet and needle jet/jet needle, they use the same location and the orifice can be pretty big at full open, so a LOT of overlap there. But the idle feed gets 100% vacuum at total closed throttle and it drops off whoppingly at even a slight opening of the throttle and why there IS overlap there into the transition but only a trace amount and virtually nil as far as comparing idle fuel to the main jet in total fuel amounts. Why the mixture needle has virtually no effect past barely opening the throttle. Of course it depends on the mixture circuit design type as well; whether it is air/fuel or pure air, the two types have different amounts of overlap.

    While it is dangerous to think of each jet position as being 100% standalone in effect, it can be just as dangerous to think of them all having the same 'overlap' effect, as they often do not depending on how the fuel circuits are designed.

    Ever heard of a 'fuel' jet exit that at a certain point becomes an air jet when the flow reverses to go backwards through it? It has been used before to get certain effects, Honda uses them to lean out off-idle to pass emission specs. That idea completely destroys the idea of overlap as understood by most people.
     
    Last edited: Apr 2, 2021
  6. Peter Ronney

    Peter Ronney XS400 Enthusiast

    Thanks for all the suggestions. I’m not sure what were the stock sizes but I’ll be pulling the carbs to do some work anyways so I’ll check then. Consensus says go up a size pilot and main with a name brand jet and go from there. The article on jetsrus says to start with the pilot jet then move to the main. This bike starts ok now that it’s spring but had trouble in cold weather, I assume a bigger pilot is in order. The real culprit I think is the main jet as running even short distances the engine would get very very hot, turning the pilot screws helped but the plugs were still white-ish indicating lean. Performance is just fine, it’s just running lean. So my hypothesis is one size larger pilot and main will be adequate but it’s up to the testing. Does this sound about right?

    A peer suggested that my recent repacking of the air box filters with free flow foam may also be a culprit, something I didn’t take into account for. Other than that the intake is stock but I don’t know how this would effect the situation, thought I’d mention it.
     
    Last edited: Apr 3, 2021
  7. sandmanred

    sandmanred XS400 Addict

    The way I understand it anything that reduces pressure drop from beginning to end makes it run leaner. So hard to say what your filter change did however many here feel that it's best to leave the intakes stock, apparently these carbs like the even flow that comes out of the stock set up. And for what it's worth I went up two sizes in pilot and main and then had to come way down on the needle to compensate so I'd try one size up like you said.
     
  8. amc49

    amc49 XS400 Junkie

    You may need to oil the foam if it is open cell and big enough pores in it or it will pass dirt like nothing in the filter box at all. The initial oiling up often screws up jetting as the filter acts like clogged until the engine pulls off the extra oil and the plugs will read funny too like too rich.

    I don't know why everybody on earth thinks you have to change pilots, I haven't on even wild race engines as the situation there is that you commonly never need to do it. Maybe it's more of the male testosterone thing of working on carbs, that I have to 'do something'. Maybe more likely pushing to sell more parts. Regardless, if you can adjust mixture screws to be too rich or lean at idle you ALREADY HAVE the correct pilot and I don't care who says otherwise. When you adjust mixture screws you are actually changing the jet there anyway but lunkheads don't grasp that. The ONLY reason to ever change a pilot is for barely off-idle transition hesitation and the mixture screws commonly will fix that too if you are good. I think of all the engines I've ever jetted and only one comes to mind as to changing a pilot and I went DOWN on that one or backwards, the needle got changed by the factory and the transition to needle was too rich. I bought a brand new RM Suzuki for pennies based on the previous owner not picking up on that, he kept going UP and the bike ran worse and worse. I bought it and fixed to run perfectly instead of a solid bang out that ran like crap. Made $1000 on re-selling the bike and not the first time I did it either. I used to buy and sell Honda DOHC 4's based on people screwing the carbs up, typically nobody ever set the valves on them and they close up to then miss worse and worse and then the 'experts' rebuilt the carbs (richer of course) to run even worse. Buy one for less than $500 and set valves and put carbs back together while REMOVING the bigger jets and instant $1000 profit for 4 hours work if not more. Bike when bought by me missed outrageously at 4000 rpm or higher, when done it ran at 10,000 rpm every gear easy as spit.

    Food for thought...........a DOHC 1100 Honda runs the exact same pilot as a DOHC 750, how can that be?

    '...turning the pilot screws helped...'

    You all say that, the only problem there is that the normal vacuum at idle of 10 inches or so at closed throttle drops suddenly to maybe 3-4 all the way down to 1 at anything with an open throttle, your pilots and mixture screws dropped flow like a rock then. More like what is happening there is off-idle got a little better and bike owner then thinks overall the bike runs better and how you melt the engine at higher loads.

    I've seen those fuel delivery maps Mikuni and other carb makers publish to show you fuel overlap of various jet positions in carbs, the problem being they can be largely bullsh-t.

    Of course yours and do as you will, it's just that this planet is a bit more complicated that people think. I'd bet nobody here has seen a big carb that put on a high output engine has to jet DOWN a whopping amount before it will run right.
     
    Last edited: Apr 9, 2021
  9. amc49

    amc49 XS400 Junkie

    'The way I understand it anything that reduces pressure drop from beginning to end makes it run leaner.'

    The problem of course is that if you have no way to measure that amount of pressure 'drop' you don't have a clue as to either the amount, the rate it fell, or or if it even was a drop at all. What if it increased instead? It CAN happen in unusual cases. AND the pressure drop of air is in no way commensurate with a pressure drop of liquid fuel either, they are wildly different. One big fat reason why they use CV carb type, to try and control some of that to a point.

    AND as to overall pressure drop, that can be a mess as well, the drop is not the same throughout the carb, it can vary wildly in different spots in carb body and then on top of that you have various exit points for fuel that can meter fuel at one point to then back off to nothing as the vacuum rises and falls in carb, it does not rise and fall evenly throughout the carb either. Even the air jets affect that, they are carefully chosen to correct for pressure drop if it is damaging to fuel delivery. You can even fuel jet the carb using the air alone if you are sharp. I've done it more than once. Like with cutting the slide bottom slightly to lean out transition when no pilots were available, it cured a balk off-idle when I used to run aftermarket pipes on small Honda twins.
     
    Last edited: Apr 3, 2021
  10. amc49

    amc49 XS400 Junkie

    'And for what it's worth I went up two sizes in pilot and main and then had to come way down on the needle to compensate so I'd try one size up like you said.'

    Exactly. All somebody did there was buy extra carb parts. Leave the pilots and needle alone and simply tune main jet first. Once you are satisfied with wide open throttle jetting THEN you go lower to cure other issues. Pilot should probably never have been touched there, it just sent somebody messing with needle too probably for nothing.

    Deal with your bigger fuel flow issues first then go after the lesser ones as the big ones often help alleviate smaller ones too. If you do the lesser first commonly bigger changes then mess the lesser ones up again.
     
  11. Peter Ronney

    Peter Ronney XS400 Enthusiast

    So these pipes have been too loud for me so I shoved two washers in the ends of them which gave me some more back pressure which gave me a better mix. Sorry for the radio silence but this is my solution for now
     
  12. amc49

    amc49 XS400 Junkie

    Too much back pressure will burn exhaust valves, the hole in washers needs to be big enough.
     
  13. Peter Ronney

    Peter Ronney XS400 Enthusiast

    There’s the same if not more more flow from the ends of these mufflers with the washers than the stock ones. Will keep that in mind, thanks for the tip.
     
  14. xschris

    xschris XS400 Guru Top Contributor

    10,316
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    The stock foam filters should not be oiled. Just blow them out with an air gun. When the yellow hair on them starts to get dark, wash them with warm soapy water and let fully dry. Make sure your bike has the stock boxes and grommets or the filters will get destroyed.
     
  15. Peter Ronney

    Peter Ronney XS400 Enthusiast

    They are not stock, the foam was deteriorating after sitting outside in the Colorado mountains for a few years so I changed them for open cell foam.
     
  16. amc49

    amc49 XS400 Junkie

    The hair takes the place of the oil.

    On pipes, all OEM ones serve as a method to slow down exhaust flow slower and slower using progressively more restriction as you go. Not nearly the same effect you get with a straight pipe with low baffling (the noise) to let top speed impulses hit a sudden perceived huge restriction to stack backwards through the pipe and what burns the valves. There's way more than one type of back pressure, the difference in blowdown time and location can kill you.
     
  17. Peter Ronney

    Peter Ronney XS400 Enthusiast

    Interesting, here’s a photo of what I’ve done. The muffler itself has some restricting properties but I’ve added to that. You can see the original diameter on the outside of the washer, it’s like that throughout the entire baffling of the muffler. Of course there’s no substitute to seeing the real thing but in your expert opinion, should I have more or less of an opening? I also have wrapped pipes now if that makes any difference.
     

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  18. amc49

    amc49 XS400 Junkie

    You should be fine with that.
     
  19. Peter Ronney

    Peter Ronney XS400 Enthusiast

    Cool thanks. Unrelated to the topic but one of these days I’d like to take this thing from Colorado to California. What I’m concerned the most about is how modified my exhaust is and if it would hinder engine health after a really really long ride. IE valves like you mentioned earlier or overheating or something of that nature. Any suggestions or concerns?
     
  20. amc49

    amc49 XS400 Junkie

    One, that while not impossible, the bike there is somewhat undersized for long trips like that. You'll have to buzz the engine too much to keep up with freeway traffic and premature wear as a result. It will take it for a while.
     

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