01ps Posted May 25, 2010 Share Posted May 25, 2010 On my 82 xs400, the mixture screw is on the engine side of the carb. To adjust, screw out to make it rich? Link to comment Share on other sites More sharing options...
HoughMade Posted May 25, 2010 Share Posted May 25, 2010 Short answer- yes. Screw in to lean, out to enrichen. Link to comment Share on other sites More sharing options...
01ps Posted May 25, 2010 Author Share Posted May 25, 2010 Short answer- yes. Screw in to lean, out to enrichen. Thanks, was not sure if it added air to the mix or limited it... Link to comment Share on other sites More sharing options...
armyofda12mnkeys Posted May 26, 2010 Share Posted May 26, 2010 Silly question... but I been a bit confused lately with the different adjustment screws on my 1978 xs400 carb after reading my manual ... When people talk about adjusting the mixture ... is this the 'Idle mixture' screw or 'Pilot screw' (not sure officially what to call it)?... and is that the screw on the back of the carb that you use to get the rpm's 1200 at idle for example. Is this also known as the 'Throttle Stop' screw as well or is that different (saw this mentioned in my manual)? Another other screw, other than back one mentioned above, is the carb exhaust balance one (not sure of the official name for it)... but it is to make sure the pressure coming out of the exhaust is the same by adjust this one screw (i guess it adjust the amount of something coming into each carb (air? or gas?) which affects the exhaust/output coming out of cylinder). and a 3rd that I think exists is another mixture screw (not sure what it is called or where its located on my 1978 XS400), but it is set at factory and has a special plastic thing to stop you from tampering with it according to my manual (guess these were from the 1978 EPA regulations). I remember there is a screw that you can't turn in too much, or the tip of it will break... Which one is that from above screws?... I guess I'm looking for an education carb tutorial if anyone wants to clarify carb adjustments (where the screws for them are and how/what they affect whats going into the carb and into the engine) Link to comment Share on other sites More sharing options...
Moderator drewpy Posted May 26, 2010 Moderator Share Posted May 26, 2010 Silly question... but I been a bit confused lately with the different adjustment screws on my 1978 xs400 carb after reading my manual ... When people talk about adjusting the mixture ... is this the 'Idle mixture' screw or 'Pilot screw' (not sure officially what to call it)?... they are the same thing and is that the screw on the back of the carb that you use to get the rpm's 1200 at idle for example. throttle stop screw, just lifts both the revs via the butterfly's in the carb Is this also known as the 'Throttle Stop' screw as well or is that different (saw this mentioned in my manual)? correct Another other screw, other than back one mentioned above, is the carb exhaust balance one (not sure of the official name for it)... but it is to make sure the pressure coming out of the exhaust is the same by adjust this one screw (i guess it adjust the amount of something coming into each carb (air? or gas?) which affects the exhaust/output coming out of cylinder). it balances the butterfly's in side the carb so the engine draws the same air/fuel mixture in both cylinders and a 3rd that I think exists is another mixture screw (not sure what it is called or where its located on my 1978 XS400), but it is set at factory and has a special plastic thing to stop you from tampering with it according to my manual (guess these were from the 1978 EPA regulations). that is the pilot/idle mixture screw I remember there is a screw that you can't turn in too much, or the tip of it will break... Which one is that from above screws?... that refers to the pilot/idle mixture screw Link to comment Share on other sites More sharing options...
HoughMade Posted May 26, 2010 Share Posted May 26, 2010 ^^^ Absolutely correct. I see that many people had to drill out a plug to access the idle/pilot mixture screws. I don't know if it is the year of my bike ('79 F model) or the fact it is U.S. spec., but mine had no plugs and never did. It is pretty clear the mixture screws were made to be accessed as the mixture screws have large heads on them that had plastic snap on covers with a tab on them. The snap on covers allow for about .25 to .5 turns adjustment without using a screwdriver, and they pop right off for greater adjustment. Anyone else have mixture screws like these? Link to comment Share on other sites More sharing options...
armyofda12mnkeys Posted May 26, 2010 Share Posted May 26, 2010 .... they are the same thing .... wow massive post, thanks drewpy, just to confirm what i think you said ... The 'Idle mixture' screw and 'Pilot' screw and the 'Throttle Stop' screw and 'the screw that is set at factory and has an anti-tamper knob' are all the same thing, the one in the back of carb? I guess then there's just 2 adjustments on the carb, that one and the butterfly-balancer(exhaust pressure check), correct? Thanks for excellent post! Link to comment Share on other sites More sharing options...
HoughMade Posted May 26, 2010 Share Posted May 26, 2010 There are 3 adjustments. This adjusts the idle speed: The screw on the spring-loaded shaft between the carbs adjusts balance. In the pic above, that would be right behind the idle speed screw I circled. The screw on each carb...on "back" as you say- the side facing the cylinder, adjust mixture. Link to comment Share on other sites More sharing options...
armyofda12mnkeys Posted May 26, 2010 Share Posted May 26, 2010 There are 3 adjustments. This adjusts the idle speed: ... The screw on the spring-loaded shaft between the carbs adjusts balance. In the pic above, that would be right behind the idle speed screw I circled. The screw on each carb...on "back" as you say- the side facing the cylinder, adjust mixture. 1.Cool, so the Idle Speed screw (i think i was confused and drewpy was saying 'Throttle Stop'= this screw, yes?) is the one facing back of bike. 2.Balance Screw is that lil screw in between on the front/(side facing front of bike). 3.And the Idle mixture/Pilot Screw is on each carb on the front too (so 2 screws total)... You wouldnt happen to have a pic highlighting the 3rd (Idle mixture) screws on yer carb? I kinda looked around there the other day and didnt see a screw sticking out to adjust and the manual seems to only show pics of the other 2 screws but never the 3rd (i guess cause they dont want you to adjust them) Link to comment Share on other sites More sharing options...
Moderator drewpy Posted May 26, 2010 Moderator Share Posted May 26, 2010 the yellow is the pilot/ idle mixture adjustment the exploded spring and screw to the left is the carb balance adjustment (it only adjusts one carb to match the others flow rate!) hope that's clearer Link to comment Share on other sites More sharing options...
HoughMade Posted May 27, 2010 Share Posted May 27, 2010 The first pic shows the left carb idle mixture screw, the second the right screw. As I said in an earlier post, I have plastic caps on mine. The brass idle screw has the off-white plastic cover with a tab snapped on top of them. Link to comment Share on other sites More sharing options...
exess4 Posted May 27, 2010 Share Posted May 27, 2010 Another other screw, other than back one mentioned above, is the carb exhaust balance one and this is how to balance the carbs Posted 08 May 2010 - 01:16 PM Are we bordering on eccentricity? Link to comment Share on other sites More sharing options...
exess4 Posted May 27, 2010 Share Posted May 27, 2010 BTW, not poking fun Hough, actually am going to try that some day (and happy happy to moderater, just went past midnght but i think it sedd-a 47? keep 'em wheels spinnin!) Good post. Link to comment Share on other sites More sharing options...
armyofda12mnkeys Posted May 27, 2010 Share Posted May 27, 2010 Thanks Drewpy and HoughMade, yes that clears it up . exess4, what does that do exactly to balance the carbs, where those tubes hooked up to carb, (seems like gas to each carb?)? I remember you posting this and drewpy photoshopping a funny pic based on this after haha. Link to comment Share on other sites More sharing options...
HoughMade Posted May 27, 2010 Share Posted May 27, 2010 Thanks Drewpy and HoughMade, yes that clears it up . exess4, what does that do exactly to balance the carbs, where those tubes hooked up to carb, (seems like gas to each carb?)? I remember you posting this and drewpy photoshopping a funny pic based on this after haha. Well, that's my apparatus. What it allows you to do is to measure the relative vacuum being pulled by each cylinder and adjust the balance screw accordingly. Here is the post that went with my pic above...and I do not take offense- there is no doubt, I am eccentric. As proof, below the quote about balancing carbs- I will post a pic of my other bike...and it's eccentric. My '79 XS 400F seemed to be idling very lean. After conversing with other ancient XS 400 owners, they confirmed what I should have known- the factory idle mixture setting are very, very lean. This led to occasional popping at idle and as revs increased. Also, it did not return to idle speed as quickly as it should. I richened the idle mixture and those problems went away. the factory setting was 2.25 turns out. At 3.5, it runs like a charm. Also, I suspected that the carbs were out of sync. I made a simple manometer- clear tubing and auto transmission fluid. Get 20 feet of tubing (whatever fits the vaccum nipple on your bike) and find the exact center. Place the center at the bottom of a 4 foot piece of wood, then attach the tubing up the length of the board (exact length is not important, but longer give you more time to shou down if you have to). I used zip ties, but I had the thicker wall tubing- whatever you do, make sure the tubing is not collapsed. The most time consuming part of this was getting the fluid to settle in the center. To get the fluid in, I put one end in the ATF bottle and sucked the other end like a straw. I drew up about 2 feet worth of fluid, the exact amount is not important. I then hung the apparatus with the tubes in the air and waited for the fluid to settle- took 15-20 minutes. I hung mine from a ladder to use it, but I was also using that to hold the gas tank (has to come off to get to the sync screw). After warming the bike up, hooked the tubing to the vacuum nipples on the carb holders, I started it up.....and the ATF nearly got sucked into the engine because the left cylinder was drawing much harder- way out of sync. Word to the wise, keep your thumb on the kill switch until you get it close to synced. Shut it down if one side starts to rise too fast. Longer tubing would also give more time to react...thicker fluid too. I adjusted it a couple times after having to shut it down, then it was close enough that the movement was very slow. I adjusted back and forth and got both sides of the fluid as even as possible. Perfect is near impossible, but I got it within about 1/2-3/4 inch. According to what I read, that's pretty good. Do not use a thin fluid as it will foam and be hard to read. I used ATF because it is easy to see and a bit (if only a bit) sucked into the engine will not hurt it. Anyhoo, here's a pic of my setup. This works great for twins with separate carbs. I have even seen these built for 4 cyls., so I imagine a 3 could be done as well. Bike runs smoother that I thought it was supposed to. It will idles easily where it is supposed to (1200rpm) and will even hold a good idle below 1000 rpm. It ain't a Yamaha...but it gets the job done...assuming the job can be done at under 40 mph: Link to comment Share on other sites More sharing options...
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