XS400 Member
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Florida, usa
everyone how's it going??? I'm having a little problem with my 1981 Yamaha xs400.... How the heck do I tune/sync these carburators..??? Can someone explain the screws to me,?? Air/fuel screw is IN more my ß fuel or air?? The screw that connects the throttle, where do I want that set it?? Please someone HELP ME!!!!
The screw in between the carbs is for syncing the carbs. You will need 2 vacuum gages or a manometer which you will hook to the vacuumed ports in the carb boots, out is richer on the mixture screws. Something to remember is that the idle circuit adjusts need tho be made at just under 1000 RPM That way only the idle circuit is in play. The idle mixtures on a set of clean carbs with no vacuum leaks (think throttle shaft seals) should start at about 2.5 turns out from lightly seated. Once you have it idling as smoothly as you can get it then turn the screw between the carbs on the throttle shafts from the top to balance the vacuum between the cylinders as close as you can. That’s why you need the 2 vacuumed gages hooked up. This will increase or decrease the overall idle speed which you can adjust with the knob between the carbs accessed from the bottom. Clear as mud right
Yes I think I understand....I'm not positive but the gas tank in the bike is not original, the tank that he put on is gravity fed (not sure what's supposed to be on it or if it matters??))
When you say start at the top with the screw between the carbs what exactly do u mean by that?? All the way out all the way in??
I also have to let u know, my exhaust broke and a friend of mine cut them off at the bottom bend where they straighten out and welded on HOOKER tinted flow exhaust to it.. It runs on both cylinders when I hit a certain rpm then when I get going the right one stupid firing until I down shift??? Please help
Once you have the idle mixtures set then adjust the screw that attaches the throttle shafts together from the top (the tank has to be removed to see and reach this adjustment screw) to adjust the sync or balance as it’s sometimes called. There is also a knob on the bottom between the cards used to adjust the idle speed. From your description you have something else going on if one cylinder is misfiring above a certain rpm. Possibly a misadjusted float level, or incorrectly sized/clogged intermediate or main jet or your carburetors have an electrical issue . On these bikes changes to the flow of the exhaust system can and will require some creative tuning and all XS 400s have gravity fed fuel systems. What the previous owner may have done is removed the automatic vacuum control fuel shut off and replaced it with a manual petcock.
No he definitely changed the gas tank because he had to cut the center of the frame so that the petcock would fit he welded the front of the tank to the frame but anyway where should I start test all my electrical?OR Do another cleaning of the carb I've already cleaned it several times?? I had to replace the diaphragms as well... I'm supposed to get paid today so I'm going to buy the meter I need so the question is WHERE DO I START????
First I’d try to figure out which is it issue. Electrical or carburetor. Have you ever performed a “ plug chop” to determine what the fuel mixture is at speed? Start the bike, ride it at the rpm where that cylinder drops out or misses. The longer you ride like that the better the indication. Holding the throttle at that position reach over and hit the kill switch and pull in the clutch at the same time. After drifting to the side of the road pull out the spark plugs and observe the business ends. If the electrodes are black and or wet you might have a electrical or ignition issue. If the plug looks white/very light in color and dry you are running lean and have a fuel delivery issue. This might be a start and point you in the right direction Good luck
Well I forgot to mention that the right side of the exhausted is always wet and the left side always dry.. Same with plug... If I grab that could write wire when it's running it zaps the shit out of me and it has plenty of spark on the plug nice and blue... My intake flames may be the problem but I'm not sure I'm going to buy or order a meter tonight
No I have not done that I didn't want to puncture the brass floats could u walk through it for me please
If you search the forum there are some good discussions on the process. Basically with the machine on the center stand you attach a pice of hose to the drain port of the carburetor in question. Bending the tube to run up the side of the carburetor and open the bowl drain valve. The level of the fuel in the tube should be even with the mating flanges where the carburetor bowl attaches to the carburetor body +/- 3 mm I believe. Yamaha says this should be done while the engine is running but I’ve never had to go that far to archive results. To high and the carburetor will overflow into the air box, to low and you will have a lean condition under acceleration. If the levels are incorrect the adjustment is made by slightly ( a little goes a long way here) bending the tab where the needle valve contacts on the float assembly. This process can also be done with the carbs off of the machine. A couple of old carb boots attached to a 2x4 and leveled works well. I have seen replacement needle valves have different lengths and cause problems and require big changes to the float flange. So if someone has been in your carbs and used non Mirkuni parts it could explain you always wet side. Another test you should do is with the carbs off, dry and up right blow into the fuel inlet, air should pass freely. Turn the assembly upside down and blow thru the fuel inlet. If you hear air moving you know you have a float level/ leaking needle valve.
Sorry it took so long to right back thank you for the help I really appreciate it....So I was cruising down the road going about 45 and all of a sudden I lost spark my bike shut down when the bike finally stopped I kicked it over a couple times and you got nothing I took the left coil wire off the plug and put my finger in it kicked it over no spot I took the right side off put my finger in it kicked it over and God's zapped so just to see what would happen I changed the wires going to the coil wire and now I got spark on the left side and not right so it's obviously not the coil what would stop the left side from getting a signal.... So I left the bike on the side of the road for a couple hours came back with the trailer to pick it up just for s**** and giggles I kicked it over and it started and shut it off and put it in the trailer brought it home couple hours later tried to kick it over and I didn't get nothing please help me thank you
You can also bench sync the carbs to get them to a good starting point. Pull the carbs then use anything you like for a gauge. Could be a plastic brake clean straw or a small drill bit. Turn the main idle screw until you can put the "gauge" between the blade and carb body of the carb without the sync screw. Try to count the turns to do this to return it back close to the original idle setting. There should be just a slight drag on the gauge, no play. Then adjust the sync screw so the gauge feels the same on that carb. Return the main idle screw to its original setting.