Bike troubles....78 xs400

Thanks oliver!!! I have another stupid question ....I know the bike is a 1978 xs400 but how do I tell if its xs400e... or what ever it is!!!

I looked for the proper float height but not sure which one to go by... hope someone can shed some light to which model this bike is!!

The vin# 2L0-102886 AND MANUFACTURE DATE IS 6/78
some pics of the bike in my album!!1
 
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I've dealt with alot of downdraft and sidedraft CV Carbs on inline4 sport bikes, and usually the float setting is between 11.5mm and 16.5mm depending. If I recall correctly. I set my float on the XS400 to 12.5, because the bike is open pipe and open stacks. Im not an expert by any means. Best thing is trial by error to understand what does what, and what it affects on the rev range. The best way to tune it is 2nd gear pulls. Try this site link.
http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
 
scooter123 said:
I hope that will do the trick......One other question, if the bike idles fine but dies when on the throttle....could it be off timing???
I haven't checked the timing yet maybe something I should also take a look at!!
Click to expand...

When trying to fix carb problems you should ALWAYS check the timing is spot-on first !!
 
And ALWAYS check the actual liquid level with clear tube to make sure your floats are at the proper level and they are seating properly.
 
Well got the new air/mix screws today and installed them in the carbs......AND STILL THE SAME PROBLEM:wtf:
Not too sure what else to do with this bike.....it idles fine......and if I pull out the choke it revs up like a bastard...so I assume the choke is working right!!!

How do I know when the carbs are in sync....I looked in the online manual and the picture is very dark and you cant seee it!!!!

I have worked on a few honda cb500 fours and honda cb750 four so im not new to the bike thing but this is my first yamaha and its getting the best of me!!!:shrug:
 
I can't remember if these bikes have diaghrams in the carbs. If they do that could be exactly your problem. Ik the kz440's have them. Anyway if there is a rip in the diaghram you will never get the bike tuned and will act crazy. If your carbs have diaghrams it will be on top (the disc cover). Take that off and very carefully look for rips.
 
Been through these carbs now at least 5 times and all looks brand new including the diaframs!!!! Not to sure what else to do!!!!

If someone has any pics of how the butterfly's are when the carbs are sync...from what I read they both should open evenly at the same time...but how far should they be open at idle????

I have soaked these carbs in varasol two to three times and removed all the internals and I also sprayed them with carb cleaner and blew them out with 80 psi compressed air and all passage ways seem to be clear!!!

If anyone here has pics of their carbs from a 1978 to compare mine to would be great!!!
 
As to the float height, it should be at 26mm from the lip of the carb body where the float bowl seats to the bottom of the float when the carb is upside down and the float is held so it just touches, but does not depress the float needle.

Mikuni BS34 carbs have diaphragms.

You can do a rough synch on the carbs just making sure, by sight or with a wire, that they open at the same time. By sight, just look at both and make sure they open from a closed position at exactly the same time. If you use a thin wire, you hold the throttle plate just open enough produce a slight drag on the wire which is inserted under the plate, then, with the throttle at the exact same spot, see if the wire produces the same slight drag.

A more precise method is to make your own manometer...but try to get it running on the methods above, that will come later.

What are the idle mixture screws set at? As someone mentioned earlier, go 3 to 3.5 full turns, not flats, full 360 degree turns out to start. The manual says something like 1.25 turns- don't do that.

There is no way to tell you how far the plates should be open at idle. If you can start it, let it idle and screw the idle air screw (between the carbs, the finger adjustable screw, not the screw on the linkage that connects the throttle plates) screw it in for faster, out for slower- it should idle at 1200 rpm. Here is a pic from my '79:

314p40p.jpg


Sorry, don't have a pic from the other side.
 
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Thanks for your help!!!! I think I just need to take a break from this bike and go back at it later!!!
 
The diaghrams even if new could be ripped or not sealing correctly. To me it sounds like a diaghram but houghmade could be onto something. Hough those carbs are shiny!
 
And you DID check the timing, yes ?

If the PO had set it way too retarded it could idle but not rev.
 
http://www.xs400.com/forum/showthread.php?t=4619

according to the thread above, you have a 1978 XS400-E.

Houghmade said to set your floats at 26mm. The yamaha manual says for this model and year it should be 32mm. I have always wondered about that, it seems high. It is what I use though.

Heres how I sync my butterflies initially. Turn the carbs upside down so that you are holding the flat tops away from you. look up in to the barrels. Do you see the holes where you punched your broken air screws through?

There should be several other holes in there. I like to use the bottom "point" of the three hole triangle but it doesn't really matter. Just make sure you pick the same one on both carbs. The holes are only being used here as an aid to help you line up the butterflies.

Now use the butterfly sync screw and the large idle screw to line the butterflies up with your chosen holes. Its important to really line them up. Use a magnifying glass if you have to. You want to see the butterflies cut a perfect half circle on both carbs.

I hope that helps. I wish I had a picture to help you. My carbs are on my bike now, I just got them dialed in and I am not taking them off.:laugh: If you take a shot the butterfly side of your carbs I will label it for you.

+1 on setting your timing too. Now is the time to do it.
 
I think my floats are supposed to be 32mm too, but it didn't run right at that, so i went with 26mm and it works...but I guess the default position is to go with the manual first and adjust from there.
 
Drewpy says he uses 26mm too. I'm beginning to think that 32mm was a misprint in the original Yamaha manual that was referenced when printing the Haynes. It just seems way off. 32mm actually puts the fuel level very close to mating surface.

I trust you two and will be moving my floats down next time my carbs come off.
 
Mcrowell75>>
""Back to those air filters, sorry if I'm beating this. You said they are new aftermarket and they fit in the stock airbox? I'm curious. Where on earth did you get them? I have been looking forever. I didn't think anyone made those. I'd like to see a pic of them. Are they metal and oiled foam?""

i just bought 2 of these off ebay http://cgi.ebay.com/ebaymotors/Yama...8106774QQptZMotorcyclesQ5fPartsQ5fAccessories for 21.00 each i checked bikebandit and they sell genuine yamaha filters for 38.00 and change so i thought 21.00 was reasonable
 
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Your are not to oil the stock filters but to leave them dry. I f they get dirty Blow them out with air or wash and let dry. 13 years with mine and no problems.
 

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