Bike troubles....78 xs400

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scooter123

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Hey guys im new to the yamaha bike thing and just picked up a 78 xs 400 that is in pretty good shape!!!!
the bike starts fine and idles great but it dies when i try to twist the throttle????

I have totally stripped the carbs and soaked them 3/4 times and blew them out with compressed air and still doing the same thing!!!!

The bike has great spark and great compression.....any info on how many turnes those two mixture screws have to be set at and how to get the sync close with out the gauges

Thanks!!!
 
What condition are your air filters in? Are they the original foam ones? If they are, then you have probably have some work to do...the foam is 30 years old and most likely shot.

Use the search, there are a couple of threads about air filters here.

There is a manual available for download on this site. I strongly suggest downloading it. I know there is a pic in one of them that shows how to synch close without the tool. The carbs have to be off to do it though.

Good luck with that '78. I have one too. Pics please!!!
 
Welcome and congrats on your new bike! The low speed mix screws on the top of the carbs should be turned out 3 - 3.5 full turns to start, then fine tuned from there. Factory setting was 1.5 turns out and is too lean. I tell you this assuming the plugs on top of the screws have been removed. Also, the VERY SMALL o-rings at the bottom of each jet deteriorate with age, leaking air and causing problems. They often stick down inside the shaft when you remove the jets and are missed by folks not familiar with them. You can purchase a new set of jets and o-rings from mikesxs.
 
Thanks for the help guys....when I removed all the jets fro the carbs there were no rubber o-rings to be found!!!!!
As for the air filters they are the new aftermarket ones!!!
Can someone be a little clearer on which jets have the rubber o- ring???

there are screw in jets and the back of the carb....mine has no o-ring
main jet...no o-ring
and the one you have to remove the little cap to get at it......also has no o-ring!!!

hope someone can help!!! Also thanks for the help!!
 
it won't have an O ring, its too early for that malarky! check the pilot tips haven't broken off in the carb body, that is a regular thing I've come across. you have to take out the pilot screws and hope they are really pointy!!
 
I was referring to the low speed air mix screw located on the top of each carb. Yes, the one under the plug. Drewpy knows these machines best. Perhaps on your year there was not an o-ring. I can't say but I do know there is one on my carbs and because the originals were deteriorated, I couldn't get the proper idle. When the o-ring is bad, it allows air to leak by it and leans out the mix. The engine won't idle at proper speed and even low speed performance can suffer.
 
You said aftermarket filters. What are your jet numbers? Was the bike re-jetted? I had to increase my pilot jet and main jet, and I lowered the needle clip one notch when I put new filters on.
 
THe filters are the ones that fit in the stock air boxes!!!! I removed the carbs again tonight and removed all the jets and emulsion tubes and sprayed carb cleaner in all passages and blew them out with compredded air!!!

I will take a few pics of the carbs in the morning to show you guys and maybe someone here can tell me if they are the correct carbs for that year!!!

This is like the 5th time ive removed these carbs off this bike and cleaned them....not to sure about the jets that screw in the carbs on the air box side.....ya never know what the previous owners may have drilled out... any pics you guys have to help me out would be great!!!!!

scooter!!!
 
Those jets on the airbox side are for air only and the passages are large compared to the fuel passages on the inside. I wouldn't sweat them too much. If you can blow air thru them then they are clean enough.

If you have your carbs off now you should take the jets out and check the numbers on them. If you have have "stock" air filters and stock mufflers then the jets should be the stock numbers as well.

Do you have the xs400-D or xs400-E? Yamaha made both in '78 and they used slightly different main jet sizes.

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?
 
Its your air/fuel mixture screw. You can run with no main jet needles for about minute at any rev range and feel fine. If that makes any sense. So, forget about main jets. It can be your float settings, but a few millimeters wont affect it much. And lastly it could be your float needle. It has a rubber tip on it which deteriorates with time sitting in fuel.

Visually looking at it, its hard to determine. But can be the cause of it. What happens is the needles don't shut off the fuel from the petcock, which floods your float bowls to the point where the fuel starts flooding the cylinders and kill the plugs. . . . Start with basics and move from there. I have no filters on my carbs and runs like clock work with 150 main jets. As for A/F setting screw, start with Factory 1.5 and work one half turn out each time till you get a smooth transition from idle to half tach. Anything past half tach is dictated by main jet size.

Dont feel bad. We all start somewhere. Try removing the carbs off an old oil cooled gsxr slingshot 5 times everyday for a week straight. To find out it was a pinhole fray it the vacuum line to the petcock. Hope I was some sort of help. Cheers
 
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Thanks for all the input guys!!!! I could not wait till tomorrow so I went back out to the garage and put the carbs together....I ordered two air/fuel mixture screws on friday...the pointed ones....the ones in the carbs have the tips broken off both but I wanted to try with the ones I have but its a no go!!!!

I will let you guys know the results when I get the new ones installed!!

thanks!!
 
scooter123 said:
Thanks for all the input guys!!!! I could not wait till tomorrow so I went back out to the garage and put the carbs together....I ordered two air/fuel mixture screws on friday...the pointed ones....the ones in the carbs have the tips broken off both but I wanted to try with the ones I have but its a no go!!!!

I will let you guys know the results when I get the new ones installed!!

thanks!!
Click to expand...

I bet the tips broke off and are still in the carbs, you need to press them out with a sharpened nail (yes through the carb body) it will widen the hole more, but I've done 3 sets of carbs and its the only way!
 
Ya I removed the broken bits and checked it with a flash light and I can see the light through it!!!
 
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!!
 
Wait until you get the idle mix screw thing sorted out. It may well cure the problem you just described. If these carbs behave the same as my '80 and my son's '81, the idle mix screw adjustment affects more than just idle mix. It actually has a bearing on low speed mixture and so transition from idle to lower rpms. Worked for my son's xs400 with symptom as you described. Don't forget, set each screw to about 3-3.5 turns out from seated. Make fine tune adjustments from that setting. Don't turn the screw(s) down too hard when seating them or you might damage the seats. That's also how the O.E. long, sharp tips get broken off. Replacement jets we got did not have the same delicate tip.
 

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