1977 Yamaha Xs400 Help

William

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Hi All,

I am brand new to this site! Great to see how much info is on here and how helpful everyone is! I have a 1977 Yamaha xs 400 that was passed down to me and has been sitting in storage for about 15 years. The bike did not start when I first started working on it (about a month ago). This is the first bike I have ever worked on, so it has been a great learning experience. So far I have:

-Removed rust from gas tank
-New spark plugs
-New boots for carb where carb feeds into engine
-New, stock air filters (no pods)
-Cleaned carb once
-Cleaned carb twice and replaced a couple parts (jets, gaskets)
-New battery
-Changed oil and oil filter
-New Fuse Box

About 2 weeks ago I put the carb back on and was able to get the bike to start, but it would not idle. It would stay started for about 5 seconds and then die. To get it to start, I have to full choke, open the throttle, and kick the bike at least 3 times with the ignition off - then kick it with the ignition on, and it will start. I do not know much about engines but to me it seems like it isn’t getting gas. I looked at the gas tank last night and set the petcock switch to on. I removed the fuel line to the carb and tried to suck and see if gas would come out but it did not. But, gas will freely flow through the line when the petcock switch is switched to Prime. Also, I flipped the gas tank over with a little gas in it and it started leaking through the cap - would a leak prevent the vacuum mechanism from working, therefore preventing gas from flowing into the carb like it should? There is definitely gas in the carbs. Any ideas or help is greatly appreciated! Thank you in advance!
 
ON uses the vacuum diaphragm.
PRI bypasses it, and just lets gas flow.

You could have an issue with the petcock, or the vacuum line, but that does not rule out that you also have a carb issue. Work with it on PRI to make it run, then figure out the petcock.

If you set it to PRI and it fills the bowls, but still doesn't run, your carbs aren't as clean as you think. Likely clogged idle circuit.

Carbs are finicky, with lots of little passages. Idle circuit and enrichment wells are the smallest and most easily clogged.

Enrichment circuit is "choke". It does the same thing, makes mixture richer for startup, but where a traditional choke cuts air, enricheners add gas.

Enrichment circuit draws fuel via the small brass tube you see on the side when you take off the bowl. The tube goes into the well in the bowl, and there is a tiny jet at the bottom of the well. It is pressed in and not removable. If you stick the straw of a carb cleaner spray can into the well and spray (make sure the bowl is pointed away from you, that stuff hurts in your eyes) there should be a healthy stream out the bottom of the bowl (inside). It should shoot out a coupe of feet. Tube also needs to be clean.
 
There is a vent in cap or somewhere around it and normal to leak with tank upside down. The vent if stopped up would not fuel go into carbs at all. You are fine there.
 
ON uses the vacuum diaphragm.
PRI bypasses it, and just lets gas flow.

You could have an issue with the petcock, or the vacuum line, but that does not rule out that you also have a carb issue. Work with it on PRI to make it run, then figure out the petcock.

If you set it to PRI and it fills the bowls, but still doesn't run, your carbs aren't as clean as you think. Likely clogged idle circuit.

Carbs are finicky, with lots of little passages. Idle circuit and enrichment wells are the smallest and most easily clogged.

Enrichment circuit is "choke". It does the same thing, makes mixture richer for startup, but where a traditional choke cuts air, enricheners add gas.

Enrichment circuit draws fuel via the small brass tube you see on the side when you take off the bowl. The tube goes into the well in the bowl, and there is a tiny jet at the bottom of the well. It is pressed in and not removable. If you stick the straw of a carb cleaner spray can into the well and spray (make sure the bowl is pointed away from you, that stuff hurts in your eyes) there should be a healthy stream out the bottom of the bowl (inside). It should shoot out a coupe of feet. Tube also needs to be clean.


Thank you for the help! I just took the bowl off and took a picture. Is this the correct brass tube you are speaking of? I also hooked the carb up to the gas tank with the bowl off - gas was flowing out the bottom of the carb.
 

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Thank you for the help! I just took the bowl off and took a picture. Is this the correct brass tube you are speaking of?
Yep, that's the enrichment tube.

It goes into a hole in the side of the bowl, and that's the well with the tiny jet.

It's smaller than the smallest guitar string I could find, and it's at a right angle, so you can't really poke through it. Spraying through is really the only way to tell if it's clear.

I also hooked the carb up to the gas tank with the bowl off - gas was flowing out the bottom of the carb.
I assume this was on PRI?

So you're getting gas to the carbs. Check idle jets and idle circuit.


Another thing:

If you have an air leak (like, say, bad or missing vacuum cap and the petcock vacuum hose detached) you will get too much air, and will have to use choke to compensate. Make sure your cap is good and hose is connected

I highly recommend getting an OEM cap - auto parts store caps don't last in that spot. Heat trashes them. You should be able to use them to get going though.
 
X2 on the vacuum caps, most parts store ones are Dorman brand at some point and the cheap crap Chinese rubber they use can even split when you first install the cap. I've come out the next day to find the cap on the ground when it split and fell off. Uglier looking but works, use vacuum line with a tight screw or bolt in the end and it lasts forever, just doesn't look that good.
 
Looks like your missing the pilot jet cover plug. Make sure you have them on or the bike will have issues. A cold compression test of the motor is another thing to test. On the older bikes carbs pilot mix screws can be broken off into the bodies. Make sure the tips are still in tack and the holes are clear.
 
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