XS400 SE (4G5) Project

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Bulisti

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Hi everyone!
I'm new to the forum and a new XS400se owner. I couldn't find the answer by searching (possibly due to my lack of skill), so I'm asking here.


I bought an XS400se in project condition that has been turned into a bobber. It has straight pipes and power filters as air filters.


There’s a lot of work to do, but I’ll start with the most important thing – getting the engine to run properly.


It starts well with the choke and stays running with the choke on. But when I turn the choke off, it dies – even when the engine is warm. I haven’t removed the pilot jets yet. They’re apparently located under some kind of plug? Is it necessary to put that plug back in afterwards?


I also tried adjusting the mixture screw 3–5 turns out, but it still won’t stay running. It also backfires into the exhaust and sometimes into the carbs. I checked the main jets and they are size 142.5. That sounds a bit large?


What is the stock size of the pilot jet, and what size should it be with straight pipes and power filters?


Thanks in advance to everyone, and sorry if my English isn’t perfect. I’m writing from Finland :)
 
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The pilot jets were under the brass plugs. The brass plugs were stuck and wouldn't come off intact. The jets are completely clogged. I thought about replacing the jets, but I can't find those brass plugs anywhere. Are they mandatory or can I plug them somehow?
 
Welcome to the forum!

It sounds like you need to clean the carburetors. I am not sure of the jetting you will need for your intake and exhaust setup, but clean carbs are the first order of business.

The pilot jet is under a rubber plug once you pull the fuel bowl off of the carb. That rubber plug is important, so needs to be replaced. The only brass plug I know about is probably the one that covers the idle fuel screw. That plug is there for emission regulations so people don't change the screw setting. The brass plug does not need to be replaced.
 
CaptChrome said:
Welcome to the forum!

It sounds like you need to clean the carburetors. I am not sure of the jetting you will need for your intake and exhaust setup, but clean carbs are the first order of business.

The pilot jet is under a rubber plug once you pull the fuel bowl off of the carb. That rubber plug is important, so needs to be replaced. The only brass plug I know about is probably the one that covers the idle fuel screw. That plug is there for emission regulations so people don't change the screw setting. The brass plug does not need to be replaced.
Click to expand...
I cleaned carbs. Pilot jets were fully clogged. They were under brassplugs and there was no rubber plugs. I bought rubber plugs to replace brass plugs.

But still, it is hard to start without starter spray. And still no idle. I think those powerfilters let too much air. I try to cover them tomorrow and see if it helps.

Here is also photo of carbs where u can see brass plug.

What is best option for air filters if oem is not available?
 

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I got the bike running pretty well. There was a blockage in the fuel petcock. However, at idle and low revs it stalls, but it will start again right away with the kickstarter. The lights dim/flicker a bit. A quick measurement showed the battery voltage at 10V, so it seems like it’s not charging.
Where is the rectifier/regulator located, and if it’s faulty, what would be a good replacement? Will a unit from another model fit directly? Here in Finland, XS400 parts are hard to find.
 
On your carbs I think you are mistaking what the brass "plug" is. Here is your picture annotated:
IMG_9258.jpeg


The hole where the pilot jet is needs to have the rubber plugs in them when the carb is reassembled. If you unscrew the main jet, underneath you will find the needle jet, also known as the emulsion tube. All of this stuff needs to be removed and cleaned as well as the float valve seats. It looks like the retainer clip for the seat of the bottom carb is missing also. The brass tube that sticks up from the carb body leads to the enricher circuit and is important for startup. Make sure you pull the enricher valves and clean them and find where this tube inserts into the float bowl and clean out the passage in there as it has a small stating jet in there and it will probably be clogged.

As for regulator replacement, there is a good thread by @tstidham on this site about using an automotive regulator that is readily available on the XS400. Search for that post using the search feature.
 
In the picture, there was a brass plug on top of the idle jet. It can be seen more clearly on the upper carburetor. It had to be removed in order to take out the idle jet. I replaced it with rubber plugs.





I got it running quite okay. I noticed that the battery isn’t charging and the bike stalls after running for a short while. Apparently, the voltage drops and the spark gets weak or disappears.





I have a working regulator from another bike (a Honda CBR) that I’ll install in its place. It has three yellow wires for the stator, 12V to the battery, and ground.
 
Our bikes use a separate regulator from the rectifier so I would not expect a cbr rectifier to work for the regulator. And the xs400 is actually an alternator that is designed to use a ground side regulation, very much like the old Dodge slant six powered vehicles of the late 70's, early 80's. Have you ohmed your stator assembly as per the manuals (available on this site?)
 
So, from the stator I have three yellow wires. I connected them correctly to the regulator/rectifier. I also measured the resistance of the wires coming from the stator and they were fine, and they are not shorted to ground. I measured the current coming from the stator and it was fine both at idle and at higher revs. Still, it doesn’t charge.

From the regulator, one wire goes to the battery positive, and one wire goes to the frame/ground.

If the stator is producing current to the regulator, but from the regulator onward there is only 0.4V, doesn’t that mean the regulator is faulty?

Should there be more wires?
 
Bulisti said:
This is how i connected regulator
Click to expand...
This is the correct connection to the rectifier, but there is also a black and a green wire from the alternator field coil that needs to be connected to the regulator. The regulator manages current through the field coil which is needed, otherwise the alternator does not produce a charging voltage to the battery.
 
Is it possible that a 'modern aftermarket' stator has been installed here, because I tried to check and I can't see any other wires coming from the stator except for the three yellow ones?
 
Sorry for spamming, but I’m really confused right now. From the stator, there are two wire bundles. One has three yellow wires, and the other has yellow, blue, green, and black. The previous owner replaced the stator, but I don’t know if it’s original. Which wires should be connected?
 
Bulisti said:
how i know if regulator is ground controlled? for example, hos can u tell if this is? https://www.ebay.com/itm/364615115887
Click to expand...
Since this is made for an xs400, it would have the regulator for controlling field coil. I have a 78 model where the reg. Is separate from the rectifier. This is why I used the 1980 Dodge pickup truck regulator as my stator and rectifier were both still good.
 
I just don't understand how the three yellow wires coming from the stator can carry a normal amount of current even though the green wire is not currently connected. And since the current reaches the voltage regulator, why doesn't it go to the battery?
 

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