Yamaha XS400 runs only on choke, high RPM, mixture screws have no effect

[Marradubins]

Hi everyone,


I’m trying to bring an old Yamaha XS400 back to life. The bike is an inheritance and has been sitting for around 10 years, so I would really like to save it and get it running properly again.


I’m also completely new to working on motorcycles and carburetors, so I’m learning as I go. From what I’ve found so far, it also looks like the bike may have been worked on/modified by a previous DIY owner, so well... see for yourselves


Originally, the bike was only running on one cylinder because one side had no spark. I fixed the ignition issue and now both cylinders have a good spark and both exhaust headers heat up.
I also discovered that the ignition points/points plate was loose and not properly secured in its housing when I got to it, so the ignition timing was obviously way off. I have now properly secured it and statically adjusted the ignition timing.


So far I have:

• installed new spark plugs
• cleaned the carburetors (i hope so)
• installed new intake boots
• connected the vacuum line to the fuel petcock
• checked the ignition points
• statically set the ignition timing – left cylinder opens exactly at LF and right cylinder at RF
• disconnected the throttle cable for testing; the throttle linkage is fully returned to its stop

Current symptoms:


With full choke, the engine starts, but the RPM gets very high.
With about 3/4 choke, the RPM is lower, but the engine runs poorly / sounds like it is struggling.
With no choke, the engine dies.
In any case, as soon as I try to open the throttle on the carburetor, the engine dies immediately.

I also discovered that the two pilot mixture screws currently fitted to the carbs are different. One has a much longer/thinner needle tip than the other. Since the bike seems to have been worked on before, I’m wondering if somebody installed an incorrect mixture screw in one of the carbs.
Another strange thing is that the pilot/mixture screws seem to have absolutely no effect. I can screw them in or back them out and there is basically no change in RPM or engine sound. Also i can see there is different hole on each carburetor and one of them even has a crack (that one with worse pilot screw)

At this point I’m thinking there may still be a problem with the pilot circuit/passages, pilot jets, fuel level/float height, incorrect mixture screws

Has anyone experienced the same symptoms with the early XS400 Mikuni BS34 carbs?


What would you check next? I'm pretty much at a loss


Any advice is appreciated. As I said, I’m new to this, so please bear with me if I’m missing something obvious. I’d really like to get this old XS400 back on the road rather than give up on it.

 

[Revvy Kevvy]

Those Mikonies look like when I first got mine, full of that green stuff that I think comes from a mix of rubber and fuel when motorcycles get abandoned full of fuel. It took me quite a while to clean it all out using carburetta cleaner, fuel and wire brushes. I also completely rebuilt mine with a new kit. Note, I am also having trouble with mine so this might not be a solution, but its how I started.

The mixture screws in mine are the same length, I have read that its not unusual for folk to screw them in too tight and the end breaks off. To me it looks like your mixture end has broken off. In reality I have the same issue with my mikoni, I can screw them in and out and nothing changes.

I also dont have the crack in the inlet, looks that a mixture screw got stuck in there once upon a time>

Dont take everything for granted what I say, I am still getting my XS400 running and its been a long uphill ride but thats my view on your situation. I am living in Sweden but often in Poland.