Bike runs for about 5 minutes and then dies

[XSDad]

I have a 1980 XS400 that has been parked for some time because it will not stay running. About a year and half ago it began leaking fuel out of the air cleaner on the left side. I assumed this was due to a bad/dirty/sticky valve in the carburetor. After several attempts to clean and replace the valves myself I ended up taking the bike into a shop. Although they had no desire to work on a bike so old, they did get the carb to stop leaking (although they did not address a number of other issues we requested).

Around the same time (shortly before if I recall correctly), the bike started to die after a short time of riding. I assumed that the issues were related, but now that the carburetor is no longer leaking the bike still dies after a short time.

This week I setup the following experiment to try and find out what is going on. I mounted a small test fuel tank and ran a fuel hose to the carburetors, substituted a stable power source in place of the current battery, and ran some clear tubing from the bowl drain holes so I could monitor the fuel level in each carb.

After adding fuel both carburetor bowls filled to 3/8" below the gasket line (a bit high according to the manual). I started and ran the bike and the fuel level remained constant. It ran okay until the fuel in the tank was exhausted at which point it died. By adding a small amount of fuel, I was able to get the bike to restart and run.

Am I correct in assuming that this indicates that the problem may lie in the tank and/or petcock?

Also, the bike would die as soon as the fuel in the line was exhausted - the level in the carburetor bowls did not go down at all. Is this normal? I would have expected the bike to continue drawing fuel from the bowls for a while before dieing.

Unfortunately, I have some theoretical understanding, but not much practical knowledge. So any assistance would be greatly appreciated.

 

[Highwaystar]

Good Morning,

I guess you to have two problems to cope with:

First you will have to make sure the fuel comes to your carbs: does the engine run with tank cap open? If yes there the ventilation hole in the tank cap maybe stuck with dirt. Is the fuel petcock free? It maybe stuck with dirt or rust from the tank, Does the petcock open fully? Could be the vacuum line (if you have one) has a problem with dirt or the hose is not airtight. Try to empty the tank into a canister (right word?) by leading the fuel hose from the carbs into the canister and lead it into the canister, the fuel should not stop running until the tank is empty or until you close the petcock.

Second problem to deal with are your carbs: the fuel level must sink until the main jet can suck in air, the floats must follow the fuel level instantly, there must be no mechanical resistance. If you find the fuel level not sinking at all it might be possible your engine is running on the idle jet only, and there is no fuel from the main jet/needlejet to run the engine. This should be problem of dirt. What happens if you open the throttle while the engine has its moments running?

Viele Grüße aus Wuppertal, Uli

My english is not perfect, please excuse if the words seem funny.

 

[xschris]

If you have not done it yet , pull the carbs and completely take them apart and clean/rebuild them. There is a lot of threads on this and is a must for a good running and reliable bike

 

[XSDad]

Thank you both for the feedback. I will definitely pull the carbs and try my hand at cleaning and rebuilding them. The shop I took it to said that they would re-jet it for the altitude, but I am not sure if that happened or if they stopped once they got the fuel leak fixed.

 

[XSDad]

As it is now, I have completely bypassed the fuel tank and petcock so I will have to wait until I have the carburetors rebuilt to test them. I like the test you suggest for the tank & petcock it is simple to setup and see if it is working. Thank you.

If I am following your explanation correctly, it sounds like I am running on one or both sides on the idle jet only. I will know more once I re-open the carburetors and give them a good cleaning and rebuild. When the engine is running, if i give the throttle a quick blip it will usually stumble and sometimes stall. If I advance the throttle slowly, the rpms will come up but may not slow down when I let off the throttle (cable is well lubed and responsive).

Okay, I am off to find some parts and learn about rebuilding carburetors. Thank you so very much.