runs and then dies + more

[Hottoddy230]

Okay to start off I just want to let everyone know that I am brand spanking new to motorcycles but I am fairly mechanically savvy. So to start, I bought the motorcycle a couple weeks ago thinking that it ran perfectly, which it did for the 15 minutes they had it running, but after the paperwork was done and they left, she would start and then die. And then eventually she wouldn't start. So I was stuck having to put the motorcycle on a trailer instead of riding it back home. After I got home I took the carbs completely out and took them apart and cleaned every nook, jet, and float bowl and put them back in. It ran a little smoother but would still die if the throttle wasn't applied. I then adjusted the pilot screw (which I think is in between the two carbs...?) and it ran a little better, but would still eventually die and then after a few starting attempts, stop starting altogether. I have a 12v battery charger that shows the percentage of charge when hooked up so I did and it said it was 100% charged when the motorcycle is off but shoots down to 5% after a few starting attempts. I bought this motorcycle hoping for it to be running fine and now I am stuck with no transportation! I hope this is something minor!

 

[Hottoddy230]

forgot to add that after a few weeks of letting the bike be a parking spot ornament because I haven't had time to work on it, I took the cover off and tried to start it. She started right up after a few tries and would rev really high at first and then die unless I gave it a little throttle, but then ran by itself for a few minutes but then eventually died and wouldn't start again. Oh and this is a 1982 Yamaha XS400 SH

 

[16VGTIDave]

@Hottoddy230 these are all common issues that have all be addressed here many times. Charge the battery and load test it. Replace if weak. Clean the carbs again. Adjust the valves. Replace the spark plugs. Get it running and adjust the idle mixture for each cylinder with the Pilot screws. Adjust the idle speed. Sync the carbs. Check for vacuum leaks. Etc...

 

[dgoodsy]

How do the air filters look? clogged? That could cause a rich condition which would be okay while the engine is cold but once it warms up it will cause problems. Do you use the enrichener (choke) to start the engine? How does it affect running?

 

[Hottoddy230]

The filters are new and I use choke to start it and it revs up to about 5000 rpm but once I bring it down to about half choke it idles at 1200 rpm till it dies after a couple minutes. The spark plugs are new so I will try adjusting the valves and check for vacuum leaks.

 

[xschris]

What bike do you have? Any mods to the bike? Post some pics of what you are working with. If the plugs get fouled you may need to replace again.

 

[Pavlov]

There is a real good mikuni carburetor rebuild manual on this link:http://www.cvmgestrie.com/assets/mikuni_bs-cv_carburetor_rebuild_tutorial.pdf
I always start from the basics. 1. Check that the fuel is fresh, if the bike has been siting a long time that could be one part of the problem. 2. If the bike has been siting a long time and only runs on choke it is usually a carburation problem. I know you have cleaned the carbs. I would read the carb manuals and the forum to get as much info as possible. It is possible that you missed the pilot screw (it is usually under the cover and the cover has to be removed). Anyways be patient if you get frustrated with the bike take some time off and come back, that sometimes makes a world of difference. Hope this helps.