Ignition system problem

[Pete.Viking]

Recently got an '81 xj400, the batteries been buggered since I got it, had to crash start it all the time coz it didnt have the guts to start off the electric start even though it wud turn over. Was having trouble with intermitant cutting out/starter not going at all just randomly. Went to ride it a while back and wouldn't start, diagnosed a problem with the fuel tank valve (only lets fuel out when set to prime and comes out the main line not the prime?? go figure). Bike back together and rode up and down the street part way back just cuts out, now I can't get anything out of it when I hit the starter. Lights work, just the ignition loop affected. I'm suspecting a dodge conection/earth in the high amp loop, just coz I wud expect an actual component to just crap out. Any other ideas?

[MSTRS]

Recently got an '81 xj400, the batteries been buggered since I got it, had to crash start it all the time coz it didnt have the guts to start off the electric start even though it wud turn over. Was having trouble with intermitant cutting out/starter not going at all just randomly. Went to ride it a while back and wouldn't start, diagnosed a problem with the fuel tank valve (only lets fuel out when set to prime and comes out the main line not the prime?? go figure). Bike back together and rode up and down the street part way back just cuts out, now I can't get anything out of it when I hit the starter. Lights work, just the ignition loop affected. I'm suspecting a dodge conection/earth in the high amp loop, just coz I wud expect an actual component to just crap out. Any other ideas?

The bike would seem to have a vacuum tap. There should be one (maybe 2) larger bore hoses that go from tap to carb bank. And a smaller bore hose that goes to one of the inlet manifolds (engine side of the carbs). This hose does not carry gas, only transfers the suction from the inlet to the fuel tap to open that tap. Prime postion on the tap bypasses this operation, but fuel will still only go down the 1or2 bigger hoses.
Can't help with the other problem tho.

[Pete.Viking]

Ahh that makes alot more sense, thank you for enlightening me from my ignorance on that matter. So presumably theres an issue with the vaccum line or the valve in the tap. Hopefully I don't have trouble with too much/not enough fuel on prime then, ive enough issues to deal with already.

[surfer]

Check the easy stuff first like your fuses and for loose connections/wires that are earthing out or melted through. Also check the battery to see what it is kicking out, if it is well and truly knackered get a new one. Having a working battery will make it easier to figure out where your lecky is and is not going, it also eliminates a possible cause.

[Pete.Viking]

Followed your advice, surfer, with an interesting result, I had already visually checked the earths and fuses. Took the fuel tank off, couldn't see any obvious issues. Started checking from the negative terminal with a multimeter decided to properly clean one earth, no changes. Then daunted by the huge task of checking the rest of the electrics I checked the fuses again, no voltage across the ignition fuse. Swapped it with another fuse and it was all go, noticed that most of the fuses were the wrong rating so replaced them all. The old ignition fuse looked fine and checkd out with the multimeter tried it back in the bike and it worked fine?? Considering its a glass tube fuse I don't see how a dodgy connection figures so I'm at a bit of a loss as to whats going on

[CookMySock]

The old ignition fuse looked fine and checkd out with the multimeter tried it back in the bike and it worked fine?? Considering its a glass tube fuse I don't see how a dodgy connection figures so I'm at a bit of a loss as to whats going onThe glass fuses have two failure modes, one where they corrode between the fuse cap and the fuse-holder contact, and another where the actual fuselink comes loose from inside the end cap.

Theres a good reason why they went to blade fuses.

Steve