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Discussion Starter #1
I bought a 1981 Yamaha XS400 as a starter bike a few months ago. It always idled a bit high (around 2,500 rpms) but that was the only thing wrong with it. A few weeks ago I got on it (it had only been a week since my last ride) and it wouldn't start. When pushing the start button, there was a loud clicking noise and the dashboard lights would dim down when pushing the button. I learned this was from the battery being close to dead (which was weird b/c it was a relatively new battery that I bought this spring). I re-charged the battery, put it back in, and the bike started right up. A few minutes into my ride the bike seemed like it was going to die, like either the battery was dying or the gas was blocked b/c the bike would get kinda shaky and when I switched to neutral and let off the gas the bike crashed rapidly from 500 rpms and then died completely. Luckily I was able to pull over into a gas station to check things out. The bike wouldn't start up again, I got the same clicking noise as before when the battery was close to dead which I think it weird b/c I only rode 6 blocks from my building. I kick started it but it only ran for like 30 seconds before dying again. Any suggestions on what's going on would be much appreciated!

My hypotheses:

1- an issue with the regulator not charging the battery correctly? Would the battery die after just a few blocks if the regulator wasn't working? If this is the problem, is it something that me and a friend could replace together?

2- an issue with the carbs not getting enough gas? I pulled off the gas line from the tank and gas seems to be flowing down that smoothly.

3- a spark plug issue? It's a 2-valve bike and I pulled off the cables that run to the spark plugs on each side of the bike. I was able to confirm that at least one of the two cables gives a spark when I push the start button. I wasn't able to unscrew the spark plugs so I don't know if they might need to be replaced?

4- could be something else??
 

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The best way to tell if the battery isn't charging is by using a multimeter to check the voltage. You should be getting just over 12 volts from a charged battery with the engine off and somewhere in the neighborhood of 13-14 volts with the engine running at cruising speed. (3000 RPM or so)

It's a good idea to double-check the battery connections for snugness at the battery, the main grounding point, and at the starter.
 

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Discussion Starter #3
I was able to charge the battery via a wall charger and it had a full charge when I installed it. After riding for just a few minutes it was totally drained.

Connections from battery to the starter are good. I should check the connection from battery to ground again. I know the connection at the battery end with the ground is good, but I'm not sure where the other end of the ground connects. It disappears under the motor. Where should it be connecting to?
 

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Discussion Starter #6
Ok, I installed a new battery and checked the ground -- it's fine.

I charged a new battery via a wall charger and it read 12.40 when fully charged. I installed it in the bike and it still read 12.40 volts with the bike turned off. I started the bike no problem and the multimeter reading climbed up to 12.90 volts as I revved up to around 2,000 rpm. Everything seemed good so I hopped on the bike and road it down the parking garage (about 400 ft) and started to experience the same problems as last time (intermittent power // somewhat shaky). When I got back to my parking spot the bike wouldn't even idle in neutral, it just died. I checked the voltage right away and it was around 11.85 volts. I turned the keys to off and let the bike rest for a few minutes and then checked the battery again -- it was back up to 12.40 volts so I started the bike without a problem but it wouldn't rev up past 2,000 rpms for a few seconds even through the throttle was wide open. It eventually revved up all the way but then got stuck around 2,000 rpms when I tried to rev it up again a few seconds later. The bike eventually just stopped running so I checked the battery voltage again and it was around 11.80 volts this time. All this was with the bike in neutral. Do you think maybe the alternator or regulator (or both) are bad?

I thought there might be blockage in the gas line so I disconnected it from the side of the bike just past the gas dial and gas ran out freely.

The bike also was making a kind of spinning noise when it would die each time. It sounded like it was coming from somewhere close to the motor but I couldn't tell for sure. Not sure if this is helpful info or not.

If not an alternator or regulator problem, what could it be?

Any help you can provide is much appreciated!!

Thanks again,
John
 

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The stator might not be able to put out enough voltage to charge the battery if the engine will only spin up to 2000 RPM.

My best guess at this point is that the carb needs to be soaked and cleaned to clear the contamination out of it.

I'd suggest getting the motorcycle running correctly first, then check the charging issues.
 

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Discussion Starter #8
Great, thanks, I'll look at the carbs.

Re: the stator, where on the bike is this located? The only place I can think it would be is on the left side of the bike connected to the motor (if you're sitting on it). If you can google a picture of a 1981 yamaha xs400, I'm thinking that it's the chrome circle piece with the word "Yamaha" on it on the left side of the engine. Am I correct? I started to untighten the bolts on this piece yesterday but after loosening a few of them that piece started to slowly leak oil (or something like oil) so I tightened them back up right away. Doesn't seem like the alternator/stator would leak oil so maybe I have the wrong part. Where else might it be?

Thank you,
John
 

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Discussion Starter #10
I got the carbs off and they were pretty darn clean (piston, needle, jets, float chamber, etc) except for a few little pieces of dirt or something in the float chambers which I cleaned out. I got the carbs back on and I also replaced the spark plugs and put some SeaFoam in the gas tank. At this point the bike runs perfectly when in 2nd-6th gears but keeps stalling when idling. It'll stall when idling while not moving or sometimes when I pull in the clutch while riding at any speed. I noticed the spark plug tips are sooty black and the exhaust pipes are charred blackish at the ends so, from what I've read, I think the gas/air mixture needs to be adjusted to be less gas-heavy. I'm thinking about adjusting the screws on the outside of the carbs to fix the air/fuel mixture. There are two screws on the outside of each carb above where the carbs fit into the cylinder casing. The xs400 manual doesn't say if screwing them in will create a more fuel-rich or air-rich mixture. Do you know which way I should turn them to get a more air-rich mixture?

Also, in the xs400 manual I see a picture for something called a "Starter Jet Screw" that screws into the bell mouth of the carb (the side that faces towards the back of the bike and connects to the air filters). I don't think my carbs have this screw. I read somewhere else that this screw is built-into the carbs in some bikes. Does anyone know if I should have this screw or if it's built in? What issue will I have if I need this and don't have it?
 
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