Yamaha YBR 125 Custom yellow light no priming sound

[husker]

Hey guys,

I'm new to bike maintenance / troubleshooting so please excuse the lack of correct terminology. I have a Yamaha 125 Custom and have used it without much issue for nearly a year. I have since left it parked up (covered) for the better part of 2 / 3 months and now I'm having issues starting it up. The problem is when I turn the ignition on and notice the yellow engine light come on I don't hear the priming sound it normally makes - it's completely silent. Attempting to start the bike results in it chugging away and doesn't start.

I've had this issue happen before when I left the bike parked up for a few weeks and read online that it might be trapped air in the pipe that is used by the fuel injection process; I remember a suggestion to leave the fuel cap open for a while, closing it, and then attempting to turn the bike on again. Doing this in the past fixed the problem in the end but it took a while to get there; by that I mean, every attempt at turning the ignition to on would result in seeing the yellow light come on and I could hear the fuel injection priming - but struggling to do so. With each attempt, the priming sound started to sound better and eventually (after switching the ignition on and off various times) it sounded normal and bike was able to start.

I've tried this again but to no avail - turning the ignition to on results in seeing the yellow light but I hear no priming sound whatsoever despite trying numerous times. I presume the root cause is that the bike has been left for a while and either the battery has lost a lot of power or something is wrong with the fuel injection. So before I purchase a new battery I have a few questions:

1. Can anyone explain why the priming sound would go away if the bike is left stationary for a period of time? Is it related to the battery potentially not having enough power?

2. Can anyone explain to me how my initial attempt at solving this problem worked? I can't be sure if leaving the fuel cap open to allow some airflow into the tank made any difference; my guess is my repeat attempts at turning the ignition on / off may have brought my battery back to life in some way.

It did cross my mind that it might be the spark plug; but it is fairly new - about 4 months old.

My next step is to grab my battery charger and attempt to charge my battery in the hopes its simply a case of my battery having lost most of its charge due to not being used for a few months and maybe the fuel injection process is not drawing enough power from the battery to work.

Any advice on this issue would be much appreciated.

Thanks.

[Tracer1234]

Does the yellow light flash? If so, count how many long and short flashes are. On my old YBR, this could then be used to reference a fault code?
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Riding: Yamaha MT-09 Tracer Occasionally Riding: 08 Suzuki SV650, Potato: 2011 Yamaha YBR Custom.
Used to ride: 2015 Yamaha MT-09 Tracer (smidsy) 09 Triumph Street Triple (P/X'd) 08 Yamaha YBR (Sold)
CBT 04/14. A: Mod 1 & 2 13/04/15

[husker]

[Robby]

Battery needs a charge. It's cranking slowly.

Bike batteries, particularly the smaller ones, can lose their charge fairly quickly. 3 months is usually enough time to go from starting to not starting.

Batteries also die from old age, which can be anything from 3-10 years depending on a variety of conditions. I don't know how old the bike is, but if the battery is 5 years old or more I would be tempted to get a new one.

Finally, we are now in spontaneous battery death season, or Autumn. Batteries don't like cold. This is the time of year that a lot of batteries die.

[RhynoCZ]

[ThatDippyTwat]

[ZaphodBeeble]

I'd start by checking the voltage at the terminals of the fuel pump immediately after turning the ignition on. You should see at least 12V there. If you do get that voltage, I would expect the pump has seized. In that case (or perhaps try anyway) slap the tank about a bit on the sides. It might work the pump loose.

I've included a schematic of the tank.
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Current bike: VTR1000F Firestorm. Previous: Firestorm (written off) - XJ600 Divvy - NSR125 - DT125 and a load of offroaders.

[Paulf]

I am old and suffer from brain fade. But did not somebody else have problems with a dead fuel pump and it was a known problem on 125 yams that they just pack up. Probably not but worth searching on here.
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Past bikes simsom mofa 1s, suzuki hustler 250 (£35 in 1973), suzuki ts125k, mz150, yamaha yb 100, honda pc50, honda vision 50, all 2 stroke - then cars - honda pcx 125 (sold) own honda fes 125 and Mrs 125 spares or repairs. Started 1972 all accidents walked away no insurAnce claims

[ThatDippyTwat]

I think that was something to do with direct sunlight and a hot tank. Never had an issue myself.
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'98 VFR800 (touring) - '12 VFR800 Crosrunner (Commuting) - '01 KDX220 (Big Green Antisocial Machine)

[Fizzoid]

Yes, as Dippy said, there's an issue where if the tank is exposed to heat (direct sunlight) and gets warm enough, the pump won't prime and thus the bike won't start

Happened to me during this last summer on 2 separate occasions, but it's certainly not warm enough this time of year to be that!