Preface, I am not mechanically knowledgable, everything ive been doing here ive learned in the last 2 weeks.

Hello Everyone, I made another post recently asking about batteries where i learned this bike needs an automotive battery. Another user in that thread suggested I meter the bike to see if it is charging.

To clarify, that means that the voltage on the battery will show 12-13ish volts not running, and if i give the bike throttle and measure, I should see voltage increases up to like 14ish volts, because the bike is charging the battery, correct?

I decided I would try to do that, but because the battery i have sucks, I used jumper cables to a car battery. The bike just turned over a bunch and wouldn't start. I believe this rules out the battery as the current issue, even though i still need a battery.

Digging a little deeper, I noticed a smaller Red wire, which ran from what i think is the starter relay, to the back of the bike where it had a fuse (20amps, which seemed like a lot to me) The smaller red wire atttached to this was in rough shape, I thought i might be having a grounding issue, so i pulled back the electrical tape and found out that the wire was essentially melted along most the length of it. It was a strange goey mess stuck to all the other wires, melted plastic and bare metal. No other wires were damaged, so i replaced it and changed the fuse to an inline blade fuse.

I really thought this would solve my issues, but it didnt, the bike would not start when i hooked the car battery back up, however, it turned over much faster, and sounded like it was on the cusp of starting. I was able to start the bike with a push start for about fifteen seconds.

So that was tonight, these are the other things i have done and information I have.

When i bought the bike, it started easily using the electric start. I put maybe 1.5 miles on it, and was satisfied.

This has been reoccuring, when the bike sits overnight on a charger, it tends to start the first time, run for 3-4 miles, die at low rpms, then be unable to start, electric or push.

I have cleaned the carb, main, pilot jet at least 6 times. Set the air to 1.5 turns out, and idle to moderatley fast, because tach doesn't work. I installed a new carb gasket, the old one was broken. New oring for main jet, I adjusted the float to 21 mm

To clarify the technique for this is to remove the bowl, turn the carb upside down, and measure from the gasket to the top of the floats, correct? Without pushing on the floats, just letting it rest.

The spark plug has good consistent spark when kicking over or using the electric start.

There are 2 wires hanging off the rear fender to the left side, green and brown, that are snipped. I was guessing these are turn signals, but im unsure. I dont think that should be causing my problems though.

When the bike is running, it does not like wide open throttle, there is some hesitation. At 3/4 or so it accelarates smoothly.

That is everything I can think of. If you have any ideas or suggestions,I would love to hear them. Please include lots of detail.

Thank you.

Ok taken with the caveat I've never yet worked on a 12v Yamaha, yet my first car was a '65 Chevy Corvair.. Sounds like you have done due diligence on the carb, and electrical, unless for some reason your spark plug is not sparking under compression, which is not totally unheard of. As a rule of thumb, if an air cooled engine runs and then dies for no reason (yet is hot) the coil or condenser are the prime suspects. I don't even know if the 12v Yamaha has a coil so can't help you on that. You can use compressed air upside down, sprayed on the condenser to cool it quickly and test if it failing under heat. If no positive results from this; personally, at this point, I'd start looking for an air leak and test compression. I am sure MarkT will time is as he knows more than I do, and you should always follow his advice before anyone else as he is spot on.

I wonder if you’re not losing spark because the battery is toast. What’s the voltage after it quits and won’t start? My battery is 2 yrs old and it stays around 12.35 and does fine as long as I run it every couple days.
Setting the float height is from the gasket surface, not gasket itself. Would be about 1.5mm difference. The snipped wires are for the rear turn signals. Dark green and brown. They ground to the rack.

Thanks, I will do some coil and condenser reading on this site and the manuals. My dad has a compression tester, i'll check that as well.


ill get about 12.6 volts after a charge, but after a minute of cranking it'll read 10.5. I did crank it over with a fully charged car battery, and also with the battery charger attached and on the jump setting, and neither of those things helped it start.

Thanks for the comment about the float height, i took the gasket out when i did it so it'll be a smidge off.

I guess when I was trying to clarify the instructions, I did the opposite. If you took the gasket off, then you measured correctly. Hope you get it sorted out. They’re great bikes.

Yesterday I was on my way out the door when I responded to your other post about the battery.

You were asking about an "alarm" battery. Those batteries work fine on a magneto bike where all they need to do is run a few lights. They are not up to the task of running the ignition and starter you have on the AT1. And yes, fuse is 20 amps. You need an "automotive type" which is what I meant to say... a battery designed to provide ignition and cranking power. Stock replacements or a Motobatt should be readily available.

As far as your problem not starting with the jumper battery. It might just be flooded. Please remember to turn off the petcock when parking... and also make sure the petcock is not leaking and actually turns off. Accidentally leaving the petcock on is a VERY common reason for a bike to not start after sitting overnight. Is the plug wet when you remove it?

If plug is wet, with petcock off, try push starting with throttle held wide open. When it fires, keep throttle wide open... it will smoke a lot and sort of skip and not rev up well. After a few seconds it will start to rev cleanly... as that happens, start closing the throttle. Once it's running clean, you can turn the petcock on.

Could be crud in the pilot jet or simply a bad spark plug too. And no, seeing spark outside the engine does not mean the plug is good. They can spark fine outside the engine and fail under compression.

On the burned wire... I would kind of recommend not using the electric starter so much. They are not that tough. Cranking for a minute as you mentioned... if done more or less continuously... is enough to overheat things and could be the reason for burning the wire. Even modern cars recommend only cranking a few seconds and then letting things cool off and rest before trying again.

On the float level... I don't trust manuals and specs. Many are messed up. On all Mikuni carbs, correct float level has float arms parallel or almost perfectly parallel to gasket surface. Sometimes when you go by the book, the arms end up at an angle. This is wrong. After setting level, arm should be parallel.

The carb shown below is similar to yours, not the same exact one... with arms parallel the float height is 26mm. If I had a manual that incorrectly said 21mm, the arms would be at an angle and level would be too high. If you only made a tiny adjustment to the float level, you're probably fine... if you had to move it a lot, I would recheck.



Finally, the dies after running a short distance is classic bad battery and/or charging system issue on a 12v 125. But it could be something as simple as a plugged gas cap vent.

On the charging system, the manuals in the tech library here have comprehensive troubleshooting instructions. The earlier AT1 had the same basic system and earlier manuals often have more details. There's also a 1969 Yamaha "Dealer Shop Manual" that goes over the general theory and operation of the different systems Yamaha used on their bikes.

Thanks! lot's of stuff here.

I ordered a new petcock and spark plug. The petcock doesn't work at all, so it probably has been flooded like you mentioned, which i why push start has been more reliable.

ill clean out the jets again too. I believe I had to bend the float arms to get it into that manual spec, ill play around with it leveled out instead.

WHen the bike died last time, i noticed a lot of bubles coming up from the carb into the gas tank. That makes me think i might have a plugged vent like you mentioned.

Thanks again for the suggestions!

You've learned lot in two weeks! Keep up the fight.

Thank you! It's been actually a lot more enjoyable than I thought, if rather intermittently frusturating.


Minor update, I installed a shutoff valve, since the petcock isn't working. I was able to get the bike started, Still having some issues with the float sticking, but I got a multimeter on it.

My battery read 12.5 volts after charging overnight, While running, i got readings jumping all over the place, up to 19v and negative readings so fast i could barely read them. I am guessing that is a good thing, just how the bike charges, little bursts as the charging system spins.

Would that lead you to believe the charging system is good? My current poor battery was reading 10.5 volts after 5 minutes of riding, I think it is just unable to take a charge from the bike's system. I'm hopeful it'll be good to go wednesday when the new battery comes.

Not positive, but I think it only charges above 2000 rpm. So you need to check at that rpm. The manuals are really good on the electrical trouble shooting.