Hello all, first time posting but been reading plenty of posts over the past year or two. I have a 1971 AT1 that my grandfather bought new and rode alongside my father until 1974. He put it in the garage sometime in ‘74 (with only 5,000 miles on it) and it sat there untouched until 2023 when I inherited it. 

I immediately decided I wanted to get it back on the road, and surprisingly some new gas, a new battery, and a carb clean got it running almost immediately. I’ve since put some new tires and tubes on it and changed out the oil…but otherwise it hasn’t needed anything. I originally had some trouble where it wouldn’t run for long, but I found the points were staying closed rather than cycling. Purchased some new points, but ended up having enough luck just adjusting the ones that were in there and cleaning them up a bit. Has been running consistently ever since, so I don’t plan on touching them haha. Now that I can get out on the trails, I got my son (10) a used PW-80 and we hit the trails together every few weeks. 

The bike starts real easily, even starts using the electric starter if it hasn’t been sitting more than a few weeks. It gets up to 50MPH on the roads leading to our trails no problem, and seems to be running fine as long as I keep it moving. I never rode it before 2023 so I can’t compare to how it originally ran, but to me it seems good enough for what I would expect out of 125cc’s. My only other bike is a ‘94 CBR-600 which isn’t really a useful comparison. 

However, it seems that whenever the bike gets hot and we don’t move enough it dies on me. If I ride the trails alone or hit the street (and therefore keep it moving) it runs great for an hour plus and never shuts off. But I’m finding that with my son just learning to ride I’m spending a lot of time sitting idle while I walk back to guide him through an obstacle or help him pick a line. When he does get moving he’s very cautious, maybe 5-10 MPH for a big part of the trail. On a hot day (80+) I can get about 20 minutes of this before it just flat dies. It will still turn over no problem and the electric start will crank and crank and crank, but not even a sputter. I have a clear fuel line and I can see it’s still full when this happens, so I tend to think it’s a spark issue. Each time I walk the bike or sit with it off for about 10-15 minutes and then it will fire right back up. Depending on how much we move around after it starts back up it will run for some time before it dies in the same way again. We’ve been out three times where this happened and each time at the end of the day I’ve ended with it dead, loaded his bike into the wife’s van, and then having cooled off…started it back up and driven the 10 mile trip home without any hint of an issue. 

My best theory right now is that something
in the ignition is getting hot and failing, then once it cools off it starts working again. My two best guesses are the condenser and coil but I don’t have much experience with this style ignition. Both seem to test just fine on a bench, but that’s when they are cold so I’m not sure I consider that as valid info. I also get an interesting noise when the key is turned to the on position. Almost sounds like a relay contact bouncing on and off (like a small metal piece vibrating). Does it every time and I’m not sure if that’s just a normal noise for an older bike or is somehow another symptom you guys are familiar with. 

I don’t mind throwing some parts at it, after all…it has been great fun and I love seeing it back in the trails (especially since some are the same ones my grandpa rode 50+ years ago). Would you guys recommend I replace the condenser, coil, or both? Is it best to go with the originals I see on ebay, or are some of the aftermarket ones I see on Amazon good to use? Anything else I should consider or do before heading down that path?

Thanks in advance for any help!

Greetings from Tennessee 
 I would put a new spark plug in. I had that
happen to my Ct1c one time and it fixed the problem 

 from near Salisbury, UK.

Great story, and wonderful that the bike is still in the same family.

It could well be the condenser. When buying any ignition parts, stick with original or a respected repro such as Daiichi. Some folks on here swear by Autolite spark plugs instead of NGK - NGK B8ES = Autolite 4054 and NGK B9ES = Autolite 4052. 

Regarding the clicking noise, I suspect that it is the flasher relay. Are the indicators fitted and do they work?

Photos would be great, but you need to use a third party host such as Imgur.

I'd also try the spark plug first...  and make sure gap is set correctly 0.020" to 0.024".

Second most likely in my experience on that model is the ignition coil...  especially if you're aware of a time when the key was left on without the engine running.  Coil can overheat in a few minutes on that model with key on and engine off and I've seen them melt!

I would have said charging system but you said battery isn't low or dead.  Clicking could be the voltage regulator?

Replacement electrical parts are a crap shoot...  lots of junk out there.  Economy Cycle in Texas has good OEM quality ignition coils for the RD350 Yamaha which is the same ignition coil on your bike. 

www.economycycle.com/shop/yamaha-rd25035...ignition-coil-23-99/

Welcome. Had similar where the breather pipes hanging down from the carb were blocked so check that/those. Also I'm wondering if the rings are stuck in the grooves so how does it feel for compression when it stops. Feeling by hand on the kick start turning through. No comp, some comp, great comp etc. None of my thoughts really stack up though if it goes ok when clicking along at speed. If it's electrical I'd expect erratic miss firing before stopping. Possible coil or condenser issues but ideally a video to see & hear how it plays out would be a good help. I have an AT1 1970/71 but has flywheel magneto ignition.

Thanks for the replies. I forgot to mention about the spark plug. When I first got the bike running I decided it would be cheap insurance to replace the plug, so I bought a B8ES to match what the book said. Went to install it and found the bike for some reason has a B7ES installed rather than the B8, and the head of the plug looks different from the B8 (threaded top). I tried the new B8ES in the plug wire on the bike but it doesn't fit into the wire as far as I can tell. Is the B7 similar enough in spec to keep running it? Should I replace the B7 with a new B7 to see if that helps, or am I better off trying to source an original spark wire so I can use the B8 I purchased? I read a few articles online and some suggested a hotter plug could result in overheating, so maybe you guys are onto something there.
Photo of plug:  imgur.com/eUU1KvG

As for compression, I am no expert but it sounds/feels exactly the same when kicking whether it is running normal or in one of its 15 minute periods of failing to run. I certainly don't notice it feel any easier to turn or anything like that. And yea, I have run it for 30+ minutes at highway speed without issue...it only seems to be an issue during slow rides in the trails. I'll try to get a video tomorrow of it running but to me it sounds like it's running fine. No backfiring or roughness or anything leading up to when it stops, it just basically goes from idle to off without any drama. And no matter how many kicks I give it, not even a pop comes out the exhaust for the 10-15 minutes I need to wait before it then starts right back up and runs as if nothing happened.

As for the noise, I guess it's possible it is the indicator circuit but I had assumed that relay only got power when one of the indicators was activated. This happens every time the key is turned on/off even if no indicator is set. Yes, the indicators are on the bike and working perfectly and the noise does not happen when the indicators are going (it just gives a single tick with each flash like you'd expect)
Video of the noise:  imgur.com/DcG21P3

I definitely left the ignition on for an entire 30+ hours at one point. If that's a possible way to kill the coil, I would say it's a likely possibility for me. I leave the bike in gear when I park it and I wasn't used to being able to leave the key ON without some sort of obvious indication (my only other bike headlights are on 100% of the time if key is on). Learned that lesson by killing a perfectly good battery (which has since been replaced) and now I make sure it's off before I put it away for the night.

Considering coils and condensers are what I would consider relatively cheap, I might just bite the bullet and replace both to be safe. Thanks for the link to EconomyCycle, that's where I'll go first if I decide to start throwing parts in it.

And here is a photo of the entire bike:
imgur.com/Kpl7QxH

That ally type top on the plug is removeable - just use a pair of pliers. Then it will fit fine.

If you can, humour me by removing either of the wires (brown and brown/white) to the flasher relay and see if the noise persists.

Put your hand on the relay & you'll soon know or feel something in line with the noise.
Be sure its earthed as i think those early 3 wire big flasher units can pack a punch.
If not that, is it coming from the big original regulator & cutout.

If you left the key on, the ignition coil definitely could be damaged. It's under the tank and you might even see that it melted the plastic. 

New spark plug is still critical to try first and make sure you're buying from a reputable source.  Counterfeits are out there and NGK has discontinued the B8ES (replaced by a BR8ES).  Not a great idea to use the BR plug with stock resistor cap.

Spark plugs heat range is about the heat path of the center insulator/electrode.  A hotter plug has a longer heat path so the tip gets and stays hotter.  This "heat" is about keeping the electrode hot enough to stay clean...  but not so hot that it turns into a glow plug and causes preignition and/or blisters and melts the electrode.  The whole "hotter plug makes the engine run hotter" is BS...  unless of course the plug tip overheats and becomes a glow plug.  Not likely at all in your case.  But I would use the correct heat range. 

"Counterfeits are out there and NGK has discontinued the B8ES (replaced by a BR8ES).  Not a great idea to use the BR plug with stock resistor cap."

Not that it helps the majority of members here, but these are still available in the UK.