Got a 1972 LT2 100 yamaha starts fine but cuts out after a minute or so. Just stops dead as if turned off with the key. Checked and cleaned carb and all components, new air cleaner, new spark plug, fuel taps fine and fuel flow, clean fuel tank, compression seems fine but have no tester to check.
Can anyone point me in the right direction, thanks

I'm guessing you mean LT2?. T next to R on keyboard. So to stop dead like that with no hint of leaning starving for fuel before stopping i'm guessing electrical. Heat & vibration can damage the source coil in the mag but they're pretty reliable as heavier windings than modern cdi mag coils. Be good if you could quickly check for spark after it stops. Have a spare plug in your pocket so not wasting time removing existing one. How soon will it run again--instant--couple of minutes--walk home. Check points gap & points clean but if it runs it shouldn't suddenly cut out without a misfire. Set plug gap close like 20 max & points at 12-14max just for a try. Is the lighting coil ok & not loose on its pole as think--not sure--the lighting coil might be involved to earth the ignition source coil. Could be main plug coil but unlikely although there's some cheap ones out there to go try. I forgot condenser but if faulty you could get a very large flash at the plug but a spark with no oomph in it. After all that woffle--not out of fuel lol.

Thats great thanks, plenty for us to look into after work. Will let you know. Thanks again. Ps. Defo not out of fuel lol

Gas cap vent plugged.

I am dealing with a 100cc bike that runs and idles well but as soon as you shift into second gear the bike just falls flat on it's face. ( KE 100 1978 ). I am leaning in two different directions. The bike is very clean and rust free. The odometer shows 10,000 miles.

( Either it's jetted too rich or the exhaust pipe needs to be burned out ). Soon enough it will expose itself..

Thanks for suggestion, tried running with cap open but no different

With the KE100, does it rev right out if you give it a good handful in neutral?. If it won't rev then lift the air filter out then try again & then if 'no' then pull the exhaust baffle out & try again--but not close to the neighbors haha.

The engine will rev out in neutral. It will also rev out in first gear. The brand new foam air filter is dry ( not oiled ) and I was planning on removing it - to see whether that's an obstruction of airflow, in addition to the aftermarket ( Indonesian ) gas cap that has 3 feet of hose on it. The baffle is missing. The previous owner had rigged up something out of copper and I have removed that. I am certainly appreciative and open to all suggestions regarding this rotary valve small carburetor conundrum..

Ok, let's go the other way & consider it might have a partially blocked main jet. Will rev with no load but gives out when the 'real' pressure is on. Could give the choke a flick or even leave it open at the moment of fading out. I'm too far away to be useful in NZ but they're a good little motor. On early Yamaha rotary valve little bikes the steel or alloy center drive used to rip out of the disc but they were totally fiber where i'm guessing the KE is a steel disc or something. Anyway--when the center let go it would run but barely pull itself along as the valve just randomly spun & not timed. That's all clutching at straws though. We love videos lol with good audio if pos. Just thinking back--55 years haha- we also had strange things happen if the vent hose from the carb was inside its housing & not vented to outside as under revs it could keep the floats up & not let fuel into the bowl. Stop for minute or just idle & would come right. Model i'm thinking of the vent pipe could exit out a hole underneath. All pretty unlikely events though.

Sorry AnthandEmma i've sort of wandered off your case. keep us up to date with progress, if any. Cheers.