Hello - I am troubleshooting an issue with my 1974 DT250A where it seems to die out after about 10 minutes of running, and will not restart until it cools down enough.  Including what i've tried so far but wanted to see if anyone had suggestions on other things to try. I am running out of ideas! Thanks. What I’ve Tried So Far Engine compression measured (both cold and hot, after dying).~110 PSI observed when the engine is cold. ~105 PSI measured when the engine is hot, after dying.  Both tests were completed with WOT.  Some decrease but small amount and still above expected value (14.7 x 6.8 = 99.96 PSI). Scoped inside of cylinder for excessive scoring / possible indicators for loss of compression.  Some scoring was observed but I didn't think it seemed to be anything too excessive indicating top end rebuild would be required. Photos included. Carburetor - Original 28mm carb cleaned and reassembled with original pilot jet (#60) and new main jet (#140).  Needle clip in center position.  No cracking on the intake boot.  Air set screw at 1 ½ turns approx per manual. Note (Also purchased replacement Mikuni VM28-49 carb thinking this was the issue initially, no change in symptoms between either carb.  Rebuilt original and reinstalled.) Seems to idle and accelerate well before it eventually dies. Reed Valve - Suspected original reed valves may not be seated properly when the bike is fully warmed up.  Replaced factory reed valves with aftermarket Boysen 606 Power Reed .  No change in issue. Air Filter - Factory air filter box replaced with (oiled) “two-stage” UNI UP-418AST pod filter.  Didn’t think this was necessarily the cause but no change before / after switching. Ignition Coil - Original ignition coil secondary resistance measured at ~11kΩ (manual specifies expected 5 kΩ +/- 10%).  Replaced w/ aftermarket equivalent. Note (Ignition “Condenser” capacitor and contact points replaced previously, timing set with machinist dial gauge per repair manual, i.e. just opening @ ~3.2 mm BTDC.) Fuel Cap - Thought fuel cap may not be venting properly causing vacuum / fuel starvation.  Ran bike with fuel cap open to test no change in symptoms. Next Steps / Things to Verify Exhaust Gasket - Failed exhaust gasket causing air to enter engine? Spark Plug - Spark plug was replaced recently but could spark plug be failing when hot? More Invasive Potential Repairs Crankcase Seals - failed crank case seals causing oil / air ingress to engine?Top End Rebuild - warped cylinder gaskets, piston rings causing air ingress to engine? Potentially an issue with oil injection system?

 from near Salisbury, UK.

What make are the replacement condenser and points? Loads of repro crap out there. Also fake spark plugs in circulation.

Greetings from Tennessee 

clogged fuel tank vent-10 mins is about normal for this
crappy condenser-must be really crappy if shorts in 10 mins

cliff

1st thing I would have checked is if the oil pump is working and that the feed line is not clogged.

Thanks for the recommendations so far. Checked auto-lube at banjo bolt on cylinder head and it was feeding oil so should be good there. Going to see if i can get OEM condenser from my local yamaha dealer & replace.

Blimey that a lot of reading but you explain it well thanks. So after all that my only thought is the ignition source coil in the mag is failing then recovering when cooled. I will say though that points source coils are pretty bullet proof or you drew the short straw. One more random thought is the fuel cap vent although you might've mentioned that. When it fails is it instant or starts to run ratty first. Fuel starvation would often go a bit gasps & lean before running out. Ignition is more instant misfire to the end. But wait, there's more lol-- carb vent to the bowl & up the side not blocked or trapped somewhere or as someone did recently maybe not on our site but joined bowl vent to side vent & that causes fuel issues.

While you’re at the Yamaha shop pick another spark plug. I had my Ct1c do the same thing and it was the spark plug I switched to autolite 4054= ngk b8es

Partly clogged up fuel tap?

Somewhat mind boggling reading about all that work and then a question about an easy to change $2 part that fails often.  It might not be the spark plug but I'd start there and make sure you aren't buying counterfeits.