Only runs with choke out

[Getdeeper]

Out for a longish ride this week, I sensed the engine dying on a stretch of A road. Identifying this as moving onto the reserve tank, I moved the fuel tap accordingly and it came back to life. But as I continued, the scooter was bogging dreadfully at low revs but burst back to life at at full throttle. It would not idle at lights and was effectively stalling every time I eased off the throttle. I limped to the petrol station, assuming there to be some crud in the reserve tank but after filling up, the problem remained. Oddly, pulling the choke out solved the problem instantly and now it's the only way the bike will run or tick over.

Having researched similar issues I removed the carb and gave it an extensive clean including the jets. I also replaced the carb to box gasket and the carb to crank gasket looked fine. The problem persists and I'm now struggling to think what else might be he cause. The fact that the problem started on the transition from main to reserve tank suggested to me initially that this could be a fuel quality issue. But now I'm thinking it may have been a coincidence. The suddenness of the problem manifesting is confusing me because the scoot was running perfectly up until then.

I have a persistent but minor oil leak (about the size of a 20p piece on the floor each morning) but I have put this down to carb spray back which I can see in the box.

My understanding is that running only on choke suggests a mix which is too lean. But other than that I'm stumped. I'd be grateful for any help or suggestions.

[NICK]

Air leak on the cylinder head? cheers

[Getdeeper]

Air leak on the cylinder head? cheers

Perhaps. How would I be able diagnose that do you think?

[Scooteria]

I had similar symptoms a couple of years back on two occasions with different faults.
The first time the beast would only run with the tank half full or more. Under half full it needed choke.
Turned out the fuel pipe was pinched between the side of the oil tank and the chassis. The pressure from a full tank was enough to overcome the pinch.
The second time, 2-stroke oil was getting onto the slow running jet, blocking it. This was cured by fitting a 'tuners delight' air filter with holes drilled above the jets.

If you're running fine on full throttle but need choke for lower revs, I'd suspect the slow running jet - especially as you say there's oil in the carb.

Remember there's only one fuel tank, the lower part being reserve.

Happy hunting.

[Getdeeper]

Thanks all. Great advice as always. Turns out it was indeed the slow running jet which, for reasons unknown, kept getting clogged up with something. Cleaned it out again and bingo, problem solved. I'm hoping the petrochemicals in the carb box are just spray back rather that pan anything related to this issue. I'm guessing that crud at the bottom of the tank got drawn into the carb. Thanks again.