Engine dies, then starts, but won't rev over 2k..

[Wozza]

So, when I go for a ride, it usually goes something like this:

Bike comes out garage. I spend the next few minutes getting it started (kick-start, and often bit of a bitch to get going). When started it runs ok (apart from not firing on the left cylinder at tick-over, but that's another problem), it does tick-over and does rev normally (other than bogging down before hitting 2-3k rpm). So off I go. Then a few miles down the road, I come up to a junction, I stop, and the engine dies. So I wheel the bike to the edge of the road and try to get it started again. When I do get it started, I have to hold the throttle fully open for the engine to stay running, however it doesn't go above 2k rpm. If I release the throttle, the engine just dies. (Bike is a 125 two-stroke twin)

The first time this happened to me, I parked the bike up, left it for about 10 mins, came back to it and it started up ok. However the last two times, I started up engine again straight away, held the throttle open to maintain the 2k rpm, clutch in, first gear, run down the road and gently let the clutch out so as to force the engine to rev higher. It then kicks into life and I can ride on as before, but this method is obviously potentially dangerous!

What I have done recently:
Fitted new spark plugs.
Cleaned both carbs and adjusted float levels (to raise the bowl fuel level, particularly in the left carb because of the tickover problem I mentioned earlier - no it didn't fix it)
Cleaned baffles - although not the rest of the exhausts. Also the left exhaust has a piece of connector pipe dodgily welded on, so who knows what could be going on in there. The baffle in this exhaust is also shorter, maybe something to do with the connector pipe.

Thinking it may be a tank/tap/cap problem, earlier I drained the tank (through the tap and pipe with cap in place) into a couple of jerry cans, but it came out fine (using both ON and RES).

So it would seem that fuel is getting to the carbs fine, and fuel should be getting to the cyclinders fine. I better mention that the two airbox-to-carb rubber hoses have a split each, but I've gaffa taped them up as a temporary measure so they should be pretty airtight. Also the foam element is dry and needs oiling, but I'm thinking that alone isn't likely to cause any big problems.

I do have a pair of new (used) exhausts on the way, but again I don't see how it could be the exhausts alone causing the problem as it comes and goes rather than stays.

Now I'm thinking maybe something with the ignition, but I really don't know.

Extra info: The bike is down on power, I think due to worn piston rings/pistons, and is a bit rattley. I think maybe the lack of compression is causing the left cylinder to not fire at tickover, but thats bit of a stab in the dark. I'd like to get a compression test soon to confirm if I need new rings/pistons. I haven't been able to balance the carbs due to the left cylinder tickover problem, and subsequently I haven't checked/adjusted the oil pump, which I still should do.

Bike is a Suzuki X4 125 (GT125)
Sorry for the long read, and I hope someone can help!

[Frost]

Yip sounds like the rings might be gone. Get a manual (should be able to find one online) and return everything on the carb to standard. Compression test the engine (testers cost like £15), if its down on compression new rings will cost about £20, might need a rebore (£40), replate depending on the lining, and a new set of pistons (£60). A bit annoying but at least it will be good as new and shouldnt need touching for ages.