Ok, I've been lurking here and gathering information for a while but still can't sort out the issue with my LC4. The story so far: I bought an LC4 640 engined supermono off a friend at work. He had got the engine mounted in the Aprilia rs 125 frame, installed all required running gear and had been trying to get the thing running for about 6 months with no success. During this time he had replaced the starter clutch but that was it as far as engine work. It's an all black 640 from an Enduro (apparently) but due to the sketchy history I don't know for sure which model it's from. I bought it thinking I'd have it running in a couple of weeks. 6 months later and it's still a non-runner. The list of things I've checked so far is: Mechanical: Cam timing - checked, rechecked, rechecked, flipped it 180 degrees to check that the engine was in fact sparking at every TDC. Then rechecked the timing, then flipped it back 180 degrees to the original timing. Autodecompressor - checked it each time I've had the cam out, spring is still good and the peg still has nice sharp edges with no signs of rounding. Makes a good 'CLICK' sound when turning the engine over by hand. Flywheel - each time I've had the cam out at TDCC I've checked the position of the timing mark on the flywheel and it's been at the correct point in the sight glass. I checked this to make sure the flywheel wasn't slipping and knocking the timing out. Compression test - run a compression test and got around 160psi (can't remember exact figure, it was a while ago and I remember thinking it was easily high enough). Fuelling: Carb - it's got a BST40 on it, I've stripped it 3 times, cleaned it in a parts cleaner at work, rebuilt it, checked float heights, checked the float vavle is actuating at the correct float height, checked free movement of everything. The carb had a massive jet in it when I stripped it the first time so I put a 157.5 (I think, it's the jet size recommended on here for the BST 40 with a straight through exhaust). There was a TPS on there as well but that was removed by the previous owner, leaving a peg sticking out of the side of the carb. I don't think this leaks air, but I could be wrong. Choke - replaced the crappy plastic nut where the choke cable goes into the carb, also bought a new choke cable and plunger/spring assy. The 'make shift' fuel tank I'm using is a funnel hung from the garage roof that I fill with fresh fuel for each start attempt - definitely clean, good fuel. Electrical: Flywheel pickup - resistance about 100 Ohm (from memory, it was in the middle of the allowed range of resistances, I think it's 80 to 120 Ohm.) Ignition coil - resistance wasn't correct so I bought a second hand one from a running 660 LC4 off ebay. CDI unit - the only thing I couldn't check with a multimeter so I bought a new one. It's a DC-CDI from Ignitech (Czech company). The DC bit means the CDI doesn't need the AC input from the stator like the standard unit so this eliminated two possible problems that might have been stopping the engine from running. I've put a couple of new spark plugs in (incase the first one had some sort of defect). Checked continuity between all electrical components. All of this and the bloody thing still won't go! I've been really thorough when checking/testing everything so I havn't missed anything (I don't think). I've had covers off both sides of the engine and given everything a good visual inspection and couldn't see anything that might stop it from running (no big chunks of metal/piles of swarf etc..) So currently I have a bike with fuel (plug is wet after long starting attempts). Spark - nice fat blue spark when testing with the plug out and also testing with a plug in so it's got enough power to spark and turn over under compression. Correct valve timing. And compression. Why then, will it not run? I am on the edge of pulling the head, splitting the cases and fully rebuilding the thing from the ground up. Or the other option is buying a grey engine and get a bit more power. But I don't want to give up and buy a new engine, I want to make this one work. So, any help, advice, opinions or general banter would be appreciated. As I say, I'm ready to take drastic action. Cheers, Sam.