A few years ago I bought a 1984 CR500, made sure it ran, rode it a couple times and put it away because I had things to do before I could get to it. A couple of years ago I found a 1983 CR500-same story. Earlier this summer I dug the '83 out and started to put it in order. After getting it pretty well sorted, I started to ride it like I meant it, and found out that it would "die" when I had the throttle pinned, just like it was running out of gas. Figuring it was a fuel problem, I started addressing items that could cause fuel starvation. I have done lots of jetting changes, opened up the petcock, made sure the fuel line was big enough (5/16) even modified the drain plug/main jet well to open it up around the main jet. No results. Got ahold of another 40mm carb that is in obviously good condition and went through all the usual suspects with it, nothing. Thinking of the old adage that most carb problems are really ignition, I put another Motoplat on it with no change. I've set the timing as fast as the factory spec 2.8mm to as slow as 2.0mm. This has been my summer and I'm pretty much stumped. The problem only shows up at about 5/8-3/4 throttle and above, under load, at which point it dies out. I can come to an immediate stop and it comes down to a nice even idle, so I'm assuming that the float bowl is full. I can hold the throttle wide open and it will die, then run like a striped ape for a second then die again, not like a lean condition but like its running out of fuel. Main jets have been as small as 330 up to 390, needle valve assy is a 3.3 and I've even opened it up a bit. I've set the float level from 19mm to as high as about 15mm at which point it leaks fuel out of the overflow when on the sidestand. All other jetting is pretty much factory spec. It isn't RPM related, I can pin it in first on flat ground and it continues to run until I'm scared to hold it open any more. In second or higher on a bit of an uphill it will die out before it gets close to max RPM's. It starts pretty easy considering what it is, it runs really well after all the fine tuning I've done with the exception of high throttle openings. A week or so ago I decided to quit messing with the '83 and get the '84 going. Guess what? Same damn thing exactly! What am I missing? Someone here has to know!