It just seems to me that this is a timing issue. If you have no vacuum leaks, if coolant is flowing (don't really think it would be that anyway) and if the carb is functioning properly... I think you need to take a close look at your CDI. Switch it out and see what happens with the stock unit. Just my two pennys.

Which vacuum line? The one that goes to the intake boot?
Despite my early protests to the contrary, I am going to focus on an air leak. I know I said there were no vacuum leaks, but I just cannot imagine anything else that would cause such an extreme lean condition. I mean with a 145 Main and 42.5 pilot my exhaust header is turning molten red like hot lava. The sparkplug tip is completely white. Strange thing is though; the buggy runs and accelerates great. This is a real head scratcher for me. Anyhow, I ordered a new intake boot. I am going to remove the existing one tonight. Heck, maybe it doesn’t even have an O-ring gasket. Who knows? I was also thinking about wrapping the boot with electrical tape, and replacing all the little vacuum/coolant lines on the carb.

Any popping out of the carb?

If there is a vacuum line for the petcock you could try that but then you'd have a closed petcock.

I'm with bomoco, I'd switch out the ignition. Did this problem start when you put it on?

No vacuum petcock thank God. Yea it pops like crazy when I let off the gas. I will try the CDI first, since that is an easy thing to switch out.

Let us know how that turns out.

Is it popping at the carb or exhaust. If this is timing you must be off real good. My hh25o runs best full advance. Could be though. As far as flow, take all the plates and add them to my supertrap and just watch your bottom end leave. You can only flow as much air as the cylinder can burn fuel. At some point you hurt your self. On a fully modded 150 with a stock carb and open exhaust you aint running that size jet. I know this is a 250 but still. I have seen guy's run crazy jets though and fo what ever reason it worked well.

I think that lean with a 140, your carb is messed up or your sucking air. Maybe a bad diaphram

Whether it's electrical in the CDI or it's an air leak, the one thing that is certain is that your jets are way, way fat and you still got a lean machine. Starting with the CDI makes sense as it's the easiest fix. If that doesn't fix it, then you got an air leak or something else wrong in that carb. Getting the CDI out of your head will narrow it down to the carb and all the wonders it holds for you.

This is true. I would take the mods off one at a time till you find it

if its not the cdi, look reasl close at the throat of the carb. I have seen some new carbs have a casting flaw in the throat of the inlet, it will look like a fine tiny crack in the casting