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I don't agree - I have VHB's on my Guzzis and have no problems with them.
Choke plunger rubber seals (where it presses against the fuel outlet) do go hard and get indented, so worth changing as they are cheap and available. You can't really tell by looking at them, but if they leak you'll always be rich. Also make sure the mechanism lets them seat properly, whether cable or flip-lever type.
Are you sure the carb settings are correct? According to my book, 250 with VHB29 should have:
Needle V13, on second position
Main jet 110
Pilot 40
Slide 40
Atomiser 265M

I completely agree about the choke advice, even a tiny leak of the cold-start jet will give a very rich mixture. I always make sure that there is at least an eighth of an inch free play on choke cables to be sure.

I do like to try and keep the bike original but I have had success with alternative carbs on some of my other bikes.
Will buy new choke seals and try it again before swapping out.
It may not be directly a carb issue, The first time I strpped the carb after a first run there was a fait bit of suspended sediment in the float bowl, it looked like very fine rust.
I cleaned the carb and fitted some see through fuel pipe. When I returned from a quick trip late yesterday, I never thought to check the fuel pipes, this morning I can see more fine sediment in loop sections. I think this could be an/the issue.
Draining the fuel pipes, (which have filters that pass the sediment) the bowl looked a reddish brown colour.
Here is my daft theory, bike is runs ok at low revs, as soon as I open it up the fuel flow demand is greater so picking up more of the suspended sediment and blocking the main jet? After riding it twice it does feel more like fuel starvation, open the throttle more, is just gasps with no firing.
I will need to clean the tank again before any more running.
Is it possible to get lighter throttle return springs for these Dellorto's, the one fitted looks original (compared to pics) but is far too strong for a throttle return spring.

Regards

Ha ha! It's usually Guzzi owners who complain about the throttle spring pressure. Talking of sediment and fuel starvation, I once had an issue with my 900 mis-firing, finally stopping completely. When I carried out the usual roadside checks I found that the float bowls were empty, and no fuel was coming through the taps???? Later, I found that the fuel tap filters were completely blocked by flakes of paint that had obviously been sprayed down the filler neck in the factory and over time had flaked off and settled in the bottom of the tank.

The problem with your issue is that a failure of the ignition system will give exactly the same symptoms as carb problems - the spark fails when the throttle is opened. Think of all the exercise your brain will get trying to solve it!

Just drained the 2 litres form the tank, it looked light brown pouring out.
Lots of fine rusty sediment in the tank.
Will start a new thread on how best to get the loose rust out.

Regards

I cleaned out the tank, not particles after a quick drain check.
Transparent fuel lines showed clear fuel.
Reset the needle to the correct 2d positon and started the bike.
Took it for a run and still the same after 4500 rpm, just gasping and stuttering. Stopped the bike to see how high it would rev in neutral, I could get it to about 6-7000 but not cleanly, quite rough. Under 4500 ish smooth and clean.
I really thought it was debris blocking the fuel flow at higher rpm but just proved it was never that.
Not sure what to look at next, I will try and source an alternative carb in the meantime.

Regards

I am no expert but think you need to check advance with strobe even if it's to eliminate timing fault before trying new carb.

Jordan suggested removing the air filter, and you said it was new. But its newness isn't relevant. How big is it? Have you done a test without it? My 250 Monza in daily use exhibited similar behavior one day. It started and idled nicely, as usual, but stumbled severely at 15MPH and refused to go any faster. After some puzzlement I remembered oiling the K&N the day before. Popped it off, voila!
Later, with excess oil drained from the mesh I was able to refit the filter. By removing yours you could at least eliminate it as a potential cause.

Is the ignition system original? If so, then it is getting on for half a century old...... I moan about the condition of many of the parts on my Strada, forgetting that it is over 40 years old.

It is a paper type air filter given as new when I got the bike, maybe it is not even correct. I will remove it and the filter tube.
It looks old but unused, maybe the paper has been damp and the airflow not as good as I should expect.
I think at those revs I should be on the main jet, which looks clear whenever I check it. There was no sediment in the float bowl after yesterday.
All you need to run is air, fuel, compression, spark.
Air flow/leak, possible the filter could be restrictive, I will remove the filter and re check the inlet manifold rubber.
Compression is excellent being all new components, rebore, pistion, valves ground and clearances checked etc.
Fuel, checked all fuel system, seems ok
Spark, all parts are original and of course ancient but it seems to give a very health spark with visual, starts and runs up to 4000 ish very well.
It could be that the ign advance is failing at 4500 rpm but I expect that it should keep sparking at pre advance stage and would run but with poorer acceleration not just a gasping zero.
I have a timing wheel, rather large but I don't have any adaptors to fit it onto the Ducati.
Can anyone tell me how the Ducati Eletronica ignition system does its advance, I assume it is all totally electronic with no owner adjustment.
Could it be a spark plug? it is new.
Re plugs, I have an 81 bonneville project that I fitted new NGK plugs as per forum suggestions, it run great for 50 miles then would not start.
form a great starting and running machine to a non starting backfiring wreck, I would check the plugs, usually damp with fuel from failed starts but they always sparked. I tried everything for 2 months, new coils, yet more new leads and caps, new alternative carbs (that i kept on)...nothing.
Turns out that another forum user had the same issue, 50 miles on new NGK's then backfiring, it was the wrong heat grade of plug!
Seems the post 79 models need a hotter plug or that happens, the vast majority of those bonnies are pre 79 and used the same plug type for decades.
Fitted the next hotter plug and it started and run beautufully, the old plugs looked like new but would not run.
Could plug heat range be a problem with Ducati singles?