Will it work better?

What type and kind carb is on your bike. Those are a good carb have had good luck with them. Check your jetting and clean your old carb that might be all you need to do. You can order one of those carbs through Dennis Kirk. It is worth checking into. Good luck.

very few at the site have run this carb on a berg. i ran them on my roadracer and they're complicated if they don't come right.

if you don't like fettling bikes then just stick to your old carb. i have spent a hundred hours on the keihin fcr carb and i'm only half the way there.

if you get a keihin fcr we can help.

regards

Taffy

PS> he's very keen on nailing the buyer isn't he? like it's a trap?

Sounds like you need to raise the needle a
position or two.

"Its fine on everything except the very bottom, almost like there's play in the cable but there's not." Dell'Orto! And they are even worse if the bowl level is on the low side. You could try a more progressive throttle cam and try to open the exhaust a bit. On the tight trails you are a hugely better off with an old Mikuni VM 32 or 34 and hard starts and weak idling are history. On full throttle I doubt that you would notice the difference, you wont spend a lot of time on full throttle anyway.
For some reason you seldom see dyno runs covering the very bottom while in real life you spend a lot of time in that domain.
Also, I agree with Taffy:"PS> he's very keen on nailing the buyer isn't he? like it's a trap?"

You can get the stock 38 Dellorto round slide working well if jetted correctly. But if you are looking for super response off the bottom, keep in mind that the Dellorto is better at delivering smooth power. Smooth delivery works perfect on a 501, but leaves a 400 lacking on the bottom. I tried a 38 flat slide like you were looking at on E Bay on my 400. It is a 2 stroke carb and trying to get it to work on my 400 was hopeless. Made good mid and top so a GP motocrosser would like it, but response off the bottom was poor. Finally made the thing work on tight Colorado single track trails by installing a 36 Mikuni pumper designed as an XR 400 hop up. Sudco sells a carb kit which includes a new throttle assembly and cables. Everything fits except the air filter boot. I had to make an adaptor on the lathe that I pressed on the carb. Ended up also raising the needle one groove and upping the main one size. With the 36 Mikuni pumper, the bike was a different animal completely. Small throttle openings would lift the front end with ease over logs and mid range was XR like. Lost some snort on top. Miss that bike.

Dan

Yes, the carb is responsible for a lot of how the engine is behaving, "a different animal completely" is no exaggeration.
Some people dislike the pump in their carbs since they find that in very tight and very bumpy conditions they get over pumped. But that may be a matter of carb tuning.