I took a vehicle I owned to a performance orientated dyno shop about 15 years ago. It had a worked 400 CID small block Chevy in it. The engine had previously made 445 BHP naturally aspirated with a 750 vac secondary Holley. I picked up a Powerdyne centrifugal supercharger for a good price so I fabricated some brackets and a drive pulley, made a carb hat, and set it up blowing through a 850 double pumper Holley. The engine would obliterate the tyres at low rpms in any gear, but as soon as it hit 3K it would fall on its face. I couldn't find anything clearly wrong (it wasn't my first boosted rodeo) so I was forced to let the moths out of my wallet and engage a "professional". The dyno guy assured me he had lots of experience with blow through Holleys, and indeed the showcase in the foyer had a range of blow through carby hats for sale. First pull it made 240 odd HP at 2500 rpm, flatlined to 3000 rpm then fell off a cliff. AF ratio was in the high 11s until 3K then it plummeted to 5 to 1 (at this point the exhaust pipes just poured out thick black smoke) and the engine wouldn't rev any further. (I couldn't see the black fuel smoke when I was road testing it because of the tyre smoke

). Over the next 4 hours, multiple times old matey would say "I know what it is" and proceed to make changes. Jetting, power valves, spark plugs, ignition timing etc, and with each change it lost power.

At the 4 hour mark (after a 140 BHP pull) I decided enough was enough. I asked the guy to take it off the dyno and said I would go home and put my tune back in it and do some research. He wasn't keen, and said "I'm pretty sure I know what......", I cut him off and said "I want to take it home, If you lose any more power it won't get up my driveway".

Anyhoo he said he'd do me a deal, and "only" charge me $100 an hour, seeing how he'd made it run worse.
I got home, put it back how I had it, (which as far as I was concerned should have been a close tune) and jumped on the net. I came across a Turbo Mustangs forum in the States and most of these guys' turbos were blowing through a carby. I found a thread where the guy had pretty much the same problem as me. He had a side entry carby hat too and worked out that the boost air blowing across the top of the float bowl vents was actually siphoning fuel out of the float bowls once the air speed was high enough. He cured his setup by pushing rubber hoses over the vents and running them down into.the boost pipe facing the incoming air. I did the same mod, pulled out the gate, nailed the throttle, and the car instantly stalled and wouldn't start again. I must of had a bit more boost than the internet guy as the incoming air had instantly forced all the fuel out of both of the float bowls via the main jets, and drowned the engine.
I pulled the hoses off, fabricated a couple of little brass elbows and fitted them with restrictors (a couple of old jets) and mounted them to the vents tube facing the boost pipe. Hey presto, problem solved, the engine would now cleanly hit the redline in the blink of an eye.
As all of my subsequent builds have also run carburettors, that was also the last time I've taken a vehicle to be dyno tested. I'm not in the habit of spending good money to go slower.

As daughproto said, make sure the dyno guy knows what a carby is before you let him anywhere near it.