This is a direct quote from Jim Inglese's technical discussion on his website:
"THE IMPORTANCE OF IGNITION TIMING
This is one of those things I can't stress enough. I wish I had a dollar for every time somebody had a Weber setup that ran terrible, that was corrected by timing. This is the single most common issue with a badly-running Weber system, bar none. Very few people have an understanding of, nor pay enough attention to ignition timing. So I will say it plainly: WEBERS NEED A LOT OF IGNTION TIMING! IT IS ABSOLUTELY CRITICAL. Set the total advance at 38 degrees BTDC and forget about "intial" timing; whatever it is, that's what it is. Initial timing means nothing; total advance is EVERYTHING. If it's too retarded, you'll have a lazy, rich, engine with farting and popping carburetors, and way too much heat in the heads, which can make the carburetors boil the fuel in the bowls...and drip. It will cause all sorts of issues...none to do with the carburetors...but they'll get the blame most of the time."
Cam selection is critical to gain the maximum from Webers. Again, from Jim:
"Any cam is fine as long as you pay attention to the lobe separation angle. A 110-114 degree lobe separation is ideal for Webers. I generally discourage the use of anything tighter, like 108, or 106. The problem with a lot of overlap is that it causes "reversion"...reverse pulses from the exhaust stroke that have nowhere to go but up into the carburetor. This disturbs high rpm airflow and causes the engine to think it's too lean at high rpm’s. Without a plenum, the flow is pulsed backward up into the carburetors because there is noplace else for the reversion pulse to go. To find your cam's lobe separation angle, check your cam card. Just add the intake centerline and exhaust centerline and divide by 2."
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I followed his guidances to the letter, and now the Webers on my Lola are terrific. My suggestion: Call Jim. He'll give you the straight story.
Hope this helps.