[914Ghost]

which webers are you using?
I run dual 36mm dellortos with 28mm Venturis on my 1600 cc VW truck.
runs perfect and dell's flow better than webers- so my 36mm dell's flow like 40 mm webers. I run 40mm dell's on both my 914 and my 912- they run FLAWLESS. I haven't seen the engine in the 914 in 4 months!!
You can probably get smaller venturis (throats) for your webers and if you're running anything larger than 30mm then it could help.
Dont listen to anybody that says "GO BACK TO THE STOCK CARBS" without even tryting to figure out what your problem is. Sure put the Zeniths back on then try and find parts..good luck. ANYONE can tune a weber carb setup with a little patience and care.
1st Step. Make sure EVERYTHING ELSE is set correctly- valves, timing, points, etc....
2nd Step Any exhaust/ intake leaks? THat can give you some backfiring.
Now your problem- does it backfire through the intake *POP or exhaust *BANG! Those have different causes.
3rd Step : NOW GET A WEBER TECH BOOK. They're like $15. Seriously..why not? it will tell you what carbs you have whats wrong with them and how to fix it.
4th-You need to tune the carbs- Synchronize the MOVEMENT of the carb linkage- PRECISELY- this is an under valued procedure. Both carbs must work exactly the same.
They must open and close at EXACTLY the SAME TIME and the same amount.
There's your start, get that far and come back. Get the book and you wont need to come back and ask anymore questions....I know from experience.
-Bob O