Can someone give me a hand with 3500gt?

It sounds like the popping is happening at the carbs and not the exhaust ... correct?

My advice ... take it to a mechanic that knows Webers. This is an area you do not want to mess with unless you feel comfortable in what you are doing.

Sometimes popping though the carbs is cause by too lean of a mixture. Perhaps a carb is out of adjustment, perhaps there is an air leak somewhere, or perhaps it is an ignition timing problem, or perhaps it is something else.

Carb backfiring can be very dangerous, especially when the air cleaner has been removed.
Each time a carb pops some fuel is being sprayed out. Many years ago I almost burned my house while starting a poorly adjusted Vignale spyder.

Ivan

 

Rob, I had a couple of issues with my Webers....one of them was similar to yours. I had a fuel starvation issue that required the float levels to be adjusted. The other issue was with my front carb. I had the same popping and backfiring coming from the carbs and we isolated it to the front carb. In one afternoon, a friend (who specializes in Webers) and I had the carbs off the car 3 or 4 times trying to resolve the issues. Since my restoration took so long, I had fuel sitting in my freshly rebuilt Webers for over a year. The fuel formed deposits in the float chamber and a piece of debris plugged up the accelerator pump jet on my front carb. As soon as we removed the debris from the jet and adjusted the float levels correctly, the Webers ran perfect. It took us about 5 hours to sort out the problem. I rebuilt my Webers and was surprised to find an obstruction in a carb that I rebuilt. It was due to the carb sitting for so long with fuel in it. Since your restoration took a while, you might have a similar issue. Good luck.

 

Ivan has pretty much covered the problem with his "or perhaps it is something else"
Mark has offered the prospect of a clogged jet or other orface. I agree with Ivan in that you should take it back to your mechanic or another competent with webers mechanic.

I highly suggest you re bush the butterfly shafts as you will never get the webers in tune when the shafts are leaking air, upsetting the air/fuel ratio.

Given your admittedly low level of skill, I highly recommend that you do not fool around with it.
My 02 Lira,
FGM

 

You may also want to simply spray some WD40 or brake clean on your throttle shafts and see if the idle changes. This will give you an idea of how good or bad the throttle shafts have worn over the last 50 years.
All you weber equipped 3500 owners may want to try this test.
Ciao,

FGM

 

Hi Rob,

A lot ca be said about possible causes: Plugs, ignition, leads, caps, contact breakers, condensor, fuel pump and pressure, dirt in the carbs; a lot!
Did it happen at once or worsened over time?
Did you already check if you have any faul plugs?

Ciao,

Bart