Webers backfiring?
 

I did a search and didn't see this particular issue so I am asking for help. I have a problem with backfiring through the carburetors on my 70 911T. I bought this car a month or so ago and it hadn't been running since 2000. I took it straight to my mechanic and he got it mostly sorted out and running well except for this backfiring issue. It idles fine and when you get on the throttle it seems fine. The backfiring happens just above idle when you are not giving it a lot of gas. Also, it is worse as the engine gets warmer. Already checked for air leaks around the carb stacks etc. We tried richening up the idle jets. Checked timing as well. My next thought is to check valves? Any ideas?

Thanks for the help.
Kirk

where are you?

are throttle shafts tight?

i THINK valves out of adjustment can contribute.

I am in Illinois.

It could be alot of things but start with the basics - Take them apart and clean all the jets and mixture screws, etc.... I have 44's in my speedster replica and they are notorious for getting dirty. Floats may need adjusting as well. I am having exactly the same issue with mine right now and plan to do a thorough cleaning in the next few weeks before I put it away for the rest of the winter. If a good cleaning with carb cleaner doesn't do it then I'll be ordering rebuild kits. My car sat outside pretty much all summer. A lot of dust and debris can accumulate from simple exposure as well as gaskets and seals rotting. Again, give them a good once over before anything.
Remember to pay attention to where your settings are so you can return them to where they were when you put them back in. (i.e. count the turns to close before you unscrew them and remember to close them up and unscrew them back the equal number of turns).

I hope that made sense.

I should have mentioned that we took them off and rebuilt them as well. One thing we noticed was that these have a size 32 choke rather than a 30 like the specs say. I am guessing these may have come off of a 911s. It doesn't backfire on decel. Just light accelleration and sometimes pops at idle when it has been running a while.

Pops (muffler) or spits (stacks)?

Spits through stacks.

as a general rule i understand stack spits to be lean and muffler pops to be rich. BUT i think this is assuming a good valve adjustment as a starting point so absent verification that valves are adjusted right we can't say with absolute certainty that this is the case.

Can you replicate the spitting while the car is in neutral and thus hold a mirror (to collect fuel droplets) or paper over the stacks to see which throat it is? If so i'd check the adjustment for those valves.

lots of popping/spitting in my webers was resolved by fixing loose throttle shafts. no amount of tuning was going to get around it. i would further surmise that any tuning to negate these symptoms at idle would leave you with imperfect settings further up in the rev range.

note that there are experts on this forum at webers and i am not one of them. i'm more of the 'pre game show'.

We did richen it up the idle jets and that helped but didn't completely solve the issue. I think the next step will be to check valves. I will also check the throttle shafts. Thanks for the help.

kirk

Although I have very little experience with Webers I had a similar problem with the Zeniths on my 69. It turned out to be the point gap. I would definitely check that you are getting a correct spark and that your timing is right. I would look into replacing your distributor cap, rotor and wires.

I trhink I might be barking up the wrong tree. I just realized that the distributor on this car is a 2.7 dizzy. I am thinking that is the problem. How difficult is it to have it re curved or should I just buy the correct one?