Hi, all. I'm having issues with my 1973 Bavaria. A few weeks ago it started running intermittently rough at low idle (sub-2500) and occasionally backfiring when I let off the pedal. Whenever I came to a stoplight or stopsign and let off the gas, the tachometer would dive to zero, bounce around a little bit, and the exhaust would sound pretty choppy. But then about half the time it would drive absolutely perfectly for my entire drive. A couple of days ago the car was again running rough at low idle as I drove home from work. As I arrived at my house, I let off the gas and there was a very loud backfire. When I went out to start the car the next day it would not start. It just cranks but no sign that it is even close to starting. It's as if it had no gas. Any comments/suggestions? Thanks.
AG
As a supplement to the other good suggestions . . .
It might prove helpful if you described the engine’s overall condition, including maintenance and any upgrades. Without more specifics, long distance diagnosis is shooting in the proverbial dark. You could potentially have a compromised head gasket or skipped timing gear teeth - or something far easier to resolve, like a loose coil wire. If your engine will crank but not start, you need to tell us more, including whether you have decent compression and whether your spark plugs are receiving adequate firing voltage (blue spark) and whether fuel is being suppled to both the carbs. That includes clean, uncontaminated, fuel. From your symptoms I also have to question whether you might have a marginal fuel pump that barely supplies enough fuel to those thirsty carburetors? Ruling out something major, without addition information, I would consider performing a thorough tuneup with the hope that this will correct something obvious.
Assuming your engine will run, when you describe your problem as "intermittent," also resulting in the tach dropping to zero coupled with backfiring, my first inclination is to look for a poor ignition ground. Ignition cut out (intermittent rough running) has been known to occur after hard braking and sharp turns, especially with soft/worn motor mounts. Check for loose distributor/coil wiring or wiring shorting out against the bare metal bulkhead.
The secondary ignition leads “may” still hold a distributor in place despite a loose hold down assembly. Make sure things are secure and not susceptible to occasional “unwanted” movement.
Still using the original points-triggered ignition system? If so, have you examined all under-the-cap wiring, including the condenser? Maladjusted points and/or a slop in its movable mounting plate can lead to unwanted swings in ignition timing - and backfiring. Ditto, a nonfunctioning vacuum advance system.
Weber carburetors? Are they are equipped with idle shut-off solenoids that function properly? If one or both carbs have solenoid “issues,” including possible dirty idle jets, this can lead to poor idle and part throttle operation. Can also affect getting the engine to start. It is doubtful this would result in major backfiring, without additional contribution from something else that would affect either ignition timing or valve timing.
The search function can be helpful regarding common questions. Same with trouble shooting checklists found in most shop manuals ;-).
HTH