Backfires could be caused by a lot of things:
1. Low octane fuel (or lean mixture)
2. Incorrect electrical timing (electrical timing is set by rotating the distributor, and the timing light flashes on marks on the harmonic balancer). If the harmonic balancer is coming apart, the marks might be in the wrong location, so someone might have timed the engine incorrectly. Timing should be set differently for different altitudes. If you were driving in the mountains (changing altitudes), the car might have backfired because the timing was wrong at that altitude.
3. Stuck or burnt valves could cause a backfire (when the intake valve fails to close when there is combustion in the cylinder.
4. Cross talk between spark plug wires could cause a backfire. Cross talk means that one spark plug wire arcs to another, causing the other to spark when it should not. If it sparks when the intake valve is open, there will be a backfire.
5. If there is an explosion of unburnt gasoline in the catalytic converter or muffler, it could blow backwards through the carburetor (which is a backfire), and that could stall the engine. It is incorrect to call the explosion in the muffler a backfire if it doesn't blow backwards but blows down the tail pipe and out to the air.
6. A backfire could be caused by a miss (unburnt fuel could explode). But you said that you don't have a miss.
7. Vapor lock can cut off the supply of gasoline by evaporating the gasoline in the gas line. This sputtering engine might not burn all of the fuel and that could create a backfire.
8. The dwell angle might be set wrong (gap of points).
9. An electronic ignition (also known as the computer or brain) could malfunction and cause a backfire.
Since it is doing it at highway speeds, first thing to check is the fuel filter, if it is partially blocked it will starve for fuel as speed goes up. If you eliminate that as the problem, next look at fuel lines, it is getting into hotter weather and you may have a line close to the exhaust causing vapor lock. If so, move or insulate the fuel lines. Even better, do both.
Get back in the box. There is no such thing as vapor lock in a fuel injected car, what's the year,make, model, engine,and mileage?
The car is not fuel injected. Its a 1970 bug air cooled 1300 with one Solex carb.
Sounds to me like it jumped time. I don't know if your beetle has a timing chain or a belt.
A gear could be worn to the point the chain will jump off.Also if it has a belt the belt can break.
I have checked the fuel pump and it is good. I will check coil and distributor too shortly but I am just searching forany other issues. I think outside the box a lot.Thanks
