ClassicCar

Does it sound like the the missfire is a missfire in the carb or the exhaust?



If in the carb, it sounds like the engine is running lean. That could mean carb running lean due to low float or a gasket not sealing or a vacumm leak of some kind, maybe the manifold. The easy way to determine a leak is, with the car warm and idling, take a bottle of MAPP gas (you could use propane) and put a small hose on the end of it.Capping off the end of the bottle and reducing it to a small hose can be tricky. Meter as small of an amount from the bottle that you can ( Ihave a control that has a tapered needle in it to allow a very small amount to gas to be metered) (DO NOT LIGHT the gas) and direct the hose around the carb throat. If the idle increases, then you do have a lean condition. You can then direct the hose around the base of the carb, the intake manifold where it seals and anywhere else that you can see that has to do with vacumm lines and hoses. I do not suggest using carb cleaner to spray in these areas as I have seen them ignite into a fire.



If the noise is from the Exhaust you may have an exhaust leak near the manifold and the entering outside air is acting like an afterburner. This backfire usually occurs on decleration though.



Hope this helps.

Ron