Classic/Antique Car Repair: exhaust backfire, zenith stromberg, mgb roadster

Question
Hi Howard, I have a 1979 MGB roadster that I purchased about a week ago.  Everything seemed to be running fine, although there is a fairly strong gas smell, like it is running rich.  Anyway, While I was driving it yesterday, the car sort of started to shudder and lug, as if it was about to stall and die, except that I was going about 35mph in third gear up a mild incline, and my foot was no where near the clutch.  It then proceeded to backfire about every 5 to 10 seconds while still occasionally shuddering.  Once I got home, the car still started up just fine, with no shuddering, although I have noticed that the car has a fairly low idle.  The car has the Zenith/Stromberg single carburetor.  I was also wondering how to adjust the idle, and what I can do to fix the strong gas smell.  Thanks.

Answer
Hi Grant,
The symptoms you describe do not indicate what is wrong, only that something is wrong. You must do a process of elimination to locate the problem.
However, that year MG is notorious for carburetor problems. First look at the air filter. It is a large can shaped canister with its cover facing forward. Confirm that it is clean and seated correctly. You will find a hose connected to the carb and running over to a charcoal canister. Remove the hose from the carb and shake it to see if there are any droplets of gas in the hose. If there is you must replace the charcoal canister. If you find gas in it that means the carb float chamber overflowed and flooded the canister. If it did it is possible that it damaged the Catalytic converter (CAT).
If it is running OK now, take it out on the road and accelerate hard in first and second to see if it will run ok. If it does the CAT is probably ok and not damaged. If the extra fuel had damaged the CAT it will rev up to a point and flatten out as though it had a governor on the engine. It is difficult to get an exhaust gas pressure test but a vacuum test can be done which will help diagnose a damaged CAT. With a manifold pressure gauge on the intake at a vacuum port you rev the engine up and watch the gauge. As you open the throttle you will see manifold pressure drop but it should not ever go below 5 inch on the gauge even at a high throttle setting. A damaged (stopped up due to excess fuel in the exhaust) CAT will cause the manifold pressure to drop down to or close to "0" manifold pressure.
If the car is not running correctly now, for a test remove the hose from the carb that runs to the charcoal canister and run the engine with it off. If it runs ok with it off you will only need to take the carb off and check the float and needle and seat. Also examine the diaphragm for tares.
Other words it sounds like your float in the carb flooded and overran fuel into the charcoal canister and the excess fuel going into the exhaust may possibly have melted and damaged the CAT. This is just a guess and you need to do the above tests. Be sure to have a fire extinguisher handy while doing any work on the around the carb.
I hope this helps,let me know.
Howard