MG Car Repair: idle problem, propane bottle, manifold gaskets

Question
Hi Howard, my 1965 Sprite has an idle problem. The rpm hunts from a set(as best as possible)1000 rpm up to 1200 then down to 800 rpm over several minuets. this is with the car stopped and in neutral. The 1200 rpm is after revving the eng and over time drops down slowly. While it is dropping it still fluctuates up and down 300 rpm to 500 rpm. Most of the time it stays within the 960 rpm to 1100 rpm. When driving the car and coming to a stop the rpm has been 960 rpm sometimes 1000 rpm and even 1200 rpm. The distributor is a new Lucas repo 45D4,plugs and plug wires are new.The accelerator cable is new. The car starts up fast and runs good. I tried using a propane bottle around the carb shafts and gaskets and around the intake manifold gaskets with no rpm change other then the usual hunting. However placing the bottle at each carb inlet dropped the eng idle. I expected it to go up sharply. The smog valve hoses have been replaced, the valve inspected and cleaned. The valve has been checked by removing the oil cap and seeing a increase in rpm.

                     Thanks Al

Answer
Hi Al,

"Hunting" at idle or surging when driving as a steady speed is a symptom of a lean condition (among other things) However, propane in the intake, and an idle smoothes out or slows down is a sure sign of an intake leak.

I found for the most part working in dealerships for many years on MG, A-H, Triumph and Jaguar that symptoms can lead you to an area but other then that, they are useless. Testing is the only way to be sure of finding a problem and injecting propane into the intake is a good test.

You must at this point block off ALL vacuum from the intake manifold to pin point the cause. Then connect each one up one at a time and test run the engine each time.

Howard