I was working on a Clark c500 80 today that has an impco LP system, model 100 carburetor, cobra vaporizer (the one that looks like a model j, but different pressure), and standard vacuum operated fuel lock valve. It had an intermittent no start problem which I traced back to the fuel system, it seemed to be a faulty fuel lock, so I installed a new one and went on my way after starting and stopping the engine several times.

I was called back several hours later for the same problem, and just as before, the fuel lock wouldn't open even though the vacuum line was firmly connected on both ends and the starter was spinning the engine at a good speed.

I took a vacuum gauge reading and found that it was barely making enough vacuum to move the gauge needle, but if I used my hand vacuum pump to open the lock valve, the engine would start up normally and stay running with the vacuum hose reconnected, even though the gauge was still showing almost no vacuum. When the engine runs, it seems to run pretty good for a Jurassic age flat head I6, so I doubt that the compression is low or that the diaphragm in the carb is punctured, also the gasket between the carb and manifold is sealing well and the throttle shaft is not worn out.


So my question is how much vacuum is needed to properly operate one of these lock valves? And if any one has a good theory to explain the low vacuum problem that this engine is having I'd like to hear it.


My next question is how did Clark stay in business as long as they did building crap like this? Was there a time when an updraft carb mounted below the top sill of the frame in a narrow channel and right next to the exhaust system was actually a good idea?