Here is my Key Start Mustang I found many years ago and I have always had trouble getting it to run first go.
You pull it a few times and it starts for about 4 seconds before stalling. Pull it again once or twice and same thing. This may go on five more times, each subsequent time it runs that bit longer until eventually I stays on and I can cut my grass as normal. This is how it has always behaved.it is a beautiful smooth motor once going and has a label on the top of the engine that says QUIET in bold letters and it certainly lives up to it.
I have checked the paper filter element on the side of the engine and it looks pretty decent to me. I suspect the carb is the culprit but haven't bothered to pull it apart as I know I can eventually get the old sheila going and what a pleasure it is to use, once going.
What is the likely best remedy? Replace or try a clean out of the carby. I have heard a replacement is inexpensive, but I need to know the type it has first.
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Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!