Be SURE to use only clean filtered fuel to fill the spin on filter or else use a fuel supplement right out of the can. Also, use a clear prefilter ; thus you can place a bright light next to it and see inside the filter to see if it needs changing. I change the spin on filter every 20000-25000 miles and have no problems. The prefilter stays mostly clean and I change it only every 50,000 miles, even though it might not need to be changed. I guess that I've been lucky to get clean fuel. But, it only takes one bad load of fuel to screw up the filters! This is on the 603 engine.

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Paul 1987 300 SDL; 2000 ML; '69 MGB; '68 VW Fastback