This is a result of actor oil in your fuel. The castor bean is a relative of the soy bean, and we all know that you can make anything from a sweater, to a soda bottle to a hamburger from soy beans. It's similar with castor beans. As the fuel flows through fin mesh screens and small passages, it extrudes the fibers from the oil over time, and will build up to a point where it won't flow. I had to clean out my pre-filter on my syringe this afternoon, also. This is also why I run a filter in the models I fly also, to give this some place to happen other than the spray bar. the only way around it is to run all synthetic oil or fly electric. I just try to clean filters whenever I think about it as a preventive measure.
Type at you later,
Dan McEntee