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Having dabbled with veg oil my opinion is you have classic fuel restriction. Fit a good electric fuel pump, ( not a mini SU, a JAG SU or one of the sport electric pumps available) which will supply a good steady flow, as seen thru your clear pipe. Test run from, in my case, a two litre coke bottle 'cos you can see the fuel recirculating, and you should see no bubbles once primed. Test with half a tank and don't let it drop much below it, you will be able to see any air in the recirculating fuel, even with the engine stopped, pump running. If you have any air leaks or ingress this should show.
A while ago there was a topic about low internal IP pressure and how to deal with it. Given these pumps are getting on for 20 yrs old this may be something to do with your probs.......
The above JAG pump worked well for me. Connect to ground and the cut off valve. Keep the pump points clean!
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