Yeah I agree but don't know what it could be. It doesn't seem to be the carb - I can draw air through the hose and have looked at it and there's nothing blocking it.
I meant to check the PVC yesterday but forgot when it fired up and had a little bit of vacuum. I got happy and took a chance to drive it a few miles to the gas station. It's got a dead bog but if I baby the throttle until it's rolling it runs pretty good.