Sounds like it's just a bit out of tune but being a pumper there can be a couple of other little problems depending on the age and condition of the engine. If you go into the glow engine forum there is an OS site for factory support, you can ask Bill Baxtor. If the engine is used and been stored the diaphragm can be a bit hard or there can be some castor in the pump. Just little things but it only takes something like that to jump up and bite you. Bill should be able to tell you or give you some ideas to try. When in doubt I always ask Bill.