Yeh "they"re crap " esp on a 2 stroke",. remove it , and restrict the back of the carb, , almost completely [ strong tape.] or even someones hand,
it will then pick up fuel and fire, ,, find a stock airbox/ filter etc
Tell mike drewps " things are lookin up , http://www.ebay.co.uk/itm/HONDA-CB750-KO-1970-MOTd-NOVEMBER-2015-PLEASE-WATCH-THE-VIDEO-/261887456431?hash=item3cf9b574af:g:ZegAAOSwpDdVUx8X
lOOK AT THE PRICE OF this , still time left, . needs full resto ,
http://www.ebay.co.uk/itm/yamaha-rd-400-f-orginal-condition-to-restore/272032473957?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140107083358%26meid%3Df06daf325a4b416ab7b7ca0e69161bec%26pid%3D100011%26rk%3D5%26rkt%3D10%26mehot%3Dpp%26sd%3D151869934171