1985 Mercury Outboard V-300 V-3.4L Shop Service Manual, Page 64Get this manual
FUEL PUMP DescriptionOperation Th fuel pumps are crankcasc- pressu re-opc ratcddiaphragmtype pumpCrankcase pul sating pressu re (created by the upand-dow moveme nt of cy linde pis ton) is lransfcrrCli to the fuel pumps by way of pul se hoses between crankcase and fuel pum pThe fuel pumps work loge her in series, 1he starboard mounted fuel pumppumps the (uello the carburetors
Checking for Restricted Fuel Flow
While anti-siphon va lves in thefueltank may be helpful from II safety stand-point, they clog with debris, th ey ma he 100 small, or the may have too heavy pringSummarizingthe pressure drop across these valves can, and often doescreate opera ional problems andor powerhead damage by rest rieti ng fuel 10 the fuel pumps nd carbure torsSo me symplO ms of restricted (lean) fuel flow, which co uld be cau sed by useofa anti-siphon valve, are: Loss of fuel pump pressure Outboard (Iu its and ca nnot be restarted Outboard will not start Vapor lock Si nce any type of antisipholl dev ice must be located between the olllhoard fuel illiet ancl fu el tank oUllet, si mple method of checking rif uch device (o bad fu el) is problcm sou reel is 10 operate Ihe out board with separa te fuel uppl whi ch is known to be goods uch as 6gallon (22Y:z liler) fuel ta k