I have a 2000 Power Quest Vyper w/500efi's that are expericencing vapour locks on start up after a hot soak. The pumps sound like they are running dry. After multiple key cycles you will hear the pumps load down and start fine. This system only has one electric fuel pump under each harmonic balancer.No mech. pumps. Anyone else experiencing this problem?

Do you have a fuel filter system between the tanks and the pumps? Have you checked the tank vents to make sure they are clear? Do you have anti-siphon valves on top of the tanks?
Almost sounds like you have a suction side air leak which is loosing prime when you shut the boat off? or your vents or a filters are restricted?
Just some info needed to comment on your problems.

Best Regards,
Ray @ Raylar

They added mechanical pumps in addition to the electric pumps on the 500EFI's in 2001, for that very reason - there was a Mercury Service Bulletin for that. Easy way to tell the vintage of 500EFI's from a distance is that the early one's have a "curled" end on the oil dispstick, whereas the later one's have a "ball" on the end. Also, the early 500 EFI's have a straight piece of rubber hose connecting the headers to the tailpipes whereas the "later" 500EFI's, with the mechanical fuel pumps, have a "U" shaped rubber hose connecting the headers to the tailpipes ... how's that for some useless trivia!

yep.....a buddy has 2000 500's and ended up adding the mech pumps to fix that problem.

thanks for the tech support.....calling my mechanic now

Scarab26,
I had the exact same problem 2 years ago when I stored the boat with winter fuel(filled up in late Nov, early Dec.) it starts and runs fine then after the heat soak(maybe 1/2 hour) both engines start and run fine for 30 seconds or so and both stall at the same time. Your options are: burning off that tank of fuel and refilling with summer fuel, adding Marvel Mystery Oil to help prevent the vapor lock, OR adding an outboard primer bulb into the fuel line BEFORE the water separator/filter! I did the latter as an inexpensive troubleshooting tool. When the engines start to stall pump the bulbs and you'll hear the engines pick back up. They are still in the boat in case I run into some more bad fuel........my friends laughed at me and call me a redneck(which I guess I am!) and I now refer to the engines as the Hand Pump 500's! Let 'em laugh but it works, and once I added a gallon of Marvel Mystery Oil and filled it with a fresh tank of fuel the Hand Pump 500's have been running flawlessly. Don't waste your money buying the mechanical pumps! They did this from the factory on the 01's I believe, but it's not necessary if you add the $5 primer bulbs.
Hope this helps,
Paul in Big Rapids, Home of the "Honest Brew"
call me if you want 231 796-1316(make sure to leave a message!)