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View Full Version : 5.7l vortec carb (vapour locking?)



knufnel
08-16-2010, 03:37 PM
This motor is in a harbercraft jet. It will start fine, run for as long as you care to go, but when it gets shut off for a while it will start again, burn the fuel out of the carb and die. Then won't start for the next hour. The dealer seems to think it is vapour locking. I'm not convinced. Anyone have any ideas?

makomark
08-16-2010, 10:38 PM
electric fuel pump? maybe the oil pressured controlled switch is thermally intermittent?

Could be "vapor lock", especially given today's "fuels"....Take some IR gun readings next time it hot soaks. Most OEMs, when they spec it, don't want any temps above 150 at the carb's flame arrester level, and ~ 110 (def F) anywhere on the fuel path.

Two things that may help: 1) run the engine at fast idle a few minutes and then shut it off, & 2) when doing the warm start, slowly advance the throttle to at least half way and then crank it over. As soon as the engine starts, pull the throttle back to ~ 1500 and once the rpm stabilizes, normal idle.

knufnel
08-17-2010, 01:30 AM
when it does this there is no gas flowing through the carb. I was also wondering if it might be the oil pressure switch. Is there also a fuel pressure switch in the pump? When I ran a hot wire directly to the pump it pumps but won't shut off. But still would not start. There is a yellow wire that also comes off the oil pressure switch that I am not sure what its for.

makomark
08-17-2010, 10:47 PM
Yellow, usually with the red tracer, is the ABYC color code for the starter's starting circuit.

There is usually only one switch controlling the pump and that's typically driven by oil pressure. I've never seen a mass produced marine engine with a fuel pressure activated pump control.

Sounds like you have two separate issues. I'd make sure the fuel pump control circuit is well behaved and then tackle the 'won't start hot' issue.