5.0 XL Mercruiser possible carb/fuel pressure issue

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crazy charlie

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1988 Merc 5.0 XL 4bbl carb. 20+ years and never an issue starting .Had to switch over to an outboard fuel tank this season .Started and ran fine 1st time out. Next day started right up and stalled shortly after.Would not restart. Checked fuel water separator and fuel usually drips out when changing it but excessive amount of fuel came out while unscrewing it and it seemed pressurized.I let approximately a pint of fuel run into a jar placed under the separator.Still would not start.Tried the old screwdriver in the primaries plate and it started and ran fine.Started several times after that.Sound like it flooded?? Puzzled with the pressure of the fuel?? Charlie
 
Scott06

Scott06

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crazy charlie said:
1988 Merc 5.0 XL 4bbl carb. 20+ years and never an issue starting .Had to switch over to an outboard fuel tank this season .Started and ran fine 1st time out. Next day started right up and stalled shortly after.Would not restart. Checked fuel water separator and fuel usually drips out when changing it but excessive amount of fuel came out while unscrewing it and it seemed pressurized.I let approximately a pint of fuel run into a jar placed under the separator.Still would not start.Tried the old screwdriver in the primaries plate and it started and ran fine.Started several times after that.Sound like it flooded?? Puzzled with the pressure of the fuel?? Charlie
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you need to check the fuel pressure when cranking to see what it is. Wonder if the outboard fuel tank vent was closed and pushing fuel to filter when you were changing it.

If fuel pressure is ok you need to see if it is flooding out (too much gas), no gas, or no spark when cranking.

Why did you switch to an OB tank? I assume you had fuel contamination? if so you could also be in need of carb rebuild as well. Mikes carbs sells good quality kits. Have used him for years but I dont think he is rebuilding them anymore
 
C

crazy charlie

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Scott06 said:
you need to check the fuel pressure when cranking to see what it is. Wonder if the outboard fuel tank vent was closed and pushing fuel to filter when you were changing it.

If fuel pressure is ok you need to see if it is flooding out (too much gas), no gas, or no spark when cranking.

Why did you switch to an OB tank? I assume you had fuel contamination? if so you could also be in need of carb rebuild as well. Mikes carbs sells good quality kits. Have used him for years but I dont think he is rebuilding them anymore
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Outboard tank vent was open. I assumed closing the vent would slow the flow so tried that too. No start until I held carb open. Bad fuel tanks forced me to use outboard tank for now.Fuel is new, separator is new.Thanks for the reply, Charlie
 
Scott Danforth

Scott Danforth

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if your primary on-board tanks are dirty, assume the dirt is now stuck in the anti-siphon valve, the carb, and other places.

you may have to go thru your carbs
 
Lou C

Lou C

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That sounds to me more like a sticking needle valve in the carb inlet or one fouled with a bit of crud keeping it from closing off the fuel flow. If it seemed flooded, that's more likely unique to the carb than the fuel filter or outboard tank. What carb does the engine have?
Was the outboard tank new, or used, clean on the inside?
 
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crazy charlie

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Quadrajet I believe, new tank.Started right up today and ran for a while with no issues. I have never had an issue with this motor so never changed carb filter.May do that next if I can find the filter I bought a number of years ago. If I dont find it I can just clean the current one if dirty. Im just puzzled over the pressure I found when unscrewing the separator. Charlie
 
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cwella

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crazy charlie said:
1988 Merc 5.0 XL 4bbl carb. 20+ years and never an issue starting .Had to switch over to an outboard fuel tank this season .Started and ran fine 1st time out. Next day started right up and stalled shortly after.Would not restart. Checked fuel water separator and fuel usually drips out when changing it but excessive amount of fuel came out while unscrewing it and it seemed pressurized.I let approximately a pint of fuel run into a jar placed under the separator.Still would not start.Tried the old screwdriver in the primaries plate and it started and ran fine.Started several times after that.Sound like it flooded?? Puzzled with the pressure of the fuel?? Charlie
Click to expand...
Sounds like it may have flooded. The pressure at the separator isn’t normal for a carb setup, so check for a stuck carb float/needle and seat or fuel pressure issues. Next time it won’t start, hold the throttle wide open while cranking (don’t pump it). If it fires, it’s likely flooding. Also check for fuel dripping into the carb when the engine is off.
 
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