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2000 Mercruiser 5.7L 250 HP backfires

DCinSC

New member
2000 Mercruiser 5.7L 250 HP backfires The motor starts and runs. However if I slow down then throttle up again it will backfire through the carb it sounds like. It will bog down and then pick up speed again. I checked the coil .8ohms primary. 11.48K secondary. Any other suggestions what would cause the motor to backfire through the carb? I havent replaced the fuel filter yet this season. I will do that as soon as it arrives.

It is a Thunderbolt V. It is not EFI

Thanks!
 
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I havent replaced the fuel filter yet this season.

Ayuh,.... Backfirin' through the carb is a sign of a lean condition,.....

Examine the Contents of the filter for anything but clean fresh gasoline,.....
 
Sadly for your wallet, with boats you have to change out all the fluids and filters before you put it up for the winter, then turn around and change them again when you pull it out for use. Belts and tune up should also be a spring commissioning thing to include cap rotor wires plugs and if equipped, points and condenser. Storing a tank of gas over the winter isn't going to make it go bad, but you do want to run the carb dry or drain the float chamber.
Your Y2K marine lump may not be updated to run on ethanol-laced fuel. If you haven't already, replace all the rubber fuel lines with Shields Flex-acol or equivalent which are resistant to the effects of alcohol on older rubber.
If it was running OK before and is now exhibiting these symptoms, it's probably just fuel starvation due to glop in the line somewhere.
 
Need more info.

Is this back fire happening when accelerating from idle speed and if at crusing speed and you push trottle for more speed?

Back fire thru carb on acceleration is typically a couple issues.

1. Accelerator pump in carb is old and needs replacement. This forces raw gas iinto the carb as soon as throttle is applied. If it is not happening it will act like your deacription and it can cause a back fire...(lean condition)
Note: The above assumes no fuel delivery issues, good gas, clean filters etc.

2. A bad exhaust valve/ rounded Cam lobe.
If there is a bad valve to head seal or the valve is not opening propely it will back fire repeatedly when accelerating under load. This is rare but a possibility based on limited information.
 
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