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Loss of rpm on 5 liter mercruiser

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Bill mat

New member
1999 5. Liter mercruiser with alpha one runs well but at about 3/4 throttle it’s rpms fluctuate between 4300 down to 3800 with a hesitation. WAs thinking of doing plugs,wires,cap rotor. Also need a new prop and will change that. Any suggestions would be appreciated. It has fresh gas and new gas filter already. Thanks. Rpms go down then back up and happen quite frequently
 
Bill mat said:
1999 5. Liter mercruiser with alpha one runs well but at about 3/4 throttle it’s rpms fluctuate between 4300 down to 3800 with a hesitation. WAs thinking of doing plugs,wires,cap rotor. Also need a new prop and will change that. Any suggestions would be appreciated. It has fresh gas and new gas filter already. Thanks. Rpms go down then back up and happen quite frequently
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I would definitely inspect and replace as needed ignition components to mention also look at fuel pressure and possibly rebuild the carb if needed
When you change the fuel filter what did the contents of it look like was there anything other than fresh clean gas
 
Put a fuel pressure gage on it and have someone monitor the fuel pressure. There's a fitting for that in the system, and you can borrow a gage setup from many auto parts stores.

Jeff
 
Hi oil is at correct level. I haven’t checked the filter on the carb. I will do that and check fuel pressure and go from there. I change the water separator every other year. Changed it when I winterized the boat last fall. Thanks
 
Bill mat said:
1999 5. Liter mercruiser with alpha one runs well but at about 3/4 throttle it’s rpms fluctuate between 4300 down to 3800 with a hesitation. WAs thinking of doing plugs,wires,cap rotor.....Rpms go down then back up and happen quite frequently
Click to expand...
In 2019 I had a similar issue on my 2000 5.0L/Alpha One. We bought the boat used in 2010 and time got away before I realized I'd never touched any of the 'tune up' related components. I was going to diagnose the entire system but then realized it was all at least 9 years old and likely much older. I replaced cap, rotor, pickup sensor, wires, and plugs which immediately solved the problem.

If you know the age/history of the components I'd diagnose the system and find out exactly what needs replaced. But it might be worth replacing the parts if you're like me and don't know the history of potentially 23 year old ignition components.
 
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