Just purchased a 1997 225 HP 3 litre mercury on ebay with only said 130 hours run time. Seller said motor ran good before repowering his boat with a EFI model from this carb. motor. This motor's lower unit had been canibalized to a friends motor. This motor was sold without lower unit. I purchased one and attached to motor and tried to run it.
Motor did not even fire. found that crank position sensor was .250 in. from flywheel. I adjusted to .040. Motor ran but misfired with loud poping sounds
Sound like it was running only 2- 4 cylinders. ran with erratic idling rpms. After 15-20 minutes it sounded like it started firing on more cylinders (maybe all) and was much quieter with no popping. It ran this way for about 2-3 minutes and then went back to the original pattern. I stopped motor and restarted, and the same thing happened. Starts right away. Smooth running at first then erratic. I openned the carbs just a bit from idle, it starts erratic firing. Any ideas on what to check. Maybe I have missed something on hookup. Compression checked out @ 100-110 PSI. I got fuel, and all plugs are getting spark. Must be problems in timing or something like that. HELP

It may be lean popping which would mean the carbs need to be cleaned.

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