1987 Mercruiser 120 HP. Backfires Help
 

In water the rpm’s won’t go past 1700. On land after putting pure gas in, changing plugs, put mechanic in a bottle in with gas, it did run better with muffs on, ON LAND. Problem is, when I full throttle fast, the engine backfires a little and wants to stop running. When accelerating slowly it will get up to 5000 rpm’s with no problem. Can anyone tell me what the problem is? Timing? Too rich? Too lean? I have electronic ignition in. Thank you

With the limited info provided I would look at a carb rebuild and its running lean

Backfires
 

Yes, I think that’s my next step for this winter. Is there a link that shows how to adjust the carb? Maybe on YouTube. Btw, I’m restoring a 1987 Thundercraft 170 citation. If there is anyone in Nashville TN that would like to work together, your and my boat, I’m ready to restore.

The manual shows how to do it, but would need a serial number of the motor, or at least the carb number to see which manual to use

Backfire thru carb on acceleration, too lean
Backfire thru exhaust on deceleration, too rick
IF timing is correct.

to me it sounds like the accelerator pump in the carb is not pumping,time for a carb rebuild.

Did the boat run well previously? Or did you purchase it like this?
Might want to replace the fuel filter/water separator, dump the carb bowl, try again.
Water in fuel/carb is a very common cause for what you describe.

We are located in Madison TN.
But you'll save money and learn your boat better if you do it yourself. Just start with a real good troubleshooting guide.

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