Mercruiser V6 Backfire only at high RPM

My Mercruiser V6 backfires only at very high RPM. I?m new to boating and bought my first boat, a used 1997 sea ray 175BR series with a 4.3L V6 last winter. I?m on the east coast, so it was too cold to test the boat in the water. I had a dynamometer test performed on the engine (very low hours) and the engine performed very well according to the boat mechanic (the mechanic was not the seller). This mechanic has been caring for the boat for the previous owner since purchased and kept very good documented maintenance records. I witnessed the dyno test and the engine really performed well (high torque at very high RPM). After spending some winter days replacing simple things like spark plugs, bilge pump, fluids, adding bottom primer/paint, I was pleasantly surprised at how well the boat starts, runs, and handles in the water. I noticed a small glitch last weekend when I tried to max out the throttle and started to hear what sounds like backfires. I didn?t want to raise the sun platform up to expose the engine, since the boat is moving pretty good (around 32 GPS MPH) when this occurs. It seems like I can always start right up, quickly get up on plane, and run for long periods at a pretty good speed, but as soon as I push it to the max throttle I get the pops out of the engine, until I back it down slightly again. I do fear that there was some old gas in the tank for some time, before I filled up for the season, so I?ve been adding enzyme additives and other additives with each fill-up, which is typically only 6 ? 7 gallons at a time (thankfully). Does anyone have any idea what may be causing this problem at only the very top end of engine RPM? FYI, it does seem that I need to warm the engine up for a minute or two before putting it into gear, or I will get backfires. After warmed up, it runs like a top, and is smooth as silk, until I really push it at the top end. Other then this glitch I am very happy with my first water toy so far.

Any suggestions of what to start looking at? Boat was in storage for a few years and aways winterized before I bought it.

Thanks,
Joe

Re: Mercruiser V6 Backfire only at high RPM

How is the timing?

Re: Mercruiser V6 Backfire only at high RPM

Does anyone have any idea what may be causing this problem at only the very top end of engine RPM?

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Ayuh,....

With all the puttering you did last winter,..... I see No mention of any Fuel System Maintance......

Change the Fuel Filters,+ Your Problems just might go away.....
If Not,....
You'll need to dig Deeper into the Fuel Delivery System......

Good Luck,+ Welcome Aboard Joe.....

Re: Mercruiser V6 Backfire only at high RPM

My Mercruiser V6 backfires only at very high RPM.

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What are you calling "Very High RPM" we need numbers.

Re: Mercruiser V6 Backfire only at high RPM

I should have mentioned previously that I did change the fuel filter over the winter also. As far as the timing, I didn't check it, since I witnessed how the engine ran under load during the dyno test. I've worked a little on car engines and always lived by the fact that if it ain't broke, don't try to fix it. I'll note the RPM where backfire occurs when I take the boat out for a spin this weekend. I was too busy keeping an eye out for drifting flounder fisherpersons last weekend to note the RPM during the backfiring.

Thanks for the feedback.
Joe

Re: Mercruiser V6 Backfire only at high RPM

I should have mentioned previously that I did change the fuel filter over the winter also.

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Ayuh,..... Change it Again....