Hello, I am having slight (10%) power loss at WOT and hoping for some tips to restore it.
I have a 1997 Maxum 23ft fiberglass powerboat with a Mercruiser 5.7L V8 Alpha (sterndrive) with Thunderbolt ignition (carb) with over 1000hours.
I have had the boat for 4 seasons and the engine has always been strong and been able to WOT at 4400pm and hit 37mph on a GPS with a medium load of passengers and fuel.
The beginning of this season I noticed it had lost maybe 1mph of top speed at WOT, so I systematically performed a tune-up (as I usually do each year) starting with the distributor cap and rotor, oil and fuel water separator. After these items were changed there was no change in performance. At this point I moved onto phase two of the tune-up which included putting a 16oz bottle of Sea-foam in a tank of maybe 5 gallons of gas, letting it sit for a week of light use then filling the tank all the way up with non-ethanol gas. In addition, I sprayed in the carb pretty good with carb cleaner, and replaced 7 spark plugs. Why 7? Well because 3 seasons ago when I replaced all 8 I stripped one hole pretty bad and was afraid it would only make it worse to try and remove and replace it. So I went to replace the 7 unstripped plugs and the existing plugs were fine in great condition. I used the recommended NGK non-vortec BR6FS plug with a .040 gap to replace the 7. After these last changes I used the boat for a few weekends but didn’t notice the power loss at WOT right away until I started measuring with the GPS and found that my top speed was now 33mph (4mph less) and RPM at WOT closer to 3900 (500rpm less). I also replaced the tachometer in this time frame as the old one died. I had to assume one of the 4 things I did was the culprit. The new plugs, the Sea-form, the carb got messed up somehow, or new the tach.
In an effort to fix this, I pulled each plug to verify they were all nice and tight, had the correct gap and during this effort I replaced each spark plug wire AND double verified each wire was going to the correct cylinder (I even sharpie marker labeled each wire). I also replaced the fuel filter on the carb thinking that was plugged up with gunk after the seafoam treatment, it wasn’t. I also replaced the water fuel separator, which was also fine. None of these fixed the problem. Still only 33mph top speed with light load.
The engine idles very smooth and sounds great at 3900rpm. I simply lost about 10% of the top end power and can now go only 33mph. Also I can rev up to 4400rpm while in neutral and not under load.
I was thinking it could be growth on the hull, but every year I get some light algae growth and it only slowly me down a tad bit. Plus, it’s early in the season and not much has grown.
Can I tachometer change engine performance? There was a setting on the back to choose between engine cycles and amount of cylinders?
The prop is in the same good condition from past seasons as well, no new obvious damage.
Ideas on what to check next?
Thanks!!
I have a 1997 Maxum 23ft fiberglass powerboat with a Mercruiser 5.7L V8 Alpha (sterndrive) with Thunderbolt ignition (carb) with over 1000hours.
I have had the boat for 4 seasons and the engine has always been strong and been able to WOT at 4400pm and hit 37mph on a GPS with a medium load of passengers and fuel.
The beginning of this season I noticed it had lost maybe 1mph of top speed at WOT, so I systematically performed a tune-up (as I usually do each year) starting with the distributor cap and rotor, oil and fuel water separator. After these items were changed there was no change in performance. At this point I moved onto phase two of the tune-up which included putting a 16oz bottle of Sea-foam in a tank of maybe 5 gallons of gas, letting it sit for a week of light use then filling the tank all the way up with non-ethanol gas. In addition, I sprayed in the carb pretty good with carb cleaner, and replaced 7 spark plugs. Why 7? Well because 3 seasons ago when I replaced all 8 I stripped one hole pretty bad and was afraid it would only make it worse to try and remove and replace it. So I went to replace the 7 unstripped plugs and the existing plugs were fine in great condition. I used the recommended NGK non-vortec BR6FS plug with a .040 gap to replace the 7. After these last changes I used the boat for a few weekends but didn’t notice the power loss at WOT right away until I started measuring with the GPS and found that my top speed was now 33mph (4mph less) and RPM at WOT closer to 3900 (500rpm less). I also replaced the tachometer in this time frame as the old one died. I had to assume one of the 4 things I did was the culprit. The new plugs, the Sea-form, the carb got messed up somehow, or new the tach.
In an effort to fix this, I pulled each plug to verify they were all nice and tight, had the correct gap and during this effort I replaced each spark plug wire AND double verified each wire was going to the correct cylinder (I even sharpie marker labeled each wire). I also replaced the fuel filter on the carb thinking that was plugged up with gunk after the seafoam treatment, it wasn’t. I also replaced the water fuel separator, which was also fine. None of these fixed the problem. Still only 33mph top speed with light load.
The engine idles very smooth and sounds great at 3900rpm. I simply lost about 10% of the top end power and can now go only 33mph. Also I can rev up to 4400rpm while in neutral and not under load.
I was thinking it could be growth on the hull, but every year I get some light algae growth and it only slowly me down a tad bit. Plus, it’s early in the season and not much has grown.
Can I tachometer change engine performance? There was a setting on the back to choose between engine cycles and amount of cylinders?
The prop is in the same good condition from past seasons as well, no new obvious damage.
Ideas on what to check next?
Thanks!!
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