I just can't figure it out. It seems as though my rig has lost a couple of mph. Last year I was almost seeing 75 (74.4 GPS) with the rev limiters pinging on occasion. This year I can't get the lead out of her a$$. My best speed is 72 (with no revlimiter ping) All my operating conditions are the same EXCEPT this year the water temp is about 14 degrees cooler. Could this be my problem. I've just turned 40 hours on the motors. I was told that the 496 mags would "lossen" up if you will at around 25 - 35hrs. I didn't think that meant slow down. Any input would be useful

Just a guess but maybe you need to burn out all the old leftover gas. Maybe new gas will help.

I usually see my best speeds in early spring when the air and water temps are cooler. Then seem to loose a little in the summer months as the temps rise.

I don't think cold water is the problem. I you loaded some additional gear on the boat? A few pounds can make a difference.

cold water temps usually means the air is cooler also wich means a better running condition it should be faster!!

Water, as with all other substances, will become more dense when colder. Theoretically, this should allow the boat to float higher than in less dense water. Having said that, it should be reasonable to assume that you may actually increase speed by reducing the wetted surface. On the flip side, more dense water will have a higher coefficient of friction...The end result, who knows...

It's also a reason that seawater is said to be faster than freshwater. If you boat in both regularly, you may notice having more available freeboard in saltwater...It's not uncommon for a typical boat to sit 2"-3" higher in saltwater.

Have you weighed your boat between this year and last? Did it gain weight? Any leaks; trapped water?

Im thinking old fuel also.

Best running conditions are low temperature (air is more dense like a mini blower) and low humidity (water taking the place of air). Now that you have broken it in change plugs verify timing etc. Run higher Octane gas. Dump as much weight as possible.

Weight is the same. I've already put atleast 200 gallons of fresh gas this year. Motor's sound perfect at idle. To my knowledge I don't think there is any trapped water. I trailer the boat everytime I take her out (and pull the plug). Has anybody else had this problem with 496 mags?

What is your engine temperature,if your engine is running too cool the EFI may be putting your motor in a rich condition and it is not pulling full rpm,it happened to me a couple years ago and I had some debris stuck in my thermostat.

Need,
Seneca Lake has warmed up to 68 degrees, as of Sunday night... My daughter took a swim and said the water is "warm" (which is probably true to a 7-year-old). I threw the pool thermometer in the lake and it read 68! I noticed Sunday night and Wednesday night that the air was hot, heavy and extremely humid. I will bet the air temp has more to do with your slow-down than the water temp. Also, check your flame arrestor/air cleaner. There is still a **** load of those air-born fuzzies floating around. I cleaned off a bunch of dirty fuzz off my MPI filter Wednesday night.
Glenora