I have a 500 efi. I have made a few trips and have consistently gotten the same results. Last year I was seeing speeds of 75-78. Now I am only getting 65 and it's not coming on plane as fast. I did my fuel filter late last year and changed to moble1 synthetic 20w50 this year from a Kendall last year. I also have run a few tanks of fuel through it. Yesterday when I shut it down the engine hissed back. (Not quite a backfire) I wait for everything to warm up before I run it. Engine temp is 150-180 and oil pressure is 60s.
Any thoughts would be great. I am going to replace both fuel filters but I want to get this taken care of
Thank you all

i would compression check the cylinders and check distributor gear. i had a 500 that used to wear the distributor gear out. slows the timing till it would eventually quit all together. hope this helps

Check distributor caps

Compression and leak down test needed. That is a major loss of power.

I hope it isn't to bad of a problem, the motor only has 169 hrs on it

Any updates on the issue(s)??

How many hrs on that motor? (oh, now I see 169, not too bad) you could have a bad valve spring, worn Cam Lobe, or bad Dist Gear (as mentioned earlier). A compression test & Leak Down test will zero you in on it..Good Luck ( I hope it's a valve spring )

I haven't been able to touch it. I am going to try and change all the fuel filters. The more I think about it, I'm thinking it was right after I filled up that it started to run bad. It was just the first fresh tank since last years fuel. I'll let you know after Saturday. 118 here in phoenix. I'll be on the lake even if I have to float

i would compression check the cylinders and check distributor gear.

If u find u have no real compression issues i would check to make sure injectors are all working properly. EFI motors will run fine believe it or not that the untrained ear cannot hear and have a loss of power..try a bottle of the Quicksilver (Quickleen) if all of your compressions are good...