Can a dirty IAC muffler affect top speed in a 496 HO?
 

I've lost 7-10 MPH of my top end, can a dirty IAC muffler do this?

are You still making full rpm's?

No, I used to run at 4800 RPM and 67 MPH with a GPS. Now U I run about 4500 RPM and 58-60 MPH. I replaced the water /fuel separator but that didn't help. I just replaced the IAC muffler and it was black. I am getting ready to test it now.

There's a fuel filter in the fuel pump assy, check that.

Well that did not do it. Do you mean the cool fuel filter? Is that hard to change? I have the older gen 2.

Do you have this style pump ?
http://www.mercruiserparts.com/Show_...9+%26+Below%29

That has a filter in it

he has Gen II cool fuel system with the spin on fuel water separating filter. The diagram above is Gen III and number 23 is the most important filter in that set up that needs to be changed on top the filter # 28.

Anyways back to your gen II cool fuel system as it will have a tiny filter screen underneath the fuel pressure regulator that becomes very easily blocked. I would check that out. The first test to do is a key on fuel pressure test and then running fuel pressure tests to see what your specs are compared to your app..

Next I would have your motor scanned and drop test each cylinder to see the reduction numbers. You are looking for equal amounts across each cylinder performing that test. Also you are looking for when scanning the engine available power % plus any fault codes.

Also if a scan tool is not available for testing, you should perform a compression test and a leakdown test. 7 mph is a lot so maybe you have a motor problem and or prop problem especially with reduction of rpms so there is a lot more going on than a dirty IAC muffler.

I feel like I have very little response over half throttle, the first half is great. Is that screen hard to replace?

with diagnostic tool check for faults , for example , faulty IAC will turn on engine guardian that will reduce engine power

Shouldn't I hear beeps when it is in guardian mode?