89’ Mercruiser 454 7.4L Big Block Backfire issue

[rosstyler13]

So my dad had the block done about 2 years ago and have had problems with it since. We rebuilt most of the parts of the engine such as the block the heads and even the carburetor. Now, when getting to 2000 RPM, in gear, it starts to backfire through the carburetor and we have no clue why. We think it’s either the Carburetor, Valves, Electrical, Vacuum Leak, or even the muffler (since it is the original and has never been touched really). Any ideas?

 

[rosstyler13]

We’re going to take the muffler out and have it rebuilt too

 

[Chris1956]

This should be reposted in the MerCruiser forum. When you say muffler you mean exhaust manifold and/or riser, right? Normally they are replaced when necessary.

Those would have been replaced in 1998 in saltwater, and could still be good if used exclusively in freshwater.