Ok this sucks. my boat is a 2000 Nordic 28 Heat. To my stock 2000 502 mpi I added EMI mainfolds with thier aluminum risers, Zoomers Mufflers and changed the plugs and I lost 400 rpm and 5 mph!!!! AAAAAARRRRRRGGGGGGG! Same lake, same temp, same prop, same weight (actually lighter this time). Any one have any ideas? Bad winter storage gas - I used stabilizer. The boat ran well, fired right up but had no pull above 4000rpm. Also that sucking air sound went away at 4000 rpm as well not sure if that is significant. seems to have a stumble at fast idle 1000-1200 rpm. Plugs are the NGK BR6FS recomended by Merc. gapped to 040 - all the wires were plugged in well. I didn't have tools with me so didn't pull any plugs. I didn't really expect to gain much with the exhaust but I sure didn't think I'd be loseing anything. Last year I was 61mph @ 4900, today I was 56mph @ 4500. I'd love to hear suggestions / idea's.

If I were you I would start by changing the plugs again. Seems like you might be dropping a cylinder at high rpms. The higher the cylinder pressure the harder it is on the ignition system.

Did you have mufflers before last year when you ran 61 mph? Can you easily remove them now for a test run?

This my sound off the wall but check to make sure all the wires are hooked up. I have seen this before and you drop about 400 RPMs.

There's a problem other than the headers. Keep looking.

Fred

Nope they're new this season. They're permanent tips. I doubt they're the problem based on the tests by Hot Boat they were one of the only mufflers that didn't decrease speed and RPM. In fact on a 496 boat they actually picked up some speed.

Double and triple checked this. Pulled the plugs tonight to make sure none were cracked or had any problem - they all looked great.

Yeah Fred I tend to agree with you. I can't imagine adding a freeer flowing exhaust and a tip rated for big HP would be the cause. It seems like its got to be either fuel or spark but I'm not sure how to test.

Its very easy and common to cross #5 and #7 spark plug wires!The engine will almost run normal except at high rpms under a load(almost)!This also could be your slight stumble!

I hate to sound stupid but remind me which are 5 & 7 , I can never keep the order straight. Aft two on the port side ...right?