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Installed EMI's, mufflers and new plugs and I'm slower!!

Old 04-19-2006, 07:19 PM
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Angry Installed EMI's, mufflers and new plugs and I'm slower!!

Ok this sucks. To my stock 2000 502 mpi (in 28 Heat) 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.

Last edited by Kokopelli; 04-19-2006 at 08:18 PM.
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Old 04-19-2006, 09:47 PM
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Default Re: Installed EMI's, mufflers and new plugs and I'm slower!!

Did you try to run without the mufflers? You could have bad gas but I really don't think it would hurt the performance that much.
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Old 04-19-2006, 10:38 PM
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Default Re: Installed EMI's, mufflers and new plugs and I'm slower!!

Originally Posted by Magic Medicine
Did you try to run without the mufflers? You could have bad gas but I really don't think it would hurt the performance that much.
Nope. 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 deresae speed and RPM. In fact on a 496 boat they actually picked up some speed.
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Old 04-19-2006, 10:42 PM
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Default Re: Installed EMI's, mufflers and new plugs and I'm slower!!

Did you by chance cross one of the plug wires, put it on the wrong plug. I would believe anything that hot boat mag tests, they are always way optimistic of the speeds reported on test boats! Good luck with your search. Usually if this stuff happens to me it is something obvious that I have over looked.

Good luck
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Old 04-19-2006, 10:54 PM
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Default Re: Installed EMI's, mufflers and new plugs and I'm slower!!

Originally Posted by Magic Medicine
Did you by chance cross one of the plug wires, put it on the wrong plug. I would believe anything that hot boat mag tests, they are always way optimistic of the speeds reported on test boats! Good luck with your search. Usually if this stuff happens to me it is something obvious that I have over looked.

Good luck
Thanks. I agree with your skeptisim on hot boat tests but this seemed to be a pretty decent heads up test comparing about 10 different sets of mufflers on the same boat under the same conditions so I felt fairly confident with it.
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Old 04-20-2006, 09:25 AM
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Default Re: Installed EMI's, mufflers and new plugs and I'm slower!!

Try them without the mufflers. Hot Boat get paid advertisement so they can't say there bad. Anytime I see any magazine saying a product did not hurt performance watch out. Any time you restrict flow, you go slow.
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Old 04-20-2006, 09:30 AM
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Default Re: Installed EMI's, mufflers and new plugs and I'm slower!!


Doesn't make much sense...
I don't believe mufflers are the issue here. Your 415hp 502 is making less than 350 hp at the 4400 foot elevation which translates into "less air in / less air out"

Maybe get some fresh $3 gas and try it again.
Check all of the things you tinkered with on the exhaust install such as plug wires, vacuum lines, etc to make sure you didn't cause any collateral damage.
I may be ordering a Rinda Diacom setup here in the next week or so if you'd like me to scan it.

Dave
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Old 04-20-2006, 10:50 AM
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Default Re: Installed EMI's, mufflers and new plugs and I'm slower!!

Just thinking out loud, but is it possible that the lake water is so cold that the motor is running in full rich mode which is not allowing the engine to achieve full rpms?? What about changing your fuel water seperator and fuel filter.
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Old 04-20-2006, 06:51 PM
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Default Re: Installed EMI's, mufflers and new plugs and I'm slower!!

Possibly a defective or broken spark plug? Pull them all and see if they are indeed firing. You can have a dead or weak cylnder and not really be able to tell on an EFI engine.

Your boat ran 61(GPS) at 4400' elevation before you worked on it? That is pretty impressive! Most Heats that I've seen only run 63-64 at sea level with the stepped hull and 496HO...

Last edited by bobl; 04-20-2006 at 06:57 PM.
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Old 04-20-2006, 08:10 PM
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Default Re: Installed EMI's, mufflers and new plugs and I'm slower!!

Originally Posted by Magic Medicine
Just thinking out loud, but is it possible that the lake water is so cold that the motor is running in full rich mode which is not allowing the engine to achieve full rpms?? What about changing your fuel water seperator and fuel filter.

You may have hit the answer. Since getting the boat last year I've had a problem with it not warming up (posted here about it a few times) and could not figure out why - didn't worry too much as it ran well. Yesterday air temp was about 50 and water temps were very cold - snow run off. Ran it on the scan tool today and it would only warm to 70 and was throwing a cold temp code. Pulled the thermostat housing and the retaining sleeve that directs water was missing (I didn't even know there was supposed to be one) put one in and the boat warmed right up to 162. Also found timing to be a couple degrees off so that was reset as well. I'm going to run it out to the lake again tomrrow after work and test it. This time I'll take my tools and a fresh set of plugs in case.
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