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496 mag rpm problems

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Old 07-02-2010, 11:04 PM
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Default 496 mag rpm problems

My port motor is running about 500 rpms lower then the star side at the same throttle position I was thinking it was the props so took them to the prop shop and it still done it so then I tried a set of diffrent props and its still the same. The port motor is burning alot more fuel as well and the port exhaust of the port motor is leaving alot more black residue on the swim platform the boat runs good but the max rpm I can get out of the port motor is 4400 rpm. If anybody might know what the problem is please let me. Maybe a sensor,injector, or plugs from what I can tell I dont think theres a miss

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Old 07-03-2010, 08:39 PM
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I bet you have a fouled plug. I had a similar issue and it did what you described. Sounds like a good time to change them all. Another simple thing to do is check your cap connections on all the spark plugs, distributor cap and rotor. Im sure you will find your problem. Good Luck!
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Old 07-03-2010, 09:58 PM
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He doesn't have a distributor. But you should check for a loose plug wire. And pull all the plugs to see what they look like.
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Old 07-04-2010, 01:02 AM
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Thats what Im thinking to plug or wire or maybe an injector Im going to pull a couple of plugs in the mornin and check the connectons before I go out but I wont be able to get any parts till after the 4th
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Old 07-04-2010, 05:19 AM
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check your fuel pressure with a gage,,,the gage can be bought at any auto parts store,,and not much money,,,take out the two bolts that hold the antifreeze bottle to the bracket,,,you will see a schrader valve,right behind the bottle,,should be 40-43 psi,,compare it to the other engine,,,you might have a bad fuel pressure regulator
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Old 07-05-2010, 09:18 PM
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I put the fuel pressure gauge on it today and with the key on it was 43psi with the pump on and idleing with the hose it was about 33-35 the needle was bouncing and on the other motor it was about the same except idleing it would bounce from about 33-40 I would try to make a run up river with it on there but I dont know If It would be safe I also checked the plug wires they had good connection I guess tomorrow Ill pull the plugs and see what they look like

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Old 07-11-2010, 10:28 PM
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Ok so changed the plugs pulled some denso tj14r-p15 plugs out and put the ac delco 41-983 with a 0.060 gap thats the plug that came from the mercruiser part number so know ive pulled the plugs 4 times to check them because ive had nothing but problems since i put them in its backfiring bak to the flame arrestor know when i get over 2500 rpms ohm the wires they all ohm out maybe ill try to find a set of denso plugs and pull them again if anybody got any ideas please let me know
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Old 07-12-2010, 08:02 PM
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I wouldn't use anything but Delco or NGK. Intake backfire is caused by lean mixture, timing (highly unlikely), or valvetrain issues. If it didn't do it before, you may have cracked a plug, or got a plug that had been dropped.
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Old 07-12-2010, 09:05 PM
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yea the cameshaft sensor and crank sensor sit the timming so your right its not that im with you on the cracked plug but its not cracked where i can see it so im thinking it might be cracked on the inside they say you can shack them and here them rattle you wouldnt happen to know the ngk part number by any chance
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Old 07-13-2010, 09:46 PM
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I got the boat running back the way it was its not backfiring anymore but the rpms are still off i noticed today that the rpms stayed the same til about 2000 rpms then the port motor backs down to abot 1600 then i got to start giving it more throttle to get them at the same rpms
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