Engine not making RPM ??


vmaxhp

vmaxhp

New member
Normally, I'm making 4800 RPM on both 496 Mag engines. For a few weeks, I've only been getting 4200 out of the port and 4400 from starboard. I cleaned the bottom and got some barnacles and soft fouling off the stern drives. A little faster, but still no change in max RPM.

I'm thinking easy stuff like spark plug wires and spark plugs. Other thoughts? The plugs and wires should be a little less than 2 yrs old and 100 hours.

A couple of questions on plugs and wires. I can't find the Mercury part number for the plugs and wires. Also, what's the gap on the plugs? My engine Serial Nos are 0w676266 0w676269. Westmarine indicated the Sierra wire set is 18-8804-1 and the NGK plug is TR55 or TR55IX (Iridium). ALOT cheaper than the AC delco!

Other thoughts on cause of decreased RPM?

Thanks!
Rick
 
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I actually just ordered plugs/wires for my 496 this morning.

41-983 Delco plugs gapped to 50. I got a set of 8 for $50

RK120019 wires. They are for an 8.1L Crusader engine, but I have seen confirmation elsewhere they fit the Merc $73 or so. I couldn't justify spending $400 on a set of factory Merc plug wires (short ones at that).

Might also be prudent to change fuel filters (there are two per engine)
 
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Ok, so, plugs changed. No change in problem. Still not making RPM on port engine. I'm thinking swap plug wires between port and starboard and see if the problem follows the wires. Any other suggestions of problem resolution?

Has anyone used West Marine's Sierra plug wires? Thoughts?

Thanks,
Rick
 
I got my wires from NAPPA. Work great and only about $140 a set.

Might be stupid question but did you look at our props?
 
I swapped plug wire sets port/starboard. The problem did not follow the wires, so....

Fuel rail maybe??

Yes, props are clean.


Admin said:
I got my wires from NAPPA. Work great and only about $140 a set.

Might be stupid question but did you look at our props?
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Plugs are changed way to often. Original plugs were Denso TJ14R-P15, which became 33-863622001, cross AC 41-983 and gap at .060

Agree check fuel pressure 43 psi, might also have an antisiphon valve causing a bit to much restriction
 
What about Fuel...any chance you picked up a bit of water? You can quickly check the filters to eliminate that .....
 
I've run about 300 gallons through each engine since the problem started, so, that seems unlikely. I can certainly open the fuel filters and check.
Has anyone tried an injector cleaner added to the fuel tanks -- and seen improvement? That would be versus an injector cleaning service of pulling the injectors.

toofast said:
What about Fuel...any chance you picked up a bit of water? You can quickly check the filters to eliminate that .....
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Never found any of the snake oil which actually work, but sure wish I had stock in the company
 
Yeah, reading the actual test reviews, no benefit. Seems the more expensive pull the injector cleaning service can have some benefit per flow rate testing.
AllDodge said:
Never found any of the snake oil which actually work, but sure wish I had stock in the company
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