US MUSL

I am currently trying to dial in my 830 CFM Holleys which on are on a pair of 509s. The motors are 9:1 Compression, stock rectangular port heads with some bowl blending, Edlebrock Performer RPM intakes, full roller Comp Cams valve trains (587 intake/601 exhaust 226/234duration, and dry exhaust.

The distributors are Mallory which have been convertered to a Petronix Ignitor II system with a Flame Thrower II coil. Initial timing is 14 degrees and total is 32.

The current jetting is 82 primary and 84 secondary. The plugs are AC R42T (one heat range cooler than stock).

At idle the plugs are dark brown in color. After a 3400 RPM run (1 mile), I quickly came to a stop and shut down the motors. I checked plugs on #1 and #8 on both motors and the plugs/insulators were white in color. Same procedure after 4000 RPM run and same results.

From what I have been told, the plugs should be medium to a dark brown in color. Any suggestions on the proper color for a marine application and jet sizes I should try would be greatly apprieciated.

Thank you,
Jack

jmherbert

http://www.empirenet.com/pkelley2/sparkplugreading.html

The "real" way to read plugs

"NOTE> If the base ring has a full turn of color with some spots of heavy dry soot, then jetting is too rich, REGARDLESS, if the porcelain is "BONE-WHITE", jetting is still TOO RICH !!! NOTE> Do not look at the porcelain to read jetting !!!"

US MUSL

Very informative article, thanks.

jmherbert

No problem, got that link from a post long ago... The only trick is actually suddenly shutting down the boat under these conditions is not the safest thing in the world. Do it on a calm day and plenty of space.

BY U BOY

try 84 primary and 88 secondary
with 6.5 pv

US MUSL

JMHERBERT,

I found that article on a drag racing website. Do you think their results are applicable (1/4 mile 12 seconds or less) to pushing a 11,000 pound boat at 3500 RPM for an hour or two at a time?

Does anyone else have an engine similar to mine and know what jets they are running in their holley?

BY U BOY,

I have been told the same jets that you recommended. I was told by a friend that he thinks that the jetting on a Merc HP 500 were 84s and 88s.

Thanks

Griff

Hp 500's use staggered jetting and no secondary powervalve. 6.5 PV in the primary. A PV in the secondaries would be worth about 6-8 jet sizes.

Primaries port-81/starb-75
Secondaries port 89/starb-93

jmherbert

There are many on this board that know more than me, but I got that webpage from this board some time ago. Maybe it was the same advice on a different webpage, but the info is the same.

To me, it makes sense, as our motors have a lot in common with drag racers, as they are rode hard, with gas and not alky. Those kind of drag racers also want their motors to last more than a few passes. I was also told basically the same advice in tuning HP Harley motors, to check the plugs in the same manner, with the importance of shutting the motors off while under 3/4 to full load.

US MUSL

Griff,

I have 6.5 PV on the primary and secondary side. Do you mean that if the HP 500 had a 6.5 PV in the rear that jets would be 6-8 jet sizes higher or lower?

MDGperformance

What jets came in that carb from factory,are you using fuel with 10% methenal.I have been reading plugs using the above method for years and have had no detonation problems.Thr porcelon will only color when idling and remain white under load,if they color under full load way too rich.The fuel we use is alot different then it use to be.The only time i see porcelon turning brown under load is when i put octane boost in my dyno fuel cell.I have egt and afr gauges on my dyno set up but still play close attention to plug.Let me know the above info and i will make you my recomendation.