Ok Carb gurus....gotta a mystery carb

[RJP]

Holley list 6647. Holley book has this as a Mercury Marine 454. Fine, no CFM flow rating but jetted 68/primary, 70/sec... power valves [8.5s] in both primary and secondary. Other internet info. found has this as a 600 cfm. Wrong, has down leg boosters, "172" stamped throttle blades [pri and sec] and all indications and measurements point to a 750/780 cfm, vacuum secondaries, just like a 3310.  No "J" tube bowl vents [marine application] and a oddball throttle linkage lever, not the usual GM throttle lever found on a gazillion Holley/GM carbs. What gives? Did Mercury Marine rate the airflow different? Now I know that Mercury Marine had their own way of doing things as the Rochester Q-Jet for their marine engines are much different from the GM automotive Q-Jet as nothing from a typical rebuilding kit fits.  That kit is exclusivly M/M available from the dealers only. Found this out the hard way. BTW M/M Q-jet rebuilding kits are about $250.00 and that is jobber price.   

[My427stang]

Over the counter 396/375 horse Rat replacement carb in 1970 is what I find in my books.  Jetting all matches, over the counter replacement for a LIST 4557, but it doesn't sound like enough jetting for a hot 396.  I didn't see any marine references

I wonder if it's a gas crunch replacement or drivability replacement or something like that?   Jetting seems to be less than a 750 but more than a 600, maybe something to calm the women drivers?

[RJP]

What is throwing me off is the info is from the old Colt industries Holley Parts Illustrated Parts & Specs manual from the late 70s and the listing states "Mercury Marine 454" and lists no sec. jetting or P/V. This carb is clearly a 780 cfm with a sec. metering block drilled for a P/V and both the PVCRs are in the .060"- .070" range judging from my finely tuned and accutratly calibrated eyeball. OEM 3310s for the 396/375 HP are the only 780s that have down leg boosters, thats what make 'em 780s and not 750s with straight leg boosters. As for the OTC replacement carbs I didn't know they carried a different list number. Your referenced list 4557 has #70 pri and #76 sec jets. with 8.5 P/Vs front and rear. BTW, metering blocks are correct for this list number carb as the number "6647" is stamped into both blocks under the metering. block pt. number.