Starting problems.

[ericcarr]

Ok here’s the pic of the top of the valve covers.

 

[ScottinAZ]

ok, your serial number is on the sticker in the "middle" picture. check the tune up page that I posted last night, your serial number is in the range specified. if you need more info on the carb, its in Mercruiser Manual #3. If you need info out of the manual, let me know, Ill see what I can do to get it for you

 

[ericcarr]

Can I download the manuals?

 

[ScottinAZ]

Can I download the manuals?

messaged you

 

[ericcarr]

Is it 2 to 3 turns and them screws out once seated?

 

[ScottinAZ]

Is it 2 to 3 turns and them screws out once seated?

Yes, 2-3 turns out from lightly seated.

 

[ericcarr]

are these quadrajet carburetors vacuum or mechanical secondaries?

 

[ScottinAZ]

are these quadrajet carburetors vacuum or mechanical secondaries?

the secondaries are mechanical, the air valves at the top of the bore open in response to airflow. When these open, it raises the metering rods as well for more fuel.

 

[ericcarr]

OK, if the secondaries Are mechanical, How difficult of a job would it be to switch to a vacuum secondary carburetor?

 

[ScottinAZ]

OK, if the secondaries Are mechanical, How difficult of a job would it be to switch to a vacuum secondary carburetor?

should not be. Just need to find one that is "marine rated". IIRC Holley has a few that were used on the Mercruiser line, and IIRC one of them was vacuum secondary. Ill dig through the manual and see what I can come up with for you

digging through the manual got me zip.... here is a page from Holley though with their marine vacuum secondary carbs. Keep in mind that these are square bore carbs, and the Q-jet is a spread bore design. you will need either to change the manifold, or use an adapter plate with these carbs...

Marine Carburetors

All Holley marine carburetors are developed to meet the U.S. Coast Guard's stringent specifications. Each has a special marine carburetor fuel-metering system calibrated specifically for the unique engine loads found on watercraft and J-style vent tubes to direct fuel and fumes back into the...

www.holley.com

 

[ericcarr]

But you’re sure these 7.4 stock motors use a 750 cfm carb though right?

 

[jimmbo]

But you’re sure these 7.4 stock motors use a 750 cfm carb though right?

Most Q-Jets were capable of flowing 750 CFM, at a 1.5" pressure Drop.

However I doubt you engine needs more than just over 525 CFM @5000 RPM. I am giving the Engine a VE of 80%, which I think is Generous

Holley does market a 4175 650CFM Spreadbore Part #80402 for Marine use

Here is mine

 

[ericcarr]

How does it run though, any issues?

 

[jimmbo]

Replacing the factory Holley 500CFM(378CFM when flow rated like a 4bbl) 2bbl with the 4bbl, gave me another 500 rpm, which required a higher pitch prop to bring the RPMs down, which loads the engine higher, and reduces Hole Shot. The tiny Primaries help a tiny bit at preserving the bottom end, but the greater pitch is noticeable. I didn't have to replace the Intake, as Volvo used a 4bbl Intake and an adapter to mount the 2 bbl. While the 2bbl Holley is pretty much the Perfect size for a 5 liter spinning 5000 rpm, it is a a bit small for a 5.7 liter. At WOT the 2bbl had about 4 inches of Vacuum, with the 4bbl, it was about 1.25 inches, and the Secondaries were about 50% open. It still had more capacity, and would have opened more on a larger engine.

 

[ericcarr]

Couldn’t I run a 4160 street carb instead of a 4160 marine carb?

 

[jimmbo]

If you want to be in Violation of some Laws and want to blow up your Boat, and have your Insurance deny any Claim, go ahead.
There are reasons as to why there is USCG certification when it comes to Electrical and Fuel System parts.

 

[ScottinAZ]

this refers to the 4150 series as used on other Mercruiser engines. the 4150 is a mechanical secondary double pumper that was used on the larger 440/460 Cyclone and 475 Turbo engines. Its the only reference to a Holley carb that shows up in the service manual. Not quite what you are looking for, but there ARE a couple of part numbers to possibly track down a used unit rather than having to get a new one, or go with an automotive (not recommended for reasons stated above) unit. the 4150 is about as bulletproof and time tested as a carburetor gets, and there is a LOT of aftermarket parts for them.