Carb swap in 454 Mag
 

Because of consistent hotstart problems with the stock carter AFB carb on my 454 Mag (365hp) I am planning to replace it with an Quadrajet.
Due to the different fuel inlet location on the carbs the fuel line from the filter to the carb has to be replaced as well.
The engine came stock with the highrise manifold and all the fuel lines I found that fit the Q-Jet are designed for the lowprofile manifold of the 330s and are therefore too short.
My Info is that the 454 Mag never came originally with the Q-Jet because these carbs were discontinued from 93 on.....
Does anybody know if an original Merc part is available that fits my use (part #?) or do I have to use sth. custom made?
Thanx for your ideas. Happy new year!

I am almost positive the 454 mag (Mark 4) came with a quadrajet. Im sure that any of the Merc parts guys on here can help.

Merc used the Q-jet on the 454Mag for at least 88-92. The fuel line is Merc # 445991. You'll probably also need the longer choke linkage, Merc # 72468.

When I get back to my office on Monday, I can send the jets and rods sizes Merc used for the 454Mag. I believe it was .068/.041 & DA.

Keep the Carter and give her a quick shot of starting fluid when she's been sitting over 24 hours.
I use about 1 can a year and my 454Mag w/Carter starts like a champ plus I love the smell of ether in the morning :D

BTW:Let the engine compartmant air our before you shut the hatch.:eek:

An 800cfm Holley carb will start right up after a week and also give you a mile an hour or two over the stock Weber carb. Also much easier to tune than a Q-jet. You will have to fabricate a new fuel line and buy a linkage plate but, really a pretty straight forward swap.

Thanks everybody,
220BR if you would find out the correct jetting of the carb i would be most thankful!
MACDAD260 you are sure that just switching to an Holley 800 would give me 2 mph more? That would equal 30 hp more on my boat (Baja 272) which is hard to imagine....also I learned from this board and Dennis Moores engine bible that the intake manifold was designed for use with a spreadbore carb (--> Q-Jet) instead of an squarebore carb (Carter, Holley) and therefore performs better with the Q-Jet....
but thats theory, if the Holley in real life performs better, i will reconsider.....there was a guy on this board custom building and selling Holleys, anyone remember his name?

The Hp370 and 400 merc motors used Q jets. The 370 was the motor used before the 454 mag which is a 365hp motor. The Hp 420 and 425s I believe used holleys. So depending on which Carb your using you can definitely get a fuel inlet line and linkage from one of those motors. The 800cfm Holley marine carb would be the easiest to install and tune. Summit now carries Holley marine carbs. They sell for $460, and more than likely you will just bolt it on and forget about it, should be jetted for what you need.

I surfed into the Summit site and saw their Holley and Eldebrock carb.'s and was wondering if Demon make a Marine Carb.????

BTW, I didn't see a 800cfm carb. just 600 and 650!????

I swapped in a Gen V 502 Mag to replace a Gen V 7.4L. I used the 750cfm Weber from the 7.4L the first year. I have an Edelbrock Performer RPM intake on the 502 that is made for the square bore of a Holley or (Weber) Edelbrock carb. The original intake on the 7.4L was made for a Q-Jet and had a Mr. Gasket adapter on it for the square bore Weber. The design of the intake may make some difference but, I don't know for sure. As for the speed gain, the best I ever got from the boat when the Weber was on it was 61.5 gps. With the Holley, my last run this year was 63.5 gps. I was alone and had less than a quarter tank of gas so I would say it helped the speed by alt least a 1 to 1.5 mph if all things are equal. I was pleasantly surprised myself by the speed difference the Holley made. I put the Holley on mainly to help the starting after setting for a week problem and because the Weber ran lean even though I had it jetted just about as rich as possible. The Holley ran lean as well with the stock jetting. I am 12 sizes bigger on jetting than what the carb came with and I still have trouble getting color on the plugs. If you do put any new carb on do not assume that it is jetted correctly or you might hurt your motor. Good luck. :cool:

Demon offers Marine carburetors with or without the choke towers, and electric chokes.