I read all the information I could on changing the old Quadrajet carb to a new Holley marine unit. I found the carb at Summit Racing.
For some reason my swap to the Holley 4175 carb was not that simple. On my 1980 with the standard Mercruiser 255 – 350 GM the new carb would not mount as the thermostat housing is too high. To solve this a 1” carb spacer was required. After a mis- shipment from a supplier, I did get the correct spacer.
Then the stock steel fuel line would not fit and rather than trying to adapt it I opted to have a new flexible line made of the newer approved A1 line.
The stock linkage for the throttle cable could not be reused so a new bracket was fabricated and bolted to the intake manifold. A few minor mod’s of the bushings and spacers in the throttle hook up were required.
The new carb has an electric choke that required an ignition activated 12 volt power source. I found this in the wiring that ends in the main wiring harness plug above the starter. (full time 12V power to the choke will not work as it will stay open)
After all this the engine started and ran very well and only required a slight adjustment to the idle.
Then I found an interesting issue. While running the engine I noticed what sounded like an exhaust leak on the starboard side. On closer inspection I found that the intake manifold has a very small hole in it where the old heat activated choke was.
I removed the old choke assy. and screwed it back on to plug the hole . As I am planning a complete engine rebuild next winter I guess I’ll add an new intake manifold to the list!
So far I’m really impressed with the new carb. It seems to start faster but the real difference is the smooth acceleration when getting to boat on plane. Top end speed has increased, making me think that the old QJet did indeed have some problems with the secondaries. It also could be that the combination of the new carb with the carb spacer has given me a wee bit more horsepower to push the old boat along a bit faster!