350 mercruiser fuel economy

[fixit]

i get 2.5mpg loaded up 2.7mpg light


best ive ever seens was 2.9

smart craft numbers, not estimated
thats with a 300hp mpi with bravo 1

[Dr. Hook]

Well I got my info wrong, upon actually looking at the motor and working on it a bit I see it has a Rochester quadrajet 4 barrel carb.

This is my first season on the boat and time is limited so I'm still getting things sorted out. my fuel economy has been horrible, like 2 mpg at 30 mph.

I just cleaned the flame arrestor today so that may help. Picking away at other tune up stuff.

However I think the prop is the issue, it has a 14.25x21P aluminum prop with a big chip out of it and its in general rough shape. I can only get 4000 rpm so I think its too much prop for the boat.

Can you guys recommend a better prop? I want to go 4 blade aluminum, can't afford stainless right now. 4 blade because I like lift, control, and lower planing speeds, I couldn't care less about top speed.

[THEFERMANATOR]

You need to know exactly which engine you have. 4000 to an early engine is dead on, some turned 4400, and late model vortecs go up to 5k-5200. Also check your outdrive gearing. It sounds like your gearing may be off for a 350 as I would expect a 17 or maybe a 19 pitch prop on that boat, but not a 21 if it has the 1.5:1 lower unit.

[Dr. Hook]

The cover over the flame arrestor says WOT range 4200-4600, engine serial number OD 391290

Is there a number on the block too?

[Dr. Hook]

It also says transom serial number OD 415218 and drive serial number OD 422744

The previous owner did mention the drive was rebuilt.. What components were replaced was unclear however.