jmherbert

I don't think you are not going to gain much impovement, as motor efficency in boats is such a small factor in the economy. On a car, different story. You would see better (actual) gains per dollar by installing trim tabs to help plane at lower speeds (read; reducing load on motor), investing in a good stainless prop, and keeping the hull clean.

I personally think that you are not going to gain anywhere near 1/2 MPG by changing the carb, etc. All things equal, I doubt you'd notice any difference.

JohnS

Your time would be better spent taking weight out of the boat, clean hull... which would make a bigger impact on MPG rather than increasing your engine efficiency. I think at this point your splitting hairs. Yes your charge velocity and vacuum signal would increase with the smaller carb, but I cant imagine you would increase eff. that much...maybe better throttle response.

US1 Fountain

Your looking for a 33% improvement. That's a HUGE amount, especially for a motor that is running good now.

LFarr

Crazy,

You said that you have plenty of power out of the hole......

Try over-drive, i.e. go up 2 pitches from 17 to 19 or 19 to 21 or 21 to 23 on your Rev4.

BBlades has a test program and can provide you with a 25p Rev4 if you need it.

Cheapest, easiest, cleanest thing you can do to change / enhance performance/driveabliity is prop change.

2 cents

CrazyCat57

I tried a 23p 3 blade, the 19p rev 4 is much better. My prop came from bblades and Brett is very knowlegable. I do have hyd trim tabs with indicators and a nav man fuel flow indicator that shows around 2900 rpm to be the most efficient. The boat is trailered so the bottom is clean. It sounds like im wanting to gain something that is near impossible. Thanks for any replies.

SB

You could not load the boat with other people and coolers plus drive at 2500-3000rpm at partial throttle, but that would be no fun at all and not worth it.

Unless the motor tune is way out, there is not going to be a good jump in mileage by getting it 'spot on.' Maybe just a little tiny bit.

Drag / weight / how you drive / how much you drive is what makes/breaks fuel useage/mileage.

If economy is a real 'concern' for anyone I would suggest getting a fuel flow meter (as Crazycat has)so you can see what changes do what to fuel mileage, giving you knowledge on what rpm, trim, throttle setting, weight, etc,etc gives you the most acceptable fuel usage.

I personally just cut down on the length of my trips. I go out just as often, but picked closer spots to hang out for the day.

Fuel prices stink bad - definatley influencing things fore 99% of us.

RumRunner

CrazyCat57,

You’re not asking for the impossible, but improbable is going to be the best wording. Picture what you’d have to do to take your streetcar from 20 MPG to 30 MPG. This is what you’re asking for, and it’s not going to be very cost effective with where you are. Now that’s not to say you can’t make the engine more efficient. There are plenty of upgrades that can be done to help the engine, cam profile, cylinder heads, different coatings, etc. however you’d be better off making these changes as the engine is in need of rebuild/upgrade.
To do what you want to do there is a better approach:
Start by getting a fuel flow meter so you can see how much fuel is going into your engine while you are running the boat. Once this is done, you can run the boat at different speeds to see what RPM, Drive, Tab, settings run most efficiently. Then you can try different props to see what gives you exactly what you’re looking for. Most likely a newer prop with more blades will help your low speed efficiency.

CrazyCat57

Rumrunner thanks for your reply. Would you re read my earlier posts if you have time, as far as my trim tabs,feul flow meter, props,etc. I have tried 4 different props, a 3 blade 19p, a 3 blade 23p, a four blade 20p, and a 4 blade 19p. All were stainless. The rev 4 from bblades is by far the best. Thanks for your replies.

RumRunner

CrazyCat57,

How are you meassuring your MPG? Are you actually using a flow meter or going by tank loads?

Pourman

I would like to knoe when your power valve opens. I have the same carb and it came with a 10.5 power valve. Turns out it was open all the darn time and according to Holley it uses 25%-40% more gas witht the power valve open. I changed mine to a 3.5 and now it only opens when I really need it. Just a thought.
Ty