bcarpman

Ok, remember I'm a newbie here, but is it only me or does 1.5mpg seem incredibly low for a properly tuned 3800lb boat?

Crazy didn't state whether that's his composite mileage for an entire tank, but I assume if he's got a fuel meter he's talking his cruising mileage. If he's cruising at 2500-3000rpm I would expect 3-3.5mpg if he's got everything right.

Please tell me if I've got something wrong here, but if it was my boat, I wouldn't be looking for parts replacement, but I would be looking for something tuned wrong.

Wobble

I'd say low, my 28' single BBC averages 2mpg with mostly cruising at 40-45mph and a couple of short wot bursts.

66 miles by GPS and 32.4 gallons last time I checked.

SB

iTrader: (1)

Anybody want to comment on the 10" of vacuum at idle ?

I'm hoping the guage is faulty or atleast read incorrectly.

If this is the true reading - I'll say it again, a stock low hp 454 should be nowhere near this. There is an issue somewhere. You cannot tune around a hurt motor.

SB

iTrader: (1)

I just read it again incase I was mistaken. It looks as if I am. I apologize. Sorry.

It looks like I read wrong / or assumed wrong and made an 'azz out of me.' Doh.

Just to clarify:
Crazycat - where you saying above that you have 10" of vacuum at just over 2700rpm ? Or at idle ?

bcarpman

Yeah, that sounds reasonable for a composite. My 4200lb 26ft w/BBC seems to get about 2mpg with a fair amount of WOT. I went back and looked at a couple old magazines with boat reviews of boats about that size with BBC and they were all around 3mpg cruising at faster speeds than Crazy is talking about.

Pourman

Hey CrazyCat, did you ever open that Holley and see what power valve was in it? I have the same carb. Bought it last year and it had a 10.5 in it. My vacuume at idle is about is about 7. Very low so my power valve was open all the time. Sucked gas like there was no tomorrow! Changed it to a 4.5 and all was wel until I went to Tahoe at 6,200 ft and then it opened too soon again so I put in a 3.5. Now it settles down about 3,000 rpm. My secoundaries open at about 3,200. I have a 97-454 that I have put edlebrock aluminum heads, airgap intake, 502/502 cam, gil exhaust with captains call, crane gold roller rockers and 454 mag module. It get at least 2 miles to the gallon and is pushing 5,100 boat. I usually have another 1,000 pounds of people and gear. There is a great little pamphlet Holley put out for $5.00 on how to tune your Holley thet will help you gat that carb tunned right. I agree that you are sucking way too much gas and I think your carb has lost it's mind. Great carb though for the motor. Hope you get that thing straightened out. Ty

CrazyCat57

At idle the vacuum needle is awfull nervous. In the middle of the variation would be about 10 in, hard to tell because of the needle shake. The engine is nice and smooth,starts good runs good, no rough idle. Up and running 2800 rpm its just a little less then 10 in but with a steady needle. The needle becomes steady just a little above idle. It's a new gauge from Sumitt and sucking on the line back at the engine and plugging it with my toungue it will hold the reading until I realese it. The 1 1/2 mpg I quoted was per trip. I have taken mpg at different cruise speeds and the best I ever could get was 2.2mpg, that was trimed, trim tabs both adjusted to get my lowest gal per hr fuel burn rate and then mpg taken using gps and the fuel flow indicator. The omc dealer looked up what he had in the factory service manual and it showed 2.5 power valves stock. I'm concerned about only 10 in of vacuum but I'm having a small medical problem and havent been able to do much work on the boat. I'm getting some good replies and I hope they keep coming. Thanks

CrazyCat57

UPDATE after making some changes my gas milage went from 1.37 mpg overall trip to 2.58 mpg overall trip with about the same load. I eliminated the 750 holley and cast iron intake and went to a 600 marine edelbrock carb on a edelbrock preformer intake manifold. I also installed a homemade cold air intake system. I have more power out of the hole and better mid range, the top end stayed about the same. The money that I spent on the changes is just at the break even point right now and from here on out should be saving me quite a few $$$. Needless to say I am verry happy with the way things turne out but I have a couple more small things to change yet. Thanks to everyone for all of your past input.

Strip Poker 388

iTrader: (6)

I think your carb is to small,imo

but sounds like your saving fuel