I was just thinking about this, and I suddenly remembered reading a story online (I think it was written by FTF over on the fordsix forum) - he said that he had a Holley carb that was giving him fits once. Long story short, he eventually found out that the surface on the main body where the secondary metering plate fits was warped. No matter what he did, he always had an internal fuel leak from the secondary fuel bowl into the intake. He solved it by putting the carb body on the mill and taking a few thousandths off to straighten it out. It worked like a charm after that.

 

This issue is actually quite common on Holley carbs, but not just the secondary metering plate. It can also occur on the primary side as well.

 

Can you say billet!! I like the billet base plates and metering blocks.

On a used carb I usually run a file over the main body surfaces during a rebuild to check for low spots.

 

That sounds like a story I related recently, probably on the '80-86 forum. Had a '69 Super Bee w/a Holley. Found that the primary side of the main body was warped. Next door neighbor ran the machine shop for Conoco and put it on the mill to square it up. Problem solved.

I've since read that many of the Holley's of that era had a similar problem. Some have attributed it to the screws pulling the metal, but I don't buy it. Were that the case the metal around the screws would be the high spot, meaning you'd have two large hills on each side. Instead, the middle seems to have shrunk, and I'm guessing that happened as the casting cooled at the factory.

 

I remember him telling me this story way back when I was first trying to diagnose the thing.
I went over everything with a straight edge very carefully and never found anything that made me think it had any warp to it.
However, the AFR checked out okay when I was finally able to check it, so I don't think I had any leaks that I know of.

 

Well, my gas mileage test wasn't very successful. I had problems with pumps at different gas stations kicking off at different times. I've started getting into the habit of when the pump kicks off full, I just leave it. Everywhere I've read, topping off is a bad idea.

Before starting the trip, my fuel gauge was just a tiny hair below the F line (right after being filled up). The temp gauge and oil gauge were also reading slightly low, so I attributed it to fluctuating gauges, which happens regularly. (They all shift the same way, all slight low, all right on, or all slightly high).

Drove 135 miles to my destination and topped it off. However, I noticed that the temp and oil gauge were still on the low reading, but the fuel gauge was above the F line. So, my guess is that it filled about 1/2 to a 1 gallon higher than the last time a filled it.

15.9 mpg

On this trip I noted I had around 13 - 14hg, and around 14.0 - 14.5 AFR. Thought it was a hair rich for best highway mileage so when I got to my destination, I dropped the main jets by 2 sizes.

On the way home, another 135 miles.
After the jet change, it was a stronger 14 - 15hg, and a consistent 15.5 - 15.75 AFR. It seemed to drive very well and I don't think it ever dropped into the power valve. Pulled into the gas station when I got home and topped it off again. This time, however, the needle was pegged over on the full end of the gauge, so I'm guessing that the pump kicked off way later than it had the previous fillups, filling up way higher than where I started.

15.0 mpg


If the fuel gauge had filled to the same line every time, I think those mpg numbers would be accurate, but I honestly don't think they are. Every time I filled up, I think I got a little extra in the tank than I had the time before.


Such as, for example, I used 8 gallons driving down and got 16.8, but I was able to put 8.5 in it (making it read 15.9 mpg). Then, I used 7.5 gallons on the way back (getting 18mpg) but then and was able to squeeze 9 gallons in (making it read 15.0.)

Really screwed up my mpg test unfortunately.

 

Each pump kicks off at a different point, and the way you angle the nozzle even has a difference. The only way I've found to get consistent MPG #'s is either to average them over many tanks, which I do for all vehicles we have via an iPhone app, or to fill each tank up until the gas is showing in the neck. But, I've also read that isn't good, although I still do it as it gets me much more consistent figures tank-to-tank.

I'd be interested in knowing what you are using for the AFR info. Have you already put it in this thread?

 

I have an excel sheet I'm keeping track of things with, and I'll average it up over time. Unfortunately, without doing a lot of long distance trips, inconsistent fill ups start to blend around town mileage with highway mileage. I guess if my next around-town mpg test is abnormally high, I can attribute it to my good highway mileage.

Crazy how 1/2 a gallon difference can completely throw your mpg results out the window. Either way, even if those numbers are accurate, 15mpg is still better than what the 390 used to give me.

Right now, I'm using an actual wideband O2 sensor that's in the exhaust pipe. The numbers show up on a display in the cab.

 

What Gary said. MPG on a solo long trip can be figured rather easily but it's best to compile figures over many trips.

I stuff the tanks when filling...have done so for years...old habits are hard to break. I think it may have something to do with the evaporative system and/or avoiding the potential for spilling or percolating out especially in hot weather. I have one tank that just will not fill unless the nozzle is set in the right spot.

 

The spreadsheet thing made me laugh. Our daughter posted on facebook that she'd created a s/s to track their recent move. My wife said "who does that remind you of", and she was referring to me. I even create them for vacations.

Well, getting better mileage than a 390 isn't too difficult - been there, done that. The 6 should do pretty well in that regard, but I'm expecting to get ~15 w/the 351W I'm building. So, I'd expect you do be able to do a bit better as you get things dialed in.

What wide-band instrument do you have? I'm considering doing the same thing since I'll be tweaking this 351W as well as the 351M on Dad's truck.

 

Heh, yeah, they're very useful for keeping track, especially over a long period of time and getting a good average.

As for the 390, I was referring to the Holley 390cfm (not a 390 V8). Not sure if that's what you meant.
But yeah, considering how I seemed to get more fuel in with every fill up, I think my mpg was actually quite alright.


There's quite a few different wideband readers out there.

wideband o2 | eBay

 

"Crazy how 1/2 a gallon difference can completely throw your mpg results out the window. Either way, even if those numbers are accurate, 15mpg is still better than what the 390 used to give me", per AbandonedBronco.

Of course, the shorter the trip, the greater difference the 1/2 gallon will make!

Do any of you "old timers" remember the old SHELL gasoline commercials? Shell used to sponser gas mileage competitions and would set up the test car to have a 1 gallon glass container as the test fuel tank. Each automobile would be driven until it actually ran out of gasoline and then the fuel mileage would be calculated.

 

I think that might be the ticket. Just filling the tank until I *know* it's full. Maybe until it's visible in the neck so I know it's at the same amount.

Normally, it's not a good idea for around town and such because it'll evaporate off the top, ect. etc. but on a long trip, it's going to be dropping in a relatively short amount of time.
I might try that next time.

 

I'm not sure I agree about the evaporation issue. Evaporation occurs at the air/liquid interface, and if the gas is up in the neck the interface is much, much smaller than if down in the tank.

Having said that, if you don't drive the vehicle for a ways immediately after filling it into the neck you may have an overflow due to expansion as you've filled up the volume normally expanded into.

I try to fill up on level ground and, if it looks like there is a slope, I point the nose downhill. That's since many of the fillers come off the rear of the tank, although I don't think that is true of our trucks.

 

Ahh, Okay, I wasn't sure what the reason was. Misinformed.

I read something along the lines of vapors escaping when you top off, but looking into it further, it's that the vapor recovery system on the stations pumps will suck vapors back in to prevent them from escaping to atmosphere. But if you overfill it and leave the nozzle in there, they can actually suck some of your gas back out. It can also escape into the vehicle's vapor recovery system.
Been a while since I read up on it, so my brain fuddled it up with "evaporating."

Expansion and overspillage is another issue as well.

But, it looks like if you let it shut off automatically, pull the nozzle back, and then top off manually (and visually), you should be okay for normal purposes.