I didn’t see where you defined what type of ignition you are using . Maybe I skipped over it . Many have suggested timing issues and carb jetting . The vacuum advance is your friend on a street driven car . The factory defines the vacuum advance as “ economizer “ I would expect that for a reason . I know it doesn’t look cool and Big Daddy Don , doesn’t use one , but it is there for a reason on a street driven car . Get an adjustable unit install it , adjust it for best advance for your needs and look farther in to over fueling . I bet it will do at lease 14 MPG for you if not better .
I'd be absolutely astounded if the addition of a vacuum advance produced a gain of anything more than 1 or 2 m.p.g.
Mostly side road test drive and side roads to the next town over to get her aligned. Any yep, Muncie M21.
The new tools allow those that don't understand carburetors to tune almost as well as those who do understand carburetors. Seriously, they are quite useful, but not necessary. Lots of cars were tuned successfully prior to exhaust gas analysers. Jon.
A mechanical fuel pump does occasionally run higher than what you'd expect, so I always install a regulator, and pressure gauge inline. I use the Holley regulators as they have one inlet, and two outlets, which allows me to put the pressure gauge on one of the outlets. The high pressure you're seeing can indeed cause poor fuel mileage, but it can also wash down cylinders, and dilute the oil, which could result in bearing failure. I'd get a pressure regulator on it soon, and consider changing oil and filter once you've got the regulator on and adjusted for 4.5-5 psi. Bet your mileage with the new carb, and pressure corrected will go up a little too. Plus your new engine will appreciate not having all that gas mixed into the oil.
Pressure regulator costs $40, the plumbing for it will be another expense. How about buying a new $20 fuel pump, perhaps a different brand, that might have the correct pressure?
Vacuum advance can easily gain more than 1 or 2 mpg. However, that is only true at light throttle cruise. If the car is never driven in light throttle cruise conditions, it won't help at all.
Thought about a new mech pump, but figured may as well definitely control the pressure with a regulator. Besides, it’s shiny…lol. New Holley regulator coming in on the Amazon truck.
It’s a matter of perspective. If the OP is getting 8mpg currently, and it jumps up to 10,that’s over a 20% increase. With a 20 gallon gas tank you’d be getting an extra 40 miles out of a tankful. If it isn’t sacrificing performance, well worth it IMO.
Yeah...and after all the grief I've endured eating cams, I'd really like to know I'm not thinning my oil with excess gas.
Mark, you couldn't possibly understand the rest of us under blessed lower life people. Everything we touch doesn't turn to gold like it seems to do for you. If you don't share where you get your magic formulas to be able to accumulate any cool car and toys you want, you could at least stop torturing us underachiever's.
Oh ,oh,,,,,,I didn’t think about the cam lobes . I’m sure the bad fuel mileage is where the lobes have been rounded off . I would pull that engine and put in a late model LS mill . Lol,,,,just kidding,,,,,don’t fret the little things . I believe with some judicious tuning and insight,,,,,,you can increase your mpg a fair amount . Get the fuel pressure under control,,,,,check the distributor,,,,,,might even need to put it on a machine and get the advance dialed in . I’m not certain what your compression is,,,,,that can contribute to poor fuel economy . Is your Speedo correct,,,,,that will help dial in the mileage? And you must check it on a full tank,,,,,no guessing . Go on a nice drive,,,,,,100-150 miles round trip,,,,not too hard to do . Then check it immediately,,,,,also,,,,.check your tire pressure . Recently I had lost almost 2 mpg,,,,,turns out it was low tire pressure. Aired them all up,,,,bingo,,,,right back on the money ,,,it’s easy to overlook . Tommy
You have to check things kinda systematically and verify what is working right. I would install an O2sensor and guage because the first thing you need to know is how rich your exhaust is. Assuming or guessing that poor fuel mileage is going to be a fuel related problem is somewhat reasonable, but its always possible its an undetected fuel leak. Once you can see what your exhaust is doing, you can immediately see the result of whatever else you attempt next. Its also nice for future changes and dialing in ratios. Having a "tool" that shows you an exact result is better than guessing. Then I would start from scratch and verify that your timing mark is correctly located. Then check timing and advance to see they are actually working properly. I would put a vacuum gage on the engine while checking the timing just to see what it does......but the first thing I would want to know is whats coming out the exhaust.
This is the reason we share info on the HAMB to me. I don’t have an Edelbrock carb but had quite a few specific Carter AFB’s long ago and for the most part were very good since they were not universal fit.
I’ve had about every regulator you can think and found most to a POS. ie: Holley, Mr Gasket, Mallory, etc. only Aero Space Industries has been consistent and is well worth the cost to me. We run both a 422” and 408” on 1/2 mile dirt using an 850 at 5.5 psi with a -8 feed and -6 lines to the bowls. You do get what you pay for..
Had a customer complaining about low fuel economy. He had a new Dodge D150, slant 6 with a 5 speed manual. He claimed he was only getting 12 mpg on the highway. I brought the truck in, checked the engine with an exhaust analyzer to make sure it was running correctly and attached a fuel economy setup. It was a vial you connected to the fuel pump that held exactly 1/10 gallon. It had a valve to run the engine either on the vial or on the fuel tank. I came up with 14 mpg in town and 20 mpg on highway. Long story short, he didn't believe me, got the Chrysler zone office involved and came up with the same results. Zone office said sorry, there is nothing wrong with it. Customer comes in complaining to the sales manager. His story, which I hadn't heard, is that one time going home he got 30 mpg. He wanted us to fix it so that it got 30 mpg all the time. Turns out, he was figuring his mileage based on the position of the fuel gauge. Sales manager took him out, filled his tank and drove the truck with him in it for about an hour. Came back, re-filled the tank and figured the mileage; it was about 22 mpg. He never complained again, although I don't know that we convinced him of anything.
With only a few burnt valves and holed pistons along the way right ha ha! You're right of course, but the new tools give the novice kinda guy more confidence in experimenting and tuning, that while he's out on the highway something expensive isn't going to roast. Those old timers sometime came by their knowledge the hard way. I will say gasoline today must be quite a bit different than the leaded stuff of old as plugs don't color up a whole lot. When spark plugs today look fouled with pump gas, the AFR is WAY off.
Ignition figured out? Hottest coil, big plug wires & what size & brand of spark plugs? Also seen in real time synthetic oil/trans & rear lubes helping in your cause. And going way into the motor, a custom ground camshaft suited to both mileage & git-up-n-go will net better gains than a off the shelf grind.
The Science to his bad fuel mpg is the Op is driving South to North , Gravity Pull to the Equator!! But Seriously that Engine , Gear , trains should be 15 mpg plus. Unless you are pulling threw the gears @ every stop & go. What is the ethanol percentage ? You will get Less mpg with Ethanol mix. Stay a way from the Spectrum mechanical diaphragm fuel pumps they've been sitting on the shelf for a while they only last is Little as a few months even bad right out of the box. Not E85 correct? If not Fuel, its all in the tune , that engine will like @ a start of 15 degrees of initial timing .. & I like 1,000 plus RPMs @ Idle . what kind of air filter are you running? Some air filters can restrict / choke Dramatically.
Think i'm to old for this thread, have had holly's an edelbrocks wu ha , i prefer a q-jet because they are like old friends an yeps my timing is set by ear an reading the plugs ,the carb to, 283 stroked to 354 single plain, q-jet , 20 mpg at 3800 lbs , um was your carb a double pumper, almost any sbc gets ballpark 15 that's it's bread an butter ?
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