Carburetor-specific tips?

[jcp123]

Many of you may know by now that I'm selling my Focus and am driving a '67 Mustang Convertible as my daily driver. Last tank came in at 13.71mpg, better than I'd have expected given its past performance.

Car has a 302 bored .030" over (306ci total), C4 3-speed automatic w/o overdrive, and a 3:1 ratio rear end. The drivetrain was recently rebuilt and is past its official break-in period but probably has another 10k miles or so of loosening up to do. Car weighs approx. ~2800 lbs, give or take 150lbs. I've also noticed courtesy the '68 Hardtop I drove in high school that the cars are more or less indifferent to how they're driven. In 3 years, at 100-115 miles per week, my weekly Friday fillup never varied more than a half gallon either way, and I'd guess my standard deviation at around .4 gallons. I venture to guess that this is because they have gobs of torque for their weight and already shift at exceptionally low rpm's?

So, what things apply specifically to carb'd vehicles which I can apply?

[lunarhighway]

i've got a carbed vehicle to, so i'm interested to lear something as well. it's my experience that a carbed engine does not have to be much worse than an injected one. a carb does need propper tuneing and a worn carb that can't be tuned like it should will have a big inpact on FE.

for the moment i'm trying to figure out how to put water injection on my car. something wich at least in theory should be far simpler than on modern engines.

the only mod i did was replace my carb with an almost identical one from a bigger engine, as a replacement for the worn original. the new carb should have slightly bigger fuel jets and after installing i saw a slign improvement in fe.... than again the original carb had a worn adjustment screw seal so maybe the jets are not the reason.. if i've got the time to mess around with it i might try the original jets again

[Erik]

lunarhighway is right on the wear factor, if the carb body is worn around where the throttle shaft enters, that air gap will make it impossible to tune.

The 3:1 rear axle is a good thing

1. You can adjust your choke pull off so that it releases the choke earlier as it warms up. Or go to a manual choke knob that lets you run with as little choke as practical when its cold.

2. You could try smaller jets in the carb, or lower the float level in the bowl slightly to lean it a little

3. Advance timing slightly and be sure your mechanical and vacuum advance are working properly. There has been some debate over using venturi vacuum for advance if you want more power. I think manifold vacuum hooked to the advance mechanism will give you the best economy

4. Lower the idle speed and adjust the idle mixture properly

5. Does your car have a temperature activated valve on teh bottom of one of the exhaust manifolds that partially plugs that manifold and forces exhaust through the intake when the engine is cold? If so, make sure this valve isn't seized up, it helps heat your carb up so gas can vaporize better.

6. Try switching to a hotter thermostat- 190-195 degrees.

7. Take off the fixed radiator fan blade and install a temp activated electric fan

Edit- scratch #5, I see on your profile that you have headers, I also see that you have a clutch fan, so that's much better than a fixed blade (#7)

[kamesama980]

electric fans can still help, even with a clutch fan. BUT the less variation in the engine revs, the less it'll alter the FE.

Headers can still be used for heated intake flappers. the flapper takes in air from over the exhaust manifold so it's heated (as opposed to 20f air outside...) to regulate intake temps NOT exhaust gasses themselves

Headers and a good exhaust will help FE because the engine will breathe more easily as long as they aren't so big the torque curve is shifted to higher rpm than you use

[VetteOwner]

Originally Posted by kamesama980

electric fans can still help, even with a clutch fan. BUT the less variation in the engine revs, the less it'll alter the FE.

Headers can still be used for heated intake flappers. the flapper takes in air from over the exhaust manifold so it's heated (as opposed to 20f air outside...) to regulate intake temps NOT exhaust gasses themselves

Headers and a good exhaust will help FE because the engine will breathe more easily as long as they aren't so big the torque curve is shifted to higher rpm than you use

yes those are all good ones. when coasting, coast in neutral. anyhtign to keep the rpm's down is a good thing. (liek neutral coast down hills, dont engine brake) remember if the engien is turning its gona be sucking fuel down.

yea most older cars either had a solid fan(no cluch) or they had a heavy clutch fan. you can easily pick one up at a junkyard for pretty cheap(try to find one out of a V8 or V6, they can usually pull enough CFM's

[Erik]

Clarification on my #5 above- I was referring to the butterfly valve on only one end of one exhaust manifold (I suppose you could use one with headers just downstream of the convergence point). When one outlet is constricted, this forces exhaust gasses through a passage inside the intake manifold over to the other exhaust manifold, which is free flowing. The action of the hot exhaust through a passage in the intake manifold is what helps heat up the carb from the bottom up.

I see you are using an Edelbrock intake so I'm not sure you would still have this special crossover passage.

Russell also has a good point about using an intake air heater to draw in hot air from around the headers to also aid vaporization of fuel- this was probably part of the stock air cleaner/snorkel assembly (if you still have it).

[rvanengen]

Maybe in your budget?? But, how about thinking about a TBI megasquirt setup? You will get much better driveability, especially when cold, and better throttle response...and should be VERRRRRRY easy to setup on your car...lots of room!

[VetteOwner]

Originally Posted by Erik

Clarification on my #5 above- I was referring to the butterfly valve on only one end of one exhaust manifold (I suppose you could use one with headers just downstream of the convergence point). When one outlet is constricted, this forces exhaust gasses through a passage inside the intake manifold over to the other exhaust manifold, which is free flowing. The action of the hot exhaust through a passage in the intake manifold is what helps heat up the carb from the bottom up.

I see you are using an Edelbrock intake so I'm not sure you would still have this special crossover passage.

Russell also has a good point about using an intake air heater to draw in hot air from around the headers to also aid vaporization of fuel- this was probably part of the stock air cleaner/snorkel assembly (if you still have it).

now on my chevette, my temp switch broke so it thought it was freezing cold all thge time, meanign that flap would be stuck wide open and allowign only hot exhaust gasses. it made me get horrid milleage. but sinc ei took thet switch out and blocked off the vacum lien iv egotten alot better milleage

[kamesama980]

Originally Posted by Erik

Clarification on my #5 above- I was referring to the butterfly valve on only one end of one exhaust manifold (I suppose you could use one with headers just downstream of the convergence point). When one outlet is constricted, this forces exhaust gasses through a passage inside the intake manifold over to the other exhaust manifold, which is free flowing. The action of the hot exhaust through a passage in the intake manifold is what helps heat up the carb from the bottom up.

ohhhhhh it's not taking exhaust as part of the intake charge air, it's just running it past the carb to warm it up??? yes, that might be a good idea but I dunno.

The flapper I was referring to is stock in may older cars carb and early injection that uses a thermostatically controlled actuator to move a flap so that when the intake air temp is below a certain temperature, it pulls air from around the exhaust manifold as the intake charge thereby somewhat stabilizing the intake air temp and improving warm up time a little.

[8307c4]

My older 1986 D-250 truck was a carbed 318 cid 2bbl, standard 3 speed auto.
With a 12 foot landscaping trailer and a mower and the rest of the equipment on there the truck and trailer weighed around 3 tons, I used to get 14 mpg with it.
I just sold it in January of 2007, wasn't that long ago.

I always ran BP premium, no questions asked, 93 octane, the best.
Yes sir, tank after tank, no exceptions.
This kept my carb sparkling clean for one, the fuel burned better for another, but the below helped in the more thorough combustion process as well:

I had 10mm Taylor wires, a Mallory cap and rotor, a Firestarter coil.
Double platinum plugs, and a Mr. Gasket air filter.
Kept all tires inflated to the Max. psi as printed on the side of the tires.
Changed the oil and filter every 3,000, trans fluid and filter every 10k.
Air filter once or twice a year, also anytime the mpg got worse I'd replace pcv valve and breather cap (which I eventually got a High performance kind so as to prevent it from drawing air from my intake).
NOTHING was done to increase the fuel flow!

With carbs the rest is easy, the more gas I gave it, the worse my mpg.
This may not be the case with a car, but with a truck the faster I drove, the worse my mpg as well. Likely this was because the aerodynamics of that truck were at least as efficient as that of a billboard.

One of the biggest improvements beyond spark and airflow maximization was DWB and gliding, I'm not explaining this, other threads abound.

Beyond that a super light foot.
Try constant gas acceleration sometime.
That is, if you're standing still and get ready to go into a 45mph zone, place your foot as far down as you would have if you were already doing 45, then let the car and the engine do the work for some time. If by the time you reach 45 you have to let off you're doing it wrong lol.
As in, don't be surprised if 2 miles later you're at 38mph and you have to stop for the next intersection, that's how that goes.

You might try a thorough cleaning of the carb with some cleaner, but I wouldn't pull it off or apart for that, just spray it good... Still, 93 octane, BP, Texaco, the expensive stuff.

It's an old car, nobody will expect you to drive any which way but super slow anyhow.
Why not?

[VetteOwner]

hehe yup, i have noticed that tho, if i run midgrade(havent tried premium yet) in my chevette i instantly get 10 mpre mpg...vs regular(87 octane, mid is 93 hi is 97) im currently on my 2nd tank of testing with midgrade and its taken alot longer to use the fuel than expected so thru most if it cold weather has pinched in on the mpg. (still seems good tho). so we will see. its been like a month(driving truck lately) so it might take awhile to get it used up.

[jcp123]

Interesting stuff. A lot doesn't apply since the engine is brand spanking new rebuilt, and the carb is a brand new unit as well. Although, having sat for a couple of years the carb may need some spray - it idles rough which it didn't used to do. Berryman B12 to the rescue! It has automatic choke, which I'm not a huge fan of, but just a jab of the gas and it turns off anyway. The dyno showed the carb mixture was stoichiometrically correct already, though with the soot in the exhaust and its good-natured manner in the cold, I wonder about that. Original air cleaner assembly with the snorkel is long, long gone, and no exhaust crossover either. Already have upgraded ignition with PerTronix solid state ignition and 45k volt coil (original coil was rated 20k volts, but bench testing showed them putting out closer to 12-15k...ouch) and 8mm wires.

As for hot air intake...it's an open-element air cleaner with headers under the hood heating it all up...does that count

I may upgrade the plug wires as one already has a hole in it, and if I get ahold of a timing light I may play with that as well, but I don't want to be running anything more than 87 octane. I'll try double platinums, but I've heard these cars aren't too fond of anything other than copper-cores. I may also try the t-stat, although the clutch fan should solve the problem of it running too cool (runs @ about 160* with the solid fan). How effective are vacuum gauges? I've heard they help a little but are mediochre at best...?

The hard part is with my right foot...going from a gutless 4-banger to this car, the loud exhaust and racy lines of the Mustang have folks begging me to hit it, and it's all too pleasurable to let the engine have its head and let that jet-smooth stream of torque pull the car...but I can see the potential. With a light foot, the trans shifts @ about 1100-1200rpm, and the car easily cruises at 900rpm in top (about 20mph). That's what I'll have to concentrate on for now.

[8307c4]

It seems you're doing fine, I would replace the wires one day, Accels are pretty good too, if nothing else you'll have even more power (just what you need lol).

On the plugs the only reason I use 2x platinums is they run better, longer.
No, I don't think they're any different when new, but over time.
Same with premium, it runs better, longer... And something about carbs and higher octane, but that's your choice, might be worth the experiment for a few tanks full, I do know the trade-off can only be seen over the years and many many miles later.

As for the foot, you don't have to tell me

I'm a fan of high power rwd cars, my first was a 1984 T-bird w/ a V8, I later bought a 1988 T-bird w/ 5.0 engine, those were just the start, I also tried a 1976 Nova with a 350 cid 2bbl but it was falling apart... Now I have two lower end bmw's (still VERY fast) and a D-2500 truck (don't laugh, that 5.9liter cooks up some power, I think it has 400hp, maybe not thou, sure gets off the line thou).

But over time I did learn, it almost always takes time to get used to it, but the longer I drive them the less it tempts me anymore. It's hard, that's for sure.

Best way I can explain it is driving is about control, and control refers specifically to the ability of NOT using the power

Age helps, too.
But it does take time, I can't claim perfection by a stretch yet.

[VetteOwner]

Originally Posted by 8307c4

It seems you're doing fine, I would replace the wires one day, Accels are pretty good too, if nothing else you'll have even more power (just what you need lol).

On the plugs the only reason I use 2x platinums is they run better, longer.
No, I don't think they're any different when new, but over time.
Same with premium, it runs better, longer... And something about carbs and higher octane, but that's your choice, might be worth the experiment for a few tanks full, I do know the trade-off can only be seen over the years and many many miles later.

As for the foot, you don't have to tell me

I'm a fan of high power rwd cars, my first was a 1984 T-bird w/ a V8, I later bought a 1988 T-bird w/ 5.0 engine, those were just the start, I also tried a 1976 Nova with a 350 cid 2bbl but it was falling apart... Now I have two lower end bmw's (still VERY fast) and a D-2500 truck (don't laugh, that 5.9liter cooks up some power, I think it has 400hp, maybe not thou, sure gets off the line thou).

But over time I did learn, it almost always takes time to get used to it, but the longer I drive them the less it tempts me anymore. It's hard, that's for sure.

Best way I can explain it is driving is about control, and control refers specifically to the ability of NOT using the power

Age helps, too.
But it does take time, I can't claim perfection by a stretch yet.

certian engines run like crap with certian plugs. maybe the older ones are more "universal" but i know my truck runs liek crap on any other plugs besides the standard AC delcos, im not the only one thats noticed that either... but wires, generally the thicker the better and dont get the liek $16 cheapos either...

hehe if i drive my chevette for a week and get in my truck and have to gun it it seems liek its got a huge v8 or somehitng, even tho its just a 4 banger...

[jcp123]

Yeah, believe it or not my SVT Focus really came to life with copper core plugs - rumour is that the platinums really rob them of a lot of throttle response if not peak tq and hp. I haven't tried platinums on any of my Mustangs yet, but that's because I always figured there must be a reason nobody ran 'em on vintage steeds like mine. After a few more tanks to get a baseline MPG, I may switch over to some platinums though just to see...the wires are pretty good ones, the only reason to change them really is because of the hole in one of them. It doesn't arc, but I just don't like to have a hole in my wires. Even if nobody else sees the hole, I feel like it's trashy...

I'll try some higher octane gas too, again after I have a baseline, although we specified on the rebuild we just wanted to run 87 oct.

Now, does letting it idle in N at lights and such have the same fuel-saving effect in a carb'd vehicle as in an EFI? I can't see how it would, but I've been surprised before...

[VetteOwner]

well, some theres no sensor computer crap, slower the engines turning less vacum its makign thus less fuel beign sucked down/pumpped, so in theory maybe staying in drive would lower rpm's and save gas.... but i dunno i dont belives anyones tried since theres no real way to measure them equally.

[beatr911]

Weld in a bung for an O2 sensor and monitor your mixure for a while. Adjust mixture accordingly. Advance the ignition until you hear ping, then retard 4 degrees to get max safe cylinder pressure.

BTW I'm thinking of doing almost the same thing! Gotta love those old Mustangs. RWD with good power and handling is often a great driving experience - smiles per gallon.

[BMac]

Originally Posted by 8307c4

My older 1986 D-250 truck was a carbed 318 cid 2bbl, standard 3 speed auto.
With a 12 foot landscaping trailer and a mower and the rest of the equipment on there the truck and trailer weighed around 3 tons, I used to get 14 mpg with it.

My 98 Dakota Sport with a Magnum 318 is giving me 14mpg now in this cold with hot air and a few ignition goodies but all the other mpg mods don't take anymore. I'm getting a 95 and modding the hell out of it and going carb! This I know I can gain some! I've got rare carbs to test!

[VetteOwner]

yea im just waiting (hopefulyl not anytime soon) fo rthe 4 banger in my truck to die then in goes a 350. S-10's with a v8 are common swaps, tons of info, can snag a 350 outa almost anyhting old, LOTS of room engine bay wise. power to weight ratio is crazy

[Bruce]

Originally Posted by Erik

The 3:1 rear axle is a good thing

3:1 is okay, but if you can get longer legs and first won't be too high, go for it. My `78 Malibu had a 5L V8 and 3A/T w/o lockup; it had a 2.29:1 rear. Full-throttle shifts were 45 MPH in 1st and 65 MPH in 2nd at 4000 RPM.

[jcp123]

Had 2.73:1's in my '68 Mustang...don't care to go back. What I would like to do as far as gearing goes is to someday drop in Ford's automatic overdrive transmission and get a shorter set of gears, probably some 3.73:1's. The shorter gearing's mechanical advantage should help things out in-town and the 4th gear's .75 cog plus converter lockup should keep things in order on the highway.

[philp100]

One of the things I have not seen mentioned here relates to driving style. On a carb you have an accelerater pump that shoots extra gas in when you step on the pedal. Therefore, steady throttle position tends to yield better economy. If I remember right, P&G was first introduced by college students in the late '70s that were competing in in fuel economy challenges. All the cars were carbed and they found that if they used a steady throttle position to accelerate and then coasted they did not use the accelerator pump hardly at all.
I want to say this was covered by Popular Mechanics back in the late '70's if anybody has the motivation to try and search for it.

Phil

[jcp123]

I may try that eventually, but as mentioned in my first post, the torque to weight ratio really makes these cars almost indifferent to driving style. The '68 I had never varied more than a half gallon on its weekly fillup in three years, no matter if I flogged her or pampered her.

[philp100]

My '74 F100 with the 460 was the same way. 12mpg empty, loaded, towing, hiway, city. I had some friends with '60s vintage 'stangs over the years that consistantly got better than 20mpg with the 302 ci engines.
Proper jetting and tuning on the carb is going to be your best for mpg improvements. Playing with the spark plug gap on some Fords seemed to help also. I recall having a 400M in a '79 F150 that really woke up when gapped out to .045". With your higher voltage ignition it may be worth trying.