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Old 12-09-2007, 10:09 PM   #1
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Carburetor-specific tips?

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?
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Old 12-10-2007, 03:07 AM   #2
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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
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Old 12-10-2007, 08:16 AM   #3
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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)
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Old 12-10-2007, 08:35 AM   #4
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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
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Old 12-10-2007, 08:47 AM   #5
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Quote:
Originally Posted by kamesama980 View Post
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
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Old 12-10-2007, 11:33 AM   #6
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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).
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Old 12-10-2007, 12:33 PM   #7
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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!
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Old 12-10-2007, 01:22 PM   #8
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Quote:
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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
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Old 12-11-2007, 12:06 AM   #9
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Quote:
Originally Posted by Erik View Post
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.
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Old 12-11-2007, 03:55 PM   #10
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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?
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