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What engine? When was last tune up? Has it always been bad? Need some details...
 

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Either your engine is worn out or you have several things bad or needing adjusting: Here is a list of things to check, or adjust, or replace:

1. Initial ignition timing
2. Vacuum advance on the distributor, since you have a 68 you could have a dual diaphragm distributor, 1 is for retarding and the the other for advancing. These are controlled by temperature sensors and vacuum fittings on the engine. If there is no vacuum diagram on the top radiator support or the underside of the hood try to find one on the net and double check how the hoses are connected, since they could be wrong or the diaphragm in the distributor could be bad.
3. Air breather filter (clean and not clogged)
4. Spark plug gap (.032-.035")
5. Spark plug wires and dist HV coil wire (resistance should be no more than 1 K ohm per inch of wire)
6. Points (make sure they do not have any heavy pits or metal deposits, small ones can be cleaned with a point file) and check the gap of the points with the rubbing block on the highest point of a distributor cam lobe
7. Tire pressure
8. The power valve in the carburetor (if the diaphragm ruptures it draws too much gas, first indicator is poor mileage, worse indicator is hard to start, black smoke out the exhaust) If you have a autolite carburetor, you can remove the carb without tipping it over, remove the 4 screws on the bottom covering the power valve, if gas runs out the power valve is bad, you can get one at your local autoparts store, you may have to order it since the car is so old, but they can get it with no problem.
8. How you drive can make a difference.

I may have forgotten a few things and I'm sure some of the others will help out. Post your results. Good Luck.
 

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If you don't see clouds of black smoke in your rearview mirror when accelerating; you really need to check for fuel system leaks while the engine is running. (It would be too obvious of a puddle if it were leaking while parked.) BE CAREFUL and KEEP A FIRE EXTINGUISHER HANDY!







Disclaimer; This is my opinion only, based on available information.
 

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My 1968 mustang mileage is terrible. It only gets about 3 mpg. How car I get it better
I agree with all of the above...the 6's typically get 18-23 mpg and the V8's, depending on a lot of variables (drivetrain & foot) get 15-21 mpg.

If you are really getting 10 mpg or less...there is likely a fuel leak.
 

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suggestion

Need to know What kind of carb, engine and other specks I6 or V8 ??

Here is one situation I encountered with an Edelbrock 4 V carb.

Some one had taken the top of the carb off, and while replacing it bent both Metering Rods enough to make the Rods not go in the jet ( bent to outside of jet), therefore no closing of the jet and so, It was blowing raw fuel out exhaust...Very few MPG resulted.

Many different situations can cause problems as others have stated. Let us have a little bit of info on your engine.

Joe
 

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How did you come up with the 3 MPG? You can not just use the gas gauge. You need a measured distance and an accurate amount of gas used.

Check the oil and see if there is a gas smell. That means a bad fuel pump. Someone may have put really big jets in the carb that are not good for city driving.
 

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If you are using your gas gauge it is not accurate enough, just because it says 1/2 tank doesn't mean it is at 1/2 a tank, some are fairly close and some not so close. You need to fill the tank completely as much as you can then drive the car, some distance, then fill the tank again, then do your calculation. Warming a car up and the choke on, can use a lot of gas. And if the choke never opens up completely that will make it use a lot of gas. My 2 cts. Good Luck.
 

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Can the mixture on the carb have anything to do about it
Sure, that has a lot to do with it. But like the others, I doubt it could be as bad as 3 MPG and still run unless you have a leak.

Also, how do you know it was correctly tuned 2 weeks ago if you hadn't driven it since then?
 

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If you are using your gas gauge it is not accurate enough, just because it says 1/2 tank doesn't mean it is at 1/2 a tank, some are fairly close and some not so close. You need to fill the tank completely as much as you can then drive the car, some distance, then fill the tank again, then do your calculation. Warming a car up and the choke on, can use a lot of gas. And if the choke never opens up completely that will make it use a lot of gas. My 2 cts. Good Luck.
And in case it isn't obvious, use the amount of gas you put in it to do your calculation, NOT the gauge reading.
 

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Discussion Starter #15
Ok I will do the calculations. I'm pretty sure it has a leak but my step dad doesn't think so because it blows black exhaust out of the pipes
 

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Well your step-dad has given the clue, with the black smoke, either you are not able to burn the gas or most likely the carburetor is dumping in too much gas. Since I am not an edelbrock expert I will use generalities, like the needle and seat is not shutting off the gas could be on the primary or secondary side; I don't think the edelbrock has a power valve so if it has metering rods, either they are bent, stuck, or not sticking in the metering jets; lastly the choke is not opening up, so it is either stuck, not adjusted, or not getting the electrical or heat to open it up. My 2 cts from an old mechanic. Good Luck.
 
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