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da55

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1973 Mustang Fastback 351 V8
Morning yall! What could it be when im driving and i really wanna step on it, the engine just tops out at around 85 mph. Im thinking maybe the carb or fuel pump cant send enough fuel? Kinda bums me out that i top out at the 80s and it sounds like it wants to choke.

Thanks!
 
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1971 Mustang Mach 1 M code 351 4 speed
Could you give a better description of what it does at 85MPH? If the car can only go up to 85 MPH it is missing a TON of power. Could be as simple as the carb choke being stuck or inoperable. In a lot of carburetors, if the choke is not opening, for whatever reason, the secondaries will not open and then your primaries airflow if partially blocked by by the choke itself, you basically end with almost no airflow into the engine... On a situation like this, if the engine runs normal, and just will not go over 85 MPH, I would look at fuel supply first. Besides the choke being stuck, you could very low float levels, a bad fuel pump, a clogged fuel filter, a clogged fuel sock inside the fuel tank, a rubber fuel line that is internally collapsing, anything that can limit the amount of fuel that the engine is getting. On the ignition side a bad coil can do this, timing that is way off could also do it, but the engine would not run well at all times if the ignition timing is way off. You getting any backfires through the carburetor?
 

giantpune

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We need more specifics.
Which engine, carb, air cleaner, fuel pump, rear end gear ratio?

If it were me, I'd start out by figuring out what RPM your engine is running when it "runs out of power" at 85. Then you can insert a fuel pressure gauge in between the pump and carb, and while the car is parked, rev the motor up past that RPM and see what your fuel pressure is.

You can also run a vacuum gauge from your manifold and from the air cleaner through a 8ft rubber hose and into your car. Watch your different vacuum levels as you floor the motor. At wide open throttle, there should be essentially zero vacuum in your intake manifold. If there is any, that signifies that your engine is trying to suck air but the carburetor (or something before it) is not flowing enough air to make the engine happy. If you have any vacuum measured at the air cleaner, between the carb and the air filter, that signifies your air cleaner is too restrictive and cannot flow enough air to feed the carb or your engine.

These are tests you can do with about $20 worth of gauge and T fittings.

Something else to look at is your throttle linkage. It is possible to hook it up in such a way that no matter how hard you press the gas pedal, the carb never even opens up all the way.
 
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My all time favorite vehicle is our 1969 Shelby GT500
You have been given some good thoughts re: things to consider. We just got into our hotel room, and I am finally at my laptop. Here are some more thoughts. Remove the fuel cap, as it is possible you have a non-vented cap which will allow the fuel pump to draw enough fuel to cause a vacuum in the tank and fuel supply line going to the fuel pump. The higher the rate of fuel consumption the worse the problem If that correct the problem replace your current cap with a vented unit. If the fuel cap is, by some chance, not the original cap it is very likely this is what happened, someone got a fancy replacement cap that is not vented, like for a 1971 or later Mustang, not knowing the difference in performance behavior with the two general kinds of caps.

Others have suggested looking for and/or replacing fuel lines, to include the rubber fuel lines at the carb fuel filter, the fuel pump (both suction and pressure side), and the fuel tank. Inside the tank is a sock filter that if plugging can be blown off if you leave the fuel cap off and blow some compressed into the output fuel tube (just a quick burst, otherwise you may end up with a fuel vapor fog all around you - not safe to do that by shooting more air than needed to blow the sock off the internal steel fuel pickup line.

Testing the fuel pump pressure and output volume is very important. Both tests are equally important. With the output volume test look for debris in the liquid fuel. Check and replace the fuel filter if needed. If your filter is the kind that threads into the carburetor inlet, remove it and see if you can blow through it. Any "significant" resistance indicates it is plugging or plugged. Be aware, if it is plugged there may be still more debris in the fuel tank, fuel lines, and fuel pump. If you use a tubing cutter to cut open the filter housing (not a hack saw, a tubing cutter) you can use a magnet to see if the debris is attracted to the magnet. If it is attracted to the magnet you have rust in the system, most likely from the fuel tank. Be prepared to have more filter plugging later if you don't replace or clean out the tank, blow out the fuel lines, and replace the fuel pump.

Another are to look at is the distributor mechanical and vacuum advance, especially the mechanical advance. As the RPM of the engine is increased, with the vacuum advance diaphragm disconnected thee timing should advance how fast and how far will depend on the RPM. I do not have any books with me with the specs, but I would be looking for at least 20 degree of advance by 2,500 RPM. If you are in that area you are likely fine. Otherwise the distributor may need to be serviced, re built, or replaced. The vacuum advance diaphragm ought not have any effect on timing at Wide Open Throttle, but it is still a good idea to test it for leaks or being ruptured.

Check the primary ignition dwell angle and/or point gap. If the point gap is too small the ignition coil will not properly saturate at higher RPMs. If you adjust the point gap/dwell angle, it will directly impact the initial ignition timing. So be sure to check and adjust the initial timing as needed. Check the spark plugs for proper gap. It it is an oem system you want 0.034" - 0.035" gap. Too little or too much gap will cause higher RPM performance issues

Finally, be aware a plugged/plugging exhaust system may be the culprit. If you have an exhaust pipe that is double wall piping the inner wall can collapse and cause restrictions. It is a long sho, but if everything else checks out that is an area to not overlook.


There are so many other areas to look at (spark plug condition, burning too rich or lean comes to mind). But, the preceding is where I would attack the problem.


Once you do figure out what is wrong please report back to this forum. I know I sure the heck would like to know what happens in the end.

Good luck!
 
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