Losing power


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May 7, 2022
Reaction score
New York
My Car
1973 Fastback Mustang Mach 1 351Q 4v
Hello everyone,

Just as some background info I have a 1973 Mach 1 q code 351 Cleveland with a c6 transmission. I wanted to see if I could get peoples opinion about where to look for an issue I’m having. While driving normal speeds the car works great and accelerates fine. However I tried going to 65+ and it started to lose power and the exhaust started popping a couple times eventually when I slowed down to about 30 and barely touched the gas was I able to get it to re stabilize and get back to about 55 and it ran fine with no issues. I didn’t try to go above 65 again out of fear it would just die on me and I’d be stuck on the side of the road. I was wondering if maybe there’s a fuel or carburetor issue? I was also wondering if maybe it could be a transmission issue? My transmission shifts fine when building up to 65 but I didn’t get to see how it shifted at higher speeds because like I said my 73 would lose power and want to shut off. I only mention my transmission because while it does shift fine there are sometimes when I am switching from P-R-D etc. that it switches gears a little rough. If you could give me some ideas of where to generally start looking that would be great. Any help at all is appreciated greatly, thanks!
Welcome. A few pics of the car would be great. Some of the engine compartment as well. Finally, if you could update your location, that helps.

It could be either spark or fuel, but I’m not sure we have enough info. What happens if you start off, WOT. Will the car engine speed up and then shift, etc until that 65 mark? Do you have to “baby” the engine to get it up to the 55 speed?

If yes, my initial reaction would be to check the fuel filters. If it’s easy to access, you could change it and try again, to see if any effect. Also, you need to check the points gap and timing; those could also be contributors to the tepid engine RPMs.

Let us know what you find.
There was a similar case on here a couple years ago and, as I recall, it turned out to be a fuel pump that couldn't keep up. I don't recall if it was the pump or old fuel lines corroding from within. @Stanglover may have been on that thread.
I suggest starting by establishing a baseline that the systems providing fuel and spark are in proper order. Items to check include:
0. Freshness of fuel (did car sit and has sour fuel)
1. Fuel filter(s) for restriction
2. Rubber hoses for cracks/leaks
3. Cap and Rotor contacts for corrosion/contamination
4. Ignition points for proper gap/dwell and/or arced contacts
5. Electrical connections clean and tight contact for push-on connectors, damaged wires/insulators.

Many owners hibernate their cars over the winter and pull them out and expose problems that emerged while in storage.
With the limited amount of information I have seen so far, it is hard to tell if the situation is fuel or electrical. But, on a gut feel I would lean toward a fuel starvation issue. There are quite a few things in the fuel system that can cause grief like you describe. The ones off the top of my head include:
  • Fuel filter pugged/plugging
  • Sending Unit "sock filter" inside the fuel tank at the end of the fuel pickup tube has gotten plugged with fuel tank debris
  • Steel fuel line(s) between the fuel tank and fuel pump is crushed, bent, kinked, dented, or restricted
  • Rubber fuel lines at Sending Unit/Fuel Outlet, between steel fuel line feeding the carburetor, or at fuel filter near carburetor have collapsed internally or are restricted
  • Carburetor float level is too low
  • Fuel pump is not able to deliver adequate fuel volume and/or pressure to the carburetor (perform both the pressure and volume tests)
  • Carburetor internal passages are restricted or plugged with debris (not likely if the fuel filters have been working)
  • Fuel cap is not venting properly for vehicles without a Fuel Evaporative Control system
  • A problem with the Fuel Evaporative Control system for vehicles equipped with a Fuel Evaporative Control system
All of those conditions are fairly easy to check once you become aware of them

For these First Generation cars there is no OBD2 port you can plug into and test drive a car until a failure occurs, sadly enough. So you end up having to test the the car, perhaps with an(other) experienced technician riding along to help listen for indications of the problem, in the hope the problem manifests itself, and remains consistent enough to make a decision for the order in which you check a few things back at the shop. Or, there are some diagnostic items that can be used on a road test that can help narrow down or even find the cause(s) of the problem.

Personally, with the advent of new smaller and more capable test equipment, I would be inclined to connect a small, portable oscilloscope (like the MicSig SATO 1004 I purchased recently). In many ways, bringing a portable oscilloscope on a test drive is likely looked at as being overkill. But, it would be able to identify an ignition system problem pretty quickly if there were enough channels to connect to various circuits while having its probes in some key areas on the scope. Assuming the scope has four channels I would attach a probes to the positive and negative terminals of the ignition coil, and place an inductive ignition clamp probe on the ignition coil wire, and the 4th probe would be another inductive ignition clamp on Cylinder #1 to let the current being sent to Cylinder #1 be used as a trigger. Then you can drive until the problem occurs while a 2nd technician who understands how to use the oscilloscope dynamically watches the various channels, looking for anything that is unusual.

If the lack of power occurs, and the ignition system is found to be operating properly, it is time to look again at the fuel system. And, although this sounds like a rather eventful effort, just be glad you are living in a time where oscilloscopes are truly portable and can provide a lot of insight re: where electrical issues are relatively easy to diagnose by harnessing the power of modern diagnostic systems.

There is another power robbing problem that you may want to consider, even though in these older cars it was not a huge concern. It it entirely possible the exhaust system is plugging or plugged up with rust in the muffler or elsewhere. This is a long shot, but worth considering. The portable, 4 channel, oscilloscope I own is a MicSig SATO1004, the one with the automotive diagnostic probe kit option. Unless you are looking for an excuse to drop about $1,000.00 into such a piece of equipment, I would focus on ruling out the other possible power loss issues first.

As for me, I already had (still have) a 50+ year old, vintage sun oscilloscope in my garage. It is mounted to a large cabinet that is on rollers. There not any way I would be able to put that in a car for a road test! But, I know how powerful a scope is when it comes to diagnosing ignition and other electrical problems, so getting a portable oscilloscope unit was on my "must have" list.

Well, thereI go again. I fully intended to try to make my responsive post a brief list of things to consider looking at, and I once agin end up writing a little book. I would apologize but I would not mean it. As usual I felt compelled to provide everything I would consider and why.

If the fuel system is not causing the loss of power please post an update saying so. Between myself and many other highly experienced enthusiasts in this forum I am sure we could provide you a "nice" list of things to check out (of course, enough to fill another booklet!).you posting the resolution you find once you get this problem diagnosed and repaired.

The 1972 shop manual calls for 1 pint of fuel in 20 seconds (thru the fuel filter on a warm running engine @500 rpm,s.
Fuel pressure spec. is 5-7 psi under the same conditions.
Not telling you how to test, but SAFETY FIRST.
I concur with BoilerMaster, to include being safe. Personally, unless I am using a fuel tester designed to measure liquid fuel capacity I do not perform either test on a running engine. I find if the engine is just cranking over, with the ignition distributor rotor is removed to prevent inadvertent startup, volume and pressure tests can be dome more safely than with a running engine. If it the fuel pump passes when cranking it will certainly pass when running.

If it turns out the pump fails it is very likely the rubber diaphragm inside is leaking. Fuel leaks due to failed pump diaphragms can end up doing a few things, not pump (enough) fuel forward, pump liquid gasoline into the engine crankcase, and/or leak liquid gasoline externally onto the ground below the fuel pump. When I see a bad fuel pump I will always pull the engine oil dipstick and sniff to see if I detect any gasoline fumes. If I even "think" I smell gasoline I change the oil filter and oil, lest I end up running the engine with oil that has been contaminated with gasoline. Fuel contaminated oil will destroy an otherwise good running engine.

See page 24-30-03 for the fuel pump specs (trust the dealer, but cut the cards).


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Hi everyone sorry for the late reply I got busy with work etc and didn't have too much free time to come back and respond to the responses. All signs pointed to the carb once I started using the advice everyone gave me checking the lines, pump, etc. Once I made sure that it was most likely the carb, I readjusted the float level on the carb, which I think was too high because it was overflowing with gas when I pulled the sight plug. Once I did that it ran a little better but then had the same issues. Eventually, I decided to pull off the carb and give it a nice clean, and well it was clear the carb needed a rebuild, new gaskets, etc. I decided instead of going for a new carb to just pull the trigger on an efi system from Holley. I had already saved up because I was planning to change the carb to a new one since the one that has been on the car had seen better days I just thought I would be able to get more time out of it since it was working well for a while. However, since I plan to keep the car for a very long time I decided might as well upgrade to an efi to save some gas in the long run. This will definitely be the most challenging upgrade on my mustang I have done so far. Either way, thank you very much for all the help I greatly appreciate it. If I am being honest I am new to owning a classic and not the most mechanically inclined other than doing usual repairs on my 2003 F350 however I am still learning as I go but it's great to know there's a great community to guide me in the right direction!
If you haven’t purchased the system yet, you might ask here for a recommendation. Installing an EFI system can be challenging to someone who is not mechanically inclined. Do you intend to install it yourself! Inquiry minds want to know 😀
I do intend to install it myself but I might just get the more electrical work done by a shop or ask some friends who know more than I do to help me out with the electrical portion and do the other work needed for it myself such as mounting the included fuel pump, filters, regulator, etc and as well running the return line and install the temp sensor etc. I am obviously not going to get it done in a day let alone possibly a week but it’s something I know is possible and want to do so I am gonna take my time and do it right.
Lots of members have made the switch. If we knew your location, we might be able to offer advice of those who have a track record on the swap. And pics, we all love pics.
Here are some pics including one with my dog. Sorry, I forgot to upload some the first time lol.


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