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This may help your plug reading 3pedals:
Hello, from NYC. 73 Mach 1
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The 351 engines in 73 came with a Throttle Position Solenoid (TPS) that was intended to resolve any after-run (aka, dieseling) when ignition was turned off. It may have been removed with the non-oem carb. I have never had a demon carb, but I am on the last of our return trip back home with Lynda driving. Lots of time to research them. Once home I will put together a “care package” for you with some electrical and vacuum schematics, and some other goodies for you. We have two 73 Mustangs up in the rochester area, and a pair of shelby gt500s.
A few things to check:
Why is there a plate under the carb? Is it a spread bore manifold to holley style carb base adapter?
Was the anti dieseling idle solenoid removed?
Why is there a plate under the carb? Is it a spread bore manifold to holley style carb base adapter?
Was the anti dieseling idle solenoid removed?
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agree, but you might want to add a see-thru fuel filter. these old cars can have gunk in the gas tank.
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I strongly suggest you use ported vacuum. Test the vacuum advance diaphragm for leakage. The timing should be 6 degrees BTDC with engine idle at 600 RPM or so, and vacuum advance diaphragm unplugged. I echo the concerns Re the vibration dampener maybe having slipped and timing marks being fouled.
once back home I will provide some (more) suggestions.
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Welcome from Omaha!
Wow, that would be amazing. Thank you so much!
Yeah, there's definitely no solenoid on the throttle. I was curious how that would prevent dieseling and found this YouTube that explains it:
So, I can see how lack of that can cause what I see. I'm kind of glad to see that it's "expected" to some degree though I'll need to see if one could be added back on the Demon.
Edit: I see demon sells the solenoid and adapter, so I guess that's going on the list. I see their commonly used to increase throttle idle a bit when A/C is on. I have A/C, too, so hope I don't end up needing to address both! Only one solenoid but in theory you would need two to address dieseling and low idle with ac on.
https://www.summitracing.com/parts/dem-1950
Here's the throttle on mine:
Last edited:
The plate looks homemade and is not very thick, maybe a quarter inch. Not thick enough to be a spread bore adapter. I thought maybe it was to add a little bit of height so that the ram air seals to the hood piece? I haven't removed yet, but will take pics when I do.
The idle solenoid is definitely gone. So, that explains that issue it seems.
Thanks, seems like a good idea. Maybe add a fuel pressure gauge too. Might as well have more information available all the time!
Spike Morelli
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Welcome from west Michigan!
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Spark Plug #1 looks like it was lightly fuel fouled (sooty black coating on insulator, but with the tan color on the end of the ground electrode it looks like it has been trying to "get cleaned up" once the excess fuel in the air fuel ratio began to get out of its overly rich condition. Plug #2 looks a lot better, but there seems to have been some degree of fuel fouling that was then cleaned up with a more appropriate aid fuel ratio.
So, there appears to be an over rich air fuel ratio, at least for a while. Then it "goes away" and the plugs try to clean themselves up with a less rich air fuel ratio. This is a situation I run into with our 750CFM Holley (not Demon), and I have resolved most of the problem, and have not bothered with the rest as I have decided to wait until anything else needs to be done first.
In our case the 750 CFM Holley that came with our 73 Mach 1 when we purchased it was a unit from before 1992. That is important because in 1992 Holley began to use a Power Valve Protection approach to help resolve blown power valves due to engine backfires or "pop backs" that would send high pressure shocks into the power valve vacuum channeling, which could and would rupture power valves. Once ruptured the leaking power valves would let the engine vacuum suck as much liquid gasoline into the intake manifold as could flor though the ruptured valve(s). Clearly, that caused a very rich condition.
For a long time it seems that Holley was just trying to explain away the ruptured power valve damage do to pop backs as not plausible. But in 1992 they began to use a spring loaded checkball in the power valve vacuum channels, designed to deal off the vacuum channel in the event of a no-suction PSI level at the entrance of the power valve check valve. The Demon carburetors were built from about 1998, before Holley bought the trade name, where the BG Demons were typically Holley built units that we modified by BG, although in some cases BG did some of their own "original" machine work. My gut tells me those Demons included the power valve protection circuitry, but I do not know that for certain. It may be worth taking the Demon off the car and popping open the front bowl and metering plate to make sure the power valve vacuum chamber is indeed dry.
That said, it does not sound like you have a failing power valve, otherwise the plugs would both be cleaning up. But, if nothing else resolves the problem you may need to pull the carb and see if the vacuum side of the primary metering plate is wet from liquid fuel on the power valve side of the metering plate. That power valve chamber should be bone dry. If it has evidence of liquid fuel the power valve is ruptured, leaking, and/or the power valve gasket is leaking.
Okay, on to what else may be going on. The electric choke may be causing an issue. When the engine is running you should be getting full battery voltage to the + terminal of the electric choke cap, and 0 volts on the negative terminal of the choke cap. The positive voltage should be coming from a switched battery power circuit, NOT from the oem electric choke circuit 4 (white with black stripe wire) that is feed from the alternator's Stator terminal. That is because the Stator outputs AC, not DC current (which is fine in itself), and is also 1/2 of the alternator's output voltage sent to the battery. That Stator circuit is what oem carburetors with electric chokes (in 1973+) used, by intentional design, with the lower Stator voltage and AC current. But aftermarket carburetors specify a 12 volt switched current.
If you are tapping into the oem Stator circuit 4, you are not getting enough voltage to open the choke as fast as you should be. There are two different circuits under the hood of 73 Mustangs you can tap into. I have attached some PDF file info on this subject, and have put a YouTube video under my YouTube Channel showing how to tap into circuit 63 for switched 12 volt power.
https://youtu.be/WDcskIPfLcE
The 2nd 12 volt switched 12 volt circuit under the hoods of 1971-1973 Mustangs is Circuit 640 (red wire with yellow hash marks). Circuit 640 is used to power the Thrittle Position Solenoid (TPS) that was many Ford Mustangs in 71-73. Even if the TPS was not used on particular Mustangs, the wiring for it did exist near the carburetor unless it was removed (like someone did to our 73 Mach 1). In the following link I show how I tapped into Circuit 640 on our 73 Mustang Convertible in order to provide 12 volts of switched power to a Pertronix Electronic Ignition system. Circuit 640 can also be used to power an electric choke on an aftermarket electric choke cap:
https://youtu.be/_qGVhoFnJ50
See the video beginning at 23:33 mm:ss to jump into the voltage source aspect of the video. Then go to 46:13 mm:ss where I begin to get into wiring the TPS circuit for use by the Pertronix system (you would use it to power the electric choke). Finally I get to the TPS and show how I spliced into Circuit 640 at the TPS at 01:01:45 hh:mm:ss into the above referenced video.
This link shows how our 73 Mach 1's Holley electric choke had been incorrectly connected to the oem circuit 4 from the alternator stator.
https://youtu.be/cTOjVPo-vCo
This video shows where to tap into one of two under hood 12 volt switched power circuits to feed the electric choke (and other items).:
https://youtu.be/WDcskIPfLcE
Here is another link showing how an oem 2 barrel carburetor works, and how to adjust it in general. Your Demon carb will have some differences, but I think there is some good info in that video that you can still apply to the Demon carb.
https://www.youtube.com/watch?v=UFdg44lI-xY
If you get all that done and still have an over-rich fuel condition I suggest you then pull the front carb bowl and metering plate to see if by chance the power valve is leaking. If that is leaking replace the power valve. If it is dry just post an update and I will provide some other places to check also. Things like the float levels in the fuel bowls, fuel pump pressure to make sure it is not excessive, and some other areas as well.
Also, to reiterate what others have said, and myself no doubt, check the ignition timing, where you are NOT applying intake manifold to the vacuum advance diaphragm. And reset the idle speed with and without the TPS engaged. The following video covers the fuel bowl float adjustment on a 2100/2150 2 barrel carburetor. The technique is different on a Holley, but this may be helpful in showing how this is typically done:
https://www.youtube.com/watch?v=U-ETjhRqUwQ (01:18:17; 02:17:42; & 02:53:20 hh:mm:ss)
I hope all that helps you out. If not just post a followup in this thread and I will chime back in, again.
My recommendation is to first research/verify the centrifugal advance in the aftermarket distributor. This is usually around 26 crankshaft degrees (13 distributor degrees). The initial advance is usually around 10 crankshaft degrees. 26 + 10 gets you very close to the target maximum centrifugal advance of 36 to 37 crankshaft degrees.
Thank you for all the detailed info, @mrgmhale. I've got a lot of things to look into. A few thoughts as I ran the car yesterday. It seems to be running a little better "on its own" this weekend in that it's not dying/struggling at idle so much and it hasn't backfired through carb or exhaust again. The only difference is that it's quite a bit hotter here now than before (summer kicked in finally) and i've been giving it more gas after first starting up for a little while before I let it idle on its own. I'm able to drive around and no stalls/backfires and haven't had the run-on happen either, though I haven't run the engine for very long either so not as much heat buildup. I was able to go and fill up with 93 gas and added a bottle of fuel system cleaner:
As far as the choke, how quickly should it open up? I haven't timed it but it's definitely not fast, at least a minute or two maybe?
A real issue I have right now is the exhaust smell situation. It is really pretty bad since it's parked in my building's garage and there's not great ventilation. I can't run it very long before it's too bad to deal with, smells up my clothes, and i'm going to be getting complaints from the neighbors soon most likely. So, this all means definitely running way rich somehow. I'm going to need to start and get it out of the garage out to the street to tinker with going forward.
The fuel levels in the bowl seem within range for the demon with sight glasses, though rear is a bit higher than front. rear is exact middle and front is midway between middle and lower hash mark lines:
Another thing that's been an issue I don't think I mentioned before is with starting. In almost every case, the engine does not fire up while it's cranking with the key in start. When I release the key after cranking for a bit, then it starts. This is definitely not right, though it does start fairly reliably this way. Everything I read on this pointed to weak spark during cranking and I checked the circuit for getting "full" battery voltage during cranking, which is working. The wire hooked to "I" terminal on the solenoid is there and I measured voltage at the solenoid and at the coil when key is in start position and it is getting full voltage, then dropping when key is in run. So, despite that it's not getting enough voltage while cranking it seems and could be the starter is dropping overall voltage too much (bad starter motor? bad starter ground?) or something else is robbing voltage, maybe the choke? Ideally the choke wouldn't get voltage during cranking. Will try to figure that out when I dig into the choke wiring stuff. Otherwise, was going to try to measure the voltage drops along the starter circuit while cranking and see if something is out of line.
And, will be looking to measure the timing accurately with vacuum disconnected as @351c1971 suggested. My first attempt seemed like the advance was really low at idle (under 5 degrees with vacuum connected) given it seems to be on manifold vacuum, though also don't know how well I can trust the timing mark that's there. I presume it should be accurate given the engine work is fairly recent and I see a big white line someone put on, so i'll assume it's accurate for now.
Anyways, this is what's going on in my thought process. I knew virtually nothing about these cars (and carburetors in general) a month ago, so the learning is fun and rapid! Will report back with new findings, probably won't get to it much until the weekend. Thanks, all.
As far as the choke, how quickly should it open up? I haven't timed it but it's definitely not fast, at least a minute or two maybe?
A real issue I have right now is the exhaust smell situation. It is really pretty bad since it's parked in my building's garage and there's not great ventilation. I can't run it very long before it's too bad to deal with, smells up my clothes, and i'm going to be getting complaints from the neighbors soon most likely. So, this all means definitely running way rich somehow. I'm going to need to start and get it out of the garage out to the street to tinker with going forward.
The fuel levels in the bowl seem within range for the demon with sight glasses, though rear is a bit higher than front. rear is exact middle and front is midway between middle and lower hash mark lines:
Another thing that's been an issue I don't think I mentioned before is with starting. In almost every case, the engine does not fire up while it's cranking with the key in start. When I release the key after cranking for a bit, then it starts. This is definitely not right, though it does start fairly reliably this way. Everything I read on this pointed to weak spark during cranking and I checked the circuit for getting "full" battery voltage during cranking, which is working. The wire hooked to "I" terminal on the solenoid is there and I measured voltage at the solenoid and at the coil when key is in start position and it is getting full voltage, then dropping when key is in run. So, despite that it's not getting enough voltage while cranking it seems and could be the starter is dropping overall voltage too much (bad starter motor? bad starter ground?) or something else is robbing voltage, maybe the choke? Ideally the choke wouldn't get voltage during cranking. Will try to figure that out when I dig into the choke wiring stuff. Otherwise, was going to try to measure the voltage drops along the starter circuit while cranking and see if something is out of line.
And, will be looking to measure the timing accurately with vacuum disconnected as @351c1971 suggested. My first attempt seemed like the advance was really low at idle (under 5 degrees with vacuum connected) given it seems to be on manifold vacuum, though also don't know how well I can trust the timing mark that's there. I presume it should be accurate given the engine work is fairly recent and I see a big white line someone put on, so i'll assume it's accurate for now.
Anyways, this is what's going on in my thought process. I knew virtually nothing about these cars (and carburetors in general) a month ago, so the learning is fun and rapid! Will report back with new findings, probably won't get to it much until the weekend. Thanks, all.
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Look at the dude's face in the other car. You can just see how much he wants to have a 71-73 Mustang Mach1. He can't take his eyes off of your sled.
Yes, the timing marks on the damper are very difficult to see from above. I recommend jacking the car up and checking what advance the white mark actually at. If necessary use a little carb cleaner on a rag to make the timing marks more visible. I know this sounds like a lot of work, later you will be glad you covered all the basics first.
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I am glad you not only found the one YouTube video explaining how the TPS works, but that you also seemed to like the info. Why would I be so happy about that? Well, because that is one of the videos Lynda and I produced and posted under my YouTube channel! heh heh...
TPS devices are readily available at parts stores, or online. In your case you will need the TPS as well as its mounting bracket, if you decided to get one to help stop the engine run-on/dieseling when turning off the engine.
I will be responding to some of your other thoughts in a bit. Meanwhile, attached are some files I think you will find useful. One is the wiring schematic for 1973 Mustang relating to the TPS. Another is a collection of 67 - 72 vacuum schematics. The 3rd is the 1973 Shop Manual Volume 6 snippet, where I added the two 351 4V calibrations not included in the shop manual.
If you are interested in getting some manuals for 1973 Mustangs, my GoTo site is Forel Publications. Luckily for you they provide a nice array of decent wiring and engine vacuum line schematics (PDF files). Before 1973 the only place I could find anything re: vacuum schematics (aka, "calibrations") was a bunch of PDF files from Mustang Barn (no fee to download). I took some time, with their knowledge and permission, to consolidate all of their individual files into a single PDF grouped by year and sorted by engine. You may not need it, as the 1973 Shop Manual's Volume 6 contains some detailed emission control info to include a nice set of vacuum calibrations. But, the Shop Manual left out the two 351 4v calibrations, which I added into my Shop Manual, and into the snippet attached to this post. Feel free to share the Mustang Barn 67-72 collection of calibrations.
Following is the link for the Forel Publications site.
https://www.forelpublishing.com/clickbank/index.html
Their PDF files are very inexpensive. They do have some password protection on their PDF files, which prevent you from making any changes, or easily sharing any of the content. I found if I tried to grab a snippet to help someone with details, the resultant image was blurry. And where I wanted to annotate/enhance/correct some area in thee manuals I couldn't make any edits. That is until I licensed a PDF password removal tool. After I ran the password removal app I was able to grab sections to share with others, annotated/edit the files as needed. The app I used is Coolmuster PDF password remover for Mac
https://www.coolmuster.com/pdf-password-remover.html
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Yep, it is a little bit of work, but it pays dividends later on. In our case I used some white touch-up paint and painted a line across the full width of the vibration dampener at the 0 degree mark, and a short line a the 6 degree mark. Then from above I marked the straight edge of the timing marker. It was a PITA to mark the timing marker's edge - but I am so glad I did it. The white paint makes a huge difference in being able to easily find and look at the timing marks.
Sounds like you are on the right track.
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Hi! my responses are in blue text below:
Thank you for all the detailed info, @mrgmhale. I've got a lot of things to look into. A few thoughts as I ran the car yesterday. It seems to be running a little better "on its own" this weekend in that it's not dying/struggling at idle so much and it hasn't backfired through carb or exhaust again. The only difference is that it's quite a bit hotter here now than before (summer kicked in finally) and i've been giving it more gas after first starting up for a little while before I let it idle on its own. That is something I do for our engines when I first start them. That is typical for carburetor engines. I'm able to drive around and no stalls/backfires and haven't had the run-on happen either, though I haven't run the engine for very long either so not as much heat buildup. I was able to go and fill up with 93 gas and added a bottle of fuel system cleaner: Allegedly the fuel lines and rubber components inside the carburetor for these older cars can handle 10% ethanol, but I have read time and again that over 10% ethanol can cause problems with rubber parts (fuel line, fuel pump diaphragm, carb needle's seat rubber tip for the needle, accelerator pump diaphragm, etc.). I have opted to not take chances on our rubber parts running into problems, and use ethanol free gasoline, despite it only being 89-90 octane, When I run our engines at the factory spec of 6 degrees BTDC I get no pinging.
View attachment 77276
As far as the choke, how quickly should it open up? I haven't timed it but it's definitely not fast, at least a minute or two maybe? I have never used a stopwatch to time how quickly the electric choke begins to open our choke plate, but it is rather quickly done. 1-2 minutes sounds about like where we see the choke plate opening. On one hand it seems too soon, on the other the faster the choke plate opens the less fuel fouling I get on our plugs. For our 73 Mach 1's Holley carb I would only need to move the choke cap into a leaner initial setting and I think that would help prevent the excessively rich initial startup air fuel mixture, which would also reduce my spark plug fuel fouling I get when first starting the engine. When I run the engine at 2,000 - 2,500 RPM after it first fires up that allows the engine to clear the fuel fouling on the plugs. Once the engine is even a little warmed up the plug fouling stops and the 351W (not C) runs perfectly.
A real issue I have right now is the exhaust smell situation. It is really pretty bad since it's parked in my building's garage and there's not great ventilation. I can't run it very long before it's too bad to deal with, smells up my clothes, and i'm going to be getting complaints from the neighbors soon most likely. So, this all means definitely running way rich somehow. I'm going to need to start and get it out of the garage out to the street to tinker with going forward. Yeah, you do not want to be breathing that exhaust, especially when it is running rich.
The fuel levels in the bowl seem within range for the demon with sight glasses, though rear is a bit higher than front. rear is exact middle and front is midway between middle and lower hash mark lines: The front bowl is technically running lower (leaner) than specs would call for. But, if you are not losing power on turns, or have an acceleration hesitation you should be fine. This is one time where "close enough" is okay as long as you are not running so lean that it causes problems.
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