New 73 Convertible Questions

[mezell29]

Hi all. 

Hoping to get a little help with a new 73 Mustang Convertible I purchased a couple months ago. New to the forum as well.

Quick history on the car. This was nearly a barn find. The guy I bought it from purchased it in 2007 in California and had it shipped to Florida ( I got both the last Cali tags with the car). The car was 100% rust free and unmolested. Even all the factory air was intact. He spent a bunch of $$ on it then. I found a receipt where they went through the engine in 2007 for nearly $2500, plus they did suspension, shocks, tires, mufflers, etc. He also bought an Edelbrock RPM AirGap intake, MSD 8350 Ready to Run Dizzy and a Barry Grant Demon Carb of unspecified size. He said it never ran right and stopped registering it in 2012. The carb was off the car and in a box. The car sat since 2012 in his garage and when he went to retire he decided to sell and I bought it right. Towed it home and got to work.

While draining the tank found it full of rust, so I replaced everything. Tank, lines, carb, fuel pump. I upgraded to a Holley 670cfm Street Avenger. I also replaced the radiator and got her up and running. He put the tires on in '07 and it sat in the same place for almost 10 years. When I drove it - Whump, whump, whump so on went new tires. I had to solve the notorious vapor lock problem, but all better now. I also replaced all the factory AC so she runs smooth, no leaks from the top even in a downpour and is cool inside. My AC control was broken but I manufactured parts to fix it and all is good now. Spent last weekend r&ring the quarter-glass so everything seals well now. Oil change as well!

Among all the little stuff, I still need to replace the top, perhaps do exhaust and get paint!

So here's where help comes in. 

She runs smooth at idle and drives fine,  however she really lacks power. Even sitting in the garage on a hard acceleration there's a brief hesitation. 

I've had 2 of these 71-73 cars before all with 351c, all H code 2v motors. I just want to get the most I can without radically changing this motor  as I do have another 351C in the garage from a 73 Mach1 I sold that I plan to build for beating on, the original will go full stock in case I ever want to return it to all original condition. FYI, On the Marti its a 2.75 Convention Rear axle as well, so that's some if it I'm sure.

Starting with the Carb, brand new Holley Street Avenger 670 4 point adjustment. I fully readjusted the carb to the highest vacuum. Screws are about 1/2 turn out and even. If I go much more out its way too rich. Holley said 3/4 turn was a good start point. I get 21 in. HG. vacuum and this is the highest I can get. I checked, it has a 6.5 power valve and based on my vacuum should be fine. Not a carb expert, so maybe there's more I can do here. 

The MSD 8350 is a little unknown as it was in the car when I bought it, so wondering if maybe this is causing some issues. I have adjusted by timing light, vacuum and experienced ear. I only have the installed springs. Looks like heavy silver. And only the installed bushing (Looks like black). Its cheap to order the spring and bushing kit. Anyone running this dizzy and want to share their setup? Maybe the mechanical advance is out?  Any timing info that you are using that will help is appreciated.

I may consider a mild cam so any recommendations for a mostly bolt on and stock motor would be welcome.

Exhaust - Its all stock at the moment. I have considered that the exhaust may be the problem. Been looking at headers (Hooker 6921) and Pypes Exhaust. Wondering if the exhaust is just putting too much restriction. 

Non-Performance related, I am thinking about replacing the top. I'm leaning toward canvas rather than vinyl. I know I'm going to need pads, they look original. Any suggestions on what brand and where to buy?

Thanks in advance everyone. Let me know if you have any questions. 

Quick update: After posting this I went out and started the car. I decided to see if I lose and vacuum at speed (Indicating too much backpressure). I do lost a couple in. HG, maybe 2-3, however I also noticed it get a little rougher. Very little but still. Definitely smooth at idle and I just did a full tune up. I expected it to remain as smooth as idle. Not sure if that helps. 

 

[mezell29]

Anyone?

 

[jpaz]

Well, with a 2.75 differential and a 73 2v stockish motor you can’t expect to much. Plus you got the Worst carb for these engines. You would need to change a bunch of stuff to make it all work together. It’s not the exhaust holding back power, it’s everything. Low compression heads, small cam, stock intake, crummy carb, stock exhaust, 2.75 gear. Sounds like your other engine is the one you want for building up, so no point in doing much more than you already have done to this one, other than getting it tuned right. But if you do build your other engine with some bigger horsepower, then you will have to get rid of the highway rear gear and go to a 3.50 or more, and a higher stall torque converter.

You can’t just put a bigger cam in without making other changes too, everything needs to work together. Plus, changing a cam with engine in car is a major job. 

 

[mezell29]

Thanks jpaz, good advice. 

I'm curious about the carb comment. Yes I spent a bunch but I'm open to suggestions if something is better. At some point I'm going to test the BG Demon just because I'm curious and know its like new. 

Just for reference, running:
Edelbrock RPM AirGap intake
MSD 8350 Ready to Run Dizzy
Accel Coil

Honestly when I build the other motor, I'll also swap to a C6 or AOD. Thats what I did in my last 73. A fresh built C6 only cost me $1100 with torque converter. My thought is to keep the original motor original bagged in the garage in case I ever want it all stock for show. 

Was looking at 408 stoker kits today. $1000 bux at Summit - pistons, rods, bearing, crank. That other motor was tanked and then I went a different direction - still bagged in the garage. I'll have to get it out to the machine shop.

 

[c9zx]

The Avenger, and street HP, series of carbs are notorious for being rich at idle. Decreasing the size of the idle channel restrictor or increasing the size of the idle air bleeds is the easiest way to correct the problem as modifying the emulsion tube holes is a very risky business. These three orifices are what sets the air to fuel ratio, not what is commonly referred to as mixture screws (main jet size has a minor effect). The mixture screws determine the amount, or volume, of the mixture that is allowed to enter idle and transfer circuits of the carb. If it hesitates to rev from idle look at the adjustment and calibration of the accelerator pump circuit. Get the MSD spring and bushing kit. The as built silver springs won't allow proper advance, they are way too strong for any application I have ever seen. Chuck

 

[c9zx]

I assume that fuel pressure and float levels are correct. You can try lowering the float level 1/2 turn to see if you can get better adjustment of the mixture screws.

 

[mezell29]

Thanks c9zx.

Ordering that spring kit now. Any recommendations on the right springs and spacer?

 

[TommyK]

In my experience adjusting the idle mixture to achieve highest vacuum usually results in an idle mixture that is too lean. This may be why you are getting a stumble when goosing the throttle with the car idling in park. It could also be the accelerator pump cam and/or nozzle. Also on a 4 corner idle carb after some experimentation you may find the best idle mixture is achieved with the primary screws turned out more than the secondary ones. Carb tuning is about giving the engine what it wants to run best even if it doesn't conform exactly to what the latest magazine tuning article says. Put the screws where it idles best and continue tuning the other carb circuits. Spark plugs have the last word.

Also, the air gap manifolds are great for power on modified engines but they can have some driveability issues. For a stock or nearly stock 2V a regular Eddy performer is a better match.

 

[mezell29]

Awesome. I have an eddy performer f351 2v sitting in the garage. Maybe I'll swap this weekend and see how it goes if I can get gaskets. I looked at the cam I have on the shelf, looks like a Comp Cams 260/260. Thought about stuffing that in as well since its just sitting there. Trying to source a 9 inch rear that I can get some better gears in. 3.10 maybe

 

[mezell29]

After a lot of research I pulled the trigger on a Comp Cams 268H to wake up my sleepy pony. Turns out I had a 260H on the shelf but after talking to Comp Cams, Summit, My buddy at a BIG local speed shop and the internet, this seems like the best choice for general driving duties. The guy from Comp had a couple 351C Cars and this was his recommendation for my use/setup!

Just thought someone else might find this useful!

 

[Canted 393]

For the msd dizzy try both lite silver springs to have all the timing in by 2500 rpm if its knocks under load start adding 1 heavier spring at a time. For the timing start at 14 and have 21 degree bushing in there for a start and see how things go from there.

 

[jpaz]

That’s a single pattern cam and Cleveland’s like a dual pattern. I had a 280 H in my engine when I bought it. It was not a good cam at all. Probably one of the worst ones out there. I don’t know what your engine specs are, or what you are doing with the car, but I’m sure there is a better cam than that. I’m no expert at all, so maybe one of the engine guys on here will help you out.
I stayed away from anything that comp cams makes. But that’s just me. I went with a Crower cam and lifters,  but had a lifter failure after 1500 miles. Got a replacement set and everything is good, so far. Anyway, I hope you have good luck with whatever you decide to do!

 

[mezell29]

I ended up ordering a professional advance Timing Light and the MSD sprint/bushing kit. I went with the Blue spacer and a Light Silver and Blue spring with is just a little higher than 2 silvers. Interestingly I had to create a tool to be able to change the spacer without removing the dizzy - tough to get that nut on otherwise. Set about 12 degrees initial. I haven't taken her out but I definitely saw improvement in the garage (Smoother/faster rev). Will get her out tomorrow morning. Readjusted the Holley  as well. I went from 42 plugs to 45, that seemed to help too. Ordered new 10.5mm wires to replaced the 8mm wires. 

Cam is still in the box. It's been over 100 degrees here and honestly I didn't feel like tearing the motor apart in that.  Honestly there wasn't much difference between the Single and Dual pattern Comp Cam I considered (268H vs XE256H). A little shorter intake.



vs 



This motor is all stock and I figured the cost was cheap to get a few ponies ($135). This shouldn't cause any broken valves. Over .5 lift and you can start running into issues. 

I also did a full compression test. Good compression - 150 to 160 on all cylinders. Well within spec. I know that they put new lower bearings in this motor a few thousand miles ago as I found a receipt. 

My nemesis at the moment is the 2.75 rear end, but the good news is I discovered it is a 9" rear. This car needs a top soon (plus seals) and paint.

 

[TMAC23!]

Hi all.

Hoping to get a little help with a new 73 Mustang Convertible I purchased a couple months ago. New to the forum as well.

Quick history on the car. This was nearly a barn find. The guy I bought it from purchased it in 2007 in California and had it shipped to Florida ( I got both the last Cali tags with the car). The car was 100% rust free and unmolested. Even all the factory air was intact. He spent a bunch of $$ on it then. I found a receipt where they went through the engine in 2007 for nearly $2500, plus they did suspension, shocks, tires, mufflers, etc. He also bought an Edelbrock RPM AirGap intake, MSD 8350 Ready to Run Dizzy and a Barry Grant Demon Carb of unspecified size. He said it never ran right and stopped registering it in 2012. The carb was off the car and in a box. The car sat since 2012 in his garage and when he went to retire he decided to sell and I bought it right. Towed it home and got to work.

While draining the tank found it full of rust, so I replaced everything. Tank, lines, carb, fuel pump. I upgraded to a Holley 670cfm Street Avenger. I also replaced the radiator and got her up and running. He put the tires on in '07 and it sat in the same place for almost 10 years. When I drove it - Whump, whump, whump so on went new tires. I had to solve the notorious vapor lock problem, but all better now. I also replaced all the factory AC so she runs smooth, no leaks from the top even in a downpour and is cool inside. My AC control was broken but I manufactured parts to fix it and all is good now. Spent last weekend r&ring the quarter-glass so everything seals well now. Oil change as well!

Among all the little stuff, I still need to replace the top, perhaps do exhaust and get paint!

So here's where help comes in.

She runs smooth at idle and drives fine, however she really lacks power. Even sitting in the garage on a hard acceleration there's a brief hesitation.

I've had 2 of these 71-73 cars before all with 351c, all H code 2v motors. I just want to get the most I can without radically changing this motor as I do have another 351C in the garage from a 73 Mach1 I sold that I plan to build for beating on, the original will go full stock in case I ever want to return it to all original condition. FYI, On the Marti its a 2.75 Convention Rear axle as well, so that's some if it I'm sure.

Starting with the Carb, brand new Holley Street Avenger 670 4 point adjustment. I fully readjusted the carb to the highest vacuum. Screws are about 1/2 turn out and even. If I go much more out its way too rich. Holley said 3/4 turn was a good start point. I get 21 in. HG. vacuum and this is the highest I can get. I checked, it has a 6.5 power valve and based on my vacuum should be fine. Not a carb expert, so maybe there's more I can do here.

The MSD 8350 is a little unknown as it was in the car when I bought it, so wondering if maybe this is causing some issues. I have adjusted by timing light, vacuum and experienced ear. I only have the installed springs. Looks like heavy silver. And only the installed bushing (Looks like black). Its cheap to order the spring and bushing kit. Anyone running this dizzy and want to share their setup? Maybe the mechanical advance is out? Any timing info that you are using that will help is appreciated.

I may consider a mild cam so any recommendations for a mostly bolt on and stock motor would be welcome.

Exhaust - Its all stock at the moment. I have considered that the exhaust may be the problem. Been looking at headers (Hooker 6921) and Pypes Exhaust. Wondering if the exhaust is just putting too much restriction.

Non-Performance related, I am thinking about replacing the top. I'm leaning toward canvas rather than vinyl. I know I'm going to need pads, they look original. Any suggestions on what brand and where to buy?

Thanks in advance everyone. Let me know if you have any questions.

Quick update: After posting this I went out and started the car. I decided to see if I lose and vacuum at speed (Indicating too much backpressure). I do lost a couple in. HG, maybe 2-3, however I also noticed it get a little rougher. Very little but still. Definitely smooth at idle and I just did a full tune up. I expected it to remain as smooth as idle. Not sure if that helps.

I have the same set up in my 73 Vert and took the rear end to a 3.53 (or something like that) it had great launch but at 70!the engine ran at 4.5 k rpm - I had the 2.73 notched to posi and it launches the same and runs perfect at 90 MPH

 

[mrgmhale]

I do not know how I managed to overlook this original post, and subsequent replies for all this time. But, I am glad it has come back around. The prior owner of our 1973 Mach 1 replaced its really steep 2.79:1 rear axle gear set with a 3.5:1 TractionLok set of gears. He told me it made far more difference in performance than having the 351 beuilt for street.strip performance. It was a fairly moderate build, but on the dyno it puts out 360 HP - plenty to have some spirited fun, especially with the TractionLok 3.5:1 ratio gears.

After we had the Mach 1 for a few years I opted to replace its 3 speed auto tranny with an AOD. It was really nice to both turn fewer RPMs at highways speeds, and recapture the upper end performance that was sacrificed for the lower end performance when the TractionLok gears were installed. The AOD provided us the best of both worlds, great low end performance and terrific higher speed performance.

Anyway, more to the overall point that JPAZ made in Summer, 2020, it definitely takes a combination of changes to get these pony cars to really get up and roar. I am curious about what changes you ended up making, and how the performance turned out for you. I am just curious. We also have a 1973 Mustang Convertible which was a True Survivor barn find, after spending 40 years in a barn. It has almost 21,000 original miles on it, and is in terrific condition (prior owner replaced the roof with a canvas top, and a new set of tires as the original tires rotted out). We have made a few small emhancement to modernize it. The engine is a 302 2v, and it also hace a 2.79:1 rear axle. I have decided to not try to do anything to perk up its performance, as it does just fine with street driving. If I ever feel the need for speed I have three other Mustang Shelby vehicles I can choose to take out. The Convertible was built to be a long legged cruiser, and that is just how it will be driven.