Stronger/tighter throttle return springs

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So I have been working on my 73 Mach 1 351 Cleveland tuning. The engine is a 2v H code that is freshly rebuilt. New Pistons are flat top 30 over, new lifters, and Elgin E-954-P cam. I installed an Edelbrock Performer 2750 intake and Summit Racing 750 carb last weekend. After setting the initial timing to 17 degrees, she starts right away with no issues or after warmed up, without pumping the gas. The idle has been around 1250, idle screw made no difference really. The idle screw is actually screwed out so far it is not touching the linkage. Vacuum is only around 15-17 inches after adjusting the idle/air screws. Tonight, while it was running, I pulled forward on the throttle linkage and the idle dropped down to about 940 rpms and stayed there until I revved it again. I have a return spring on the carb, and even with it installed, if use the gas pedal or rev the engine at the carb, it still goes back to about 1250 rpms, I push the throttle linkage a little and it drops and maintains the idle around 940 or so. In gear, the idle is 825 rpms. This is all measured with my timing gun rpm display because my cluster tach is not working. Does this sound like I need a stiffer spring or possible that since the carb is brand new, it just needs to be exercised and worked out a little more. Or, something else? By the way, at either 1250 or 940 rpms and any accelerating, there is no backfires or stumbles at all and it runs very smooth without the engine shaking. Any and all thoughts appreciated, I will be looking at it again tomorrow. Thanks.

Tom
 
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Seems like maybe your throttle cable is keeping the carb butterflies open or maybe the return spring you have is just too weak, or you could have a vacuum leak. Remove your throttle cable from the carb and see if it drops down to where the normal idle should be, if it does then you have found your culprit. If you have no throttle cable attached to the carb, the carb has a spring that puts pressure on the butterflies to keep them closed, you should be able to close the idle speed to the point where the engine just dies. If you do not have a throttle cable attached to the carb, which could be forcing the butterflies to stay a little open, and the idle speed still wants to stay over 1200 RPM, something else is wrong. You either have a vacuum leak, or something that you are not seeing is not letting the throttle blades shut down completely. The rear throttle blades could be staying a little bit open, they shouldn't but I have seen it before. On some double pumper carbs there is a tiny screw that can only be accessed from the bottom of the carb that can be screwed in and will open the rear throttle blades. On vacuum carbs you can have a really screwed up vacuum canister with either a really weak spring, a cut spring (yes I have seen people cut the springs inside the cannister) or something of the sort and it will just keep the secondaries a bit open. If when you you remove the throttle cable everything goes to normal, you may just need a stronger return spring, but you probably need to adjust your throttle cable so that it does not put "opening" pressure on the carb while at idle.
I would also assume that with 17 degrees of initial timing you have the mechanical advance set up at somewhere in the 15-20 degree range correct? Also make sure that the distributor vacuum advance is connected to ported and not manifold vacuum. You could have a combination of too much mechanical advance couple with your vacuum advance connected to manifold vacuum which is also accelerating the engine at idle.
 
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Thank you for the response, very good information and I will check those things. I just picked up a return spring set at the local O'Reilly's and will check to see if my throttle cable is causing the issue first by disconnecting it and checking for binding. I have checked for vacuum leaks and have all lines disconnected and plugged except the brake booster. No leaks. The vacuum advance hose on the distributor is still disconnected and plugged, so that will be connected later this weekend and tested. This carb does have an automatic choke and vacuum secondary's on it, I will give the choke another look at this weekend as well. Your advice on the throttle cable forcing the butterflies to stay open really caught my attention, thank you for that. It will be the first thing that I check tonight. Thank you and more to follow for updates.

Tom
 
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My thoughts on it are this;
I too had a similar issue, butterfly's not fully closing, causing higher than desired idle rpms. I had installed a new t/cable (and that's another story). After going through most of what you have been through, the only solution was a double return spring. Not optimal as this puts pressure on the throttle shaft bushing and can cause premature wear and eventually a vacuum leak.
Timing can cause higher rpms (but not fluctuate) and from what little I know, 17 degrees initial is a tad high. With your engine mods, I have no idea what your motor will like, but this is what works perfectly on my mostly stock 71 351C 4V (M code); 14 degrees initial, plus 20 degrees on the crank (10L slot) for a total of 34 degrees mechanical. The adjustable vacuum canister is set for about 4-6 degrees on top, connected to timed ported vacuum. Vacuum is 17"Hg, set by the idle mixture screws @1.5 turns out approx. Curb idle for the 4 speed manual is around 800rpm.
When I first put the Holley 670 on, it ran rich at idle. I took the car to a tuning specialist who drilled 3/32" hole in each primary t/plate and that bought the entire carb into tune.
(Holley supply drilled plates by the way), so nothing new. The vacuum secondaries are set with the silver spring and that seems to work just fine.
Here's a pic of the springs. I had to make a small bracket for these springs.
 

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Hemikiller

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If you disconnect the cable from the carb, it should extend past the ball stud by a half inch or so. Also, it shouldn't be on an angle, or it'll bind, like in Geoff's pic above. It should be a straight shot to the ball from the bracket. If not, you need to use spacers under the bracket to get the geometry correct.
 
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Thank you Geoff for the pics and explanation. Hemi, thank you as well, I will check those things and the geometry tonight when I get to the car. I appreciate all of the assistance for everyone and hopefully will be driving it soon.

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If you disconnect the cable from the carb, it should extend past the ball stud by a half inch or so. Also, it shouldn't be on an angle, or it'll bind, like in Geoff's pic above. It should be a straight shot to the ball from the bracket. If not, you need to use spacers under the bracket to get the geometry correct.
Hemikiller, Thanks, but on mine the cable is actually about 1" higher that it ought to be. That is due to the 1" spacer under the carb. I would think this is about as much of an angle as one would want to see. That angle may have been part of the reason I was having trouble with the carb not closing down correctly.
 
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My thoughts on it are this;
I too had a similar issue, butterfly's not fully closing, causing higher than desired idle rpms. I had installed a new t/cable (and that's another story). After going through most of what you have been through, the only solution was a double return spring. Not optimal as this puts pressure on the throttle shaft bushing and can cause premature wear and eventually a vacuum leak.
Timing can cause higher rpms (but not fluctuate) and from what little I know, 17 degrees initial is a tad high. With your engine mods, I have no idea what your motor will like, but this is what works perfectly on my mostly stock 71 351C 4V (M code); 14 degrees initial, plus 20 degrees on the crank (10L slot) for a total of 34 degrees mechanical. The adjustable vacuum canister is set for about 4-6 degrees on top, connected to timed ported vacuum. Vacuum is 17"Hg, set by the idle mixture screws @1.5 turns out approx. Curb idle for the 4 speed manual is around 800rpm.
When I first put the Holley 670 on, it ran rich at idle. I took the car to a tuning specialist who drilled 3/32" hole in each primary t/plate and that bought the entire carb into tune.
(Holley supply drilled plates by the way), so nothing new. The vacuum secondaries are set with the silver spring and that seems to work just fine.
Here's a pic of the springs. I had to make a small bracket for these springs.
Drilling holes in the primary throttle plates is an old Holley trick for cars that have big cams in them. The Elgin cam that the OP is using is only [email protected], so he shouldn't have to drill holes in the primary plates to get it to idle correctly. How big is the cam you are running?
 
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Drilling holes in the primary throttle plates is an old Holley trick for cars that have big cams in them. The Elgin cam that the OP is using is only [email protected], so he shouldn't have to drill holes in the primary plates to get it to idle correctly. How big is the cam you are running?
Just a pretty much stock cam. Melling MTF2. It wasn't the cam that was the issue, this 670 Street Avenger carb ran really fat out of the box. It needed more air in the mixture and that is why the specialist drilled the holes to enable the transfer slots to be set in the correct position. This has been hashed out many times on here. Bottom line is it worked for my engine. The AFR is very close to being spot on now.
 
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Thanks Don, the cable feels like it moves pretty smooth now but will take another look. Anyone got the correct specs and part number for the cable if I need one? Thanks.

Tom
The best you'll get now is a repop, but the 73 is different from the 71/72.
I tried to get an NOS one from Green Sales, but although they showed stock, none could be found.
An issue I found with the repop I got for my 71 was the wire length from the firewall bracket to the eye, was an inch too long. I needed to cut it, open up the crimp and reset the wire. I also hard soldered the wire for (I hope) extra strength. So far, so good, been nearly 2 years now. Fingers crossed!
 
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Seems like maybe your throttle cable is keeping the carb butterflies open or maybe the return spring you have is just too weak, or you could have a vacuum leak. Remove your throttle cable from the carb and see if it drops down to where the normal idle should be, if it does then you have found your culprit. If you have no throttle cable attached to the carb, the carb has a spring that puts pressure on the butterflies to keep them closed, you should be able to close the idle speed to the point where the engine just dies. If you do not have a throttle cable attached to the carb, which could be forcing the butterflies to stay a little open, and the idle speed still wants to stay over 1200 RPM, something else is wrong. You either have a vacuum leak, or something that you are not seeing is not letting the throttle blades shut down completely. The rear throttle blades could be staying a little bit open, they shouldn't but I have seen it before. On some double pumper carbs there is a tiny screw that can only be accessed from the bottom of the carb that can be screwed in and will open the rear throttle blades. On vacuum carbs you can have a really screwed up vacuum canister with either a really weak spring, a cut spring (yes I have seen people cut the springs inside the cannister) or something of the sort and it will just keep the secondaries a bit open. If when you you remove the throttle cable everything goes to normal, you may just need a stronger return spring, but you probably need to adjust your throttle cable so that it does not put "opening" pressure on the carb while at idle.
I would also assume that with 17 degrees of initial timing you have the mechanical advance set up at somewhere in the 15-20 degree range correct? Also make sure that the distributor vacuum advance is connected to ported and not manifold vacuum. You could have a combination of too much mechanical advance couple with your vacuum advance connected to manifold vacuum which is also accelerating the engine at idle.
ProjectJunk, you took virtually every word right out my mouth (again <g>). Solid suggestions... I have seen one situation where one of the throttle blades was rubbing on the base of a venturi casting and kept the throttle blades from closing properly, especially when the engine was warm. I would have simply polished the area where the blades were hitting, but this was a warranty repair where the factory required me to replace the entire carburetor. What a waste... And, they never asked to have the carb returned for analysis in order to see what was wrong with it. Double waste. I kept the carb on hand for months to cannibalize linkage parts from for other carbs.

I have seen a few too many cases where the sleeve surrounding the throttle cable would constrict over time, causing the throttle cable to not fully return to idle position. It happened to one of my Mustangs, and I merely used a pair of pliers to break the smaller end point of the cable sleeve up, where the cable was rubbing on the inside of the sleeve, and it all worked great thereafter.

Ditto on the vacuum advance canister ought to be tested and connected, and looking out for over-advancing the initial timing. I usually found a 12 degree BTDC worked well, and anything past that would cause issues, unless the vacuum or mechanical advance mechanisms were not working properly (or eliminated). That said, for low octane fuel, like the non-ethanol 90 octane fuel we have in the Rochester, NY area, I end up running at the "detuned" factory timing of 6 degrees BTDC - and happy I do not need to go to a lower setting. It is bad enough at 6 degrees BTDC...
 
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Thank you for the response, very good information and I will check those things. I just picked up a return spring set at the local O'Reilly's and will check to see if my throttle cable is causing the issue first by disconnecting it and checking for binding. I have checked for vacuum leaks and have all lines disconnected and plugged except the brake booster. No leaks. The vacuum advance hose on the distributor is still disconnected and plugged, so that will be connected later this weekend and tested. This carb does have an automatic choke and vacuum secondary's on it, I will give the choke another look at this weekend as well. Your advice on the throttle cable forcing the butterflies to stay open really caught my attention, thank you for that. It will be the first thing that I check tonight. Thank you and more to follow for updates.

Tom
In 1973 the electric choke was introduced for stock engines. I see you have a non-oem carburetor. That is important for one reason re: the choke. The factory carbs used current from the alternator to heat the electric choke heating element. The stator outputs about 1/2 alternator output voltage, and it is AC current, not DC. Every aftermarket carb with electric chokes I have seen requires a full 12 volts of switched power to work properly. If you are using the factory wiring (White wire with a Black stripe) your electric choke will open too slowly and later than intended. In order to get the electric choke to be properly heated you need to either supply a different source of switched power, or use a relay that is triggered to be energized when the engine is running.

If you want to use a relay to send relay power to the choke heater, you can use the Circuit #4 White Black Stripe wire from the alternator stator terminal to activate the relay's trigger circuit. It only outputs voltage when the engine is running vs when the ignition switch is turned on whether the engine is running or not. That is how I recommend you wire it, and the stator AC current will activate the relay just fine.

If you want to use factory wiring to provide current to the electric choke, from a full battery DC voltage source, you have two easy options. The first is to tap into the carburetor Throttle Position Solenoid (TPS) wiring (Circuit #640, Red with Yellow Hash), or the Windshield Wiper Motor's Red wire Circuit #63). Both circuits are protected with a 14 amp fuse. The amperage draw from the electric choke heater ought to be low enough to not cause either circuit to become overloaded. Attached are some files showing the schematics for the TPS and Wiper Motor. That wiring for those systems is largely unchanged from 71-73.

This is a link to a YouTube video I put together to show where Circuit #640 for the TPS is located, whether you have a TPS on your carb or not.

 

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Thank you for the response, very good information and I will check those things. I just picked up a return spring set at the local O'Reilly's and will check to see if my throttle cable is causing the issue first by disconnecting it and checking for binding. I have checked for vacuum leaks and have all lines disconnected and plugged except the brake booster. No leaks. The vacuum advance hose on the distributor is still disconnected and plugged, so that will be connected later this weekend and tested. This carb does have an automatic choke and vacuum secondary's on it, I will give the choke another look at this weekend as well. Your advice on the throttle cable forcing the butterflies to stay open really caught my attention, thank you for that. It will be the first thing that I check tonight. Thank you and more to follow for updates.

Tom

I noted that you said the vacuum advance hose is plugged and disconnected. I do not know if you did that as part of the testing for a vacuum leak, or not. It is because the vacuum advance diaphragm is disconnected that you can get away with so much initial timing advance as you have. I saw that HemiKiller said he would run with up to 14 BTDC degrees initial timing. With a high enough gasoline octane that could be a good spec, but even back in the mid 70s I found 12 degrees was optimal for street engines running premium fuel. For engines running regular fuel, or lower octane levels, 12 degrees initial advance was too much - lots of pinging.

Anyway, my Old School way of power timing was to make certain the vacuum advance and mechanical advance systems were working properly (and the vacuum advance hose connected to ported vacuum). I would then use the carburetor choke's high speed cam on the carb (with the choke fully open on a warmed up engine) to get to about 2,200 - 2,400 RPM in neutral/park. I would then slowly advance the distributor timing until I found the point where the engine RPM increased to it highest point without changing the carb throttle position, then backed off that timing to reduce the RPM by 200 RPM from its highest point. I would let the engine return to idle, disconnected the ported vacuum line, then retarded the initial timing from the point where I had achieved the highest RPM level due to park advancing. Then I reconnected the vacuum advance hose. With decent fuel I would usually wind up at or near 12 degrees BTDC. I know other folks use a vacuum gauge in addition to (or instead of) a tachometer like I do. If thou can then increase it one degree to see if it pings or not. If not, lock it down. If it is pinging again back go one degree and lock it down.

As for how to connect the vacuum advance hose, in most vacuum calibrations there is a 3 port Thermal Vacuum Switch (TVS, and also called a Ported Vacuum Switch) that connects to the distributor vacuum advance diaphragm. The other two ports connect to ported vacuum, and to manifold vacuum. Other 1973 calibrations have a dual diaphragm vacuum advance diaphragm. Attached is a snippet showing the various vacuum calibrations for 1973, to include two 351 4V engine calibrations not included in the original Shop Manual. And, of course, I also have a YouTube set of videos showing what the TVS is used for and how it is connected. In some of the videos I also show how to test the vacuum advance diaphragm and even the vacuum sources. There is overlapping info in some of the videos. Sorry for the overlapping info...










And there are some interesting/useful YouTube videos on Power Timing and Ignition Timing related issues. Here is one that does aa good ob explaining why ignition timing i so important.:






I hope all that helps you get things reconnected properly.
 

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Gilbert, (mrgmhale), (I hope I am correct with your name, if not, I apologize), thank you for your response and the videos. I really appreciate it and have been watching your videos on YouTube. Great job on providing these for those of us that love this year group.
Tonight I worked on my 351 Cleveland and after looking at Geoff's pics and Hemikiller and Don's advice added washers to raise my throttle cable up (it is not completely level because I need longer bolts) and I moved the springs to the rear and mounted them at the throttle cable mount bolts. I had the springs towards the front instead of the rear. I have the engine idling at about 860 in park and 750 in gear at operating temps. I am at the point where I can stand out of the car whether cold or engine at operating temps, and turn the key and it starts right away. I will verify this in the morning though.
Thank you everyone that has helped me on this, I think after adjusting the shifter linkage and brake pedal this weekend, I will be ready to take it on it's first maiden drive since it was last registered in 1984. I am so pumped about this.

Tom
 
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Here's my setup, I used the factory 4V bracket with a couple spacers underneath to get the cable aligned. I'll make a proper one piece spacer this winter.
I like it. I suppose that means I ought to do the same thing now!! It may mean I no longer need that extra spring which will take some pressure off the t/shaft.
 
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