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Hi all! So today I’ve retuned my 68’s carb idle mixture for summer. Closed the idle screws and reset it with the leanest idle mixture possible. Made idle as low as she will go without gagging. 17 hg Z of vacuum at idle 14 in D or R. PCV suction is strong and this is all while the vacc advance is hooked up. Oh and Timing is set at 12 deg btdc. No pinging. Even with mid grade gas.

My set up is a recently rebuilt 74’ 302 with an F4B like clone “COBRA” Blue Thunder intake. Carb is a Holley 4160 4v single feed 600 CFM.

She’s showing signs of running on the rich side but no black smoke. Easy starts (even when it was cold out) and now after readjusting really smooth and nice idle.


Car drives fine had good stock like power but gets 12-13 MPG highway with a C4. With the old humble econo intake and this same carb it did about 15 -17. Ran crappy tho was on the rich side there too.

I’m hoping if I put in smaller main jets it’ll reduce the rich burn and add a few MPG back. I’ve already put in a finer accelerator pump nozzle that removed all bogging. Throttle response is crisp and deep. I kept up with 300 + modern mustangs at an all day rally last weekend visiting hospitals.

Question is if I put in new main jets what size to go to and what else might need to be done? (pulled the ones out of the old carb just to see what it entails and learn).

Do you just set mixture on the two sides like before after rejetting or is there extra tuning work I’m not aware of?
 

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Before you go "jet crazy" do a "plug read" to see if you are, in fact, running rich. Also, if you're NOT using full manifold vacuum for your vacuum advance a change to it should gain you a couple mpg's. If you DO decide to re-jet, see what you have for primary jets and reduce the size by 2 numbers. While you have the metering block out also check the rating for the power valve.

Yes, after re-jetting re-adjust your idle mixture again. Bear in mind that idle mixture and idle speed adjustment will have no noticeable effect on fuel mileage. Don't forget tire pressures though... underinflated tires can easily cost you a few mpg.
 

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Before you go "jet crazy" do a "plug read" to see if you are, in fact, running rich. Also, if you're NOT using full manifold vacuum for your vacuum advance a change to it should gain you a couple mpg's. If you DO decide to re-jet, see what you have for primary jets and reduce the size by 2 numbers. While you have the metering block out also check the rating for the power valve.

Yes, after re-jetting re-adjust your idle mixture again. Bear in mind that idle mixture and idle speed adjustment will have no noticeable effect on fuel mileage. Don't forget tire pressures though... underinflated tires can easily cost you a few mpg.
Bart,

Here is a plug pull from a few days ago. But these plugs have about 1000+ miles of winter driving mostly highway and a lot of starts and long idles as I was fixing various power steering other non related issues. These are not the break in plugs though.
I think the process has to be observed with new plugs after a short drive. Coasting to a stop after engine shut off and then pull it out... like in the Holley video?

Tires are always 28-29 PSI. Used to work in tire shops in high school days (dealership line mechanic too in my early 20s) so I’m very diligent about tires!

I am using full manifold vacuum on the distributor advance and put in a new (rebuilt) Motorcraft dizzy that was converted to Pertronix a while back not long after I got it.

One question I had today in my mind is how does one tell if the vacuum advance is working properly?
 

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The plug looks awfully clean to be considered running rich. Considering your modifications, I'd say you just need a little more fine tuning (or a little less heavy right foot)..
 

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The plug looks awfully clean to be considered running rich. Considering your modifications, I'd say you just need a little more fine tuning (or a little less heavy right foot)..
That is what I’ve set out to do here. Fine tune it. 12 MPG seems crazy low even for a 4v with those big honking F4B clone runners and 600 cfm. Should be more like 15-17 driving steady open highway 60-65 MPH, no? I mean I didn’t b get this car for its economy but still...damn 🤣!

There is also the matter that my fuel gauge is wonky. It would be great if I’m under estimating. But 16 gal tank 10 from E to F. Has a drain plug. Once drained 5 gal on empty and in think it’ll fill 1-2 gal over F Mark. Still the fuel vanishes astonishingly fast now within that rage. Stinking 10% ethanol not helping of course 😡
 

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Is the 12° with the vacuum disconnected?
That’s with it connected at idle. Think it was the same either way but I can recheck next week. When you rev it at idle though it will jump higher. I marked my 10 and 12 deg spots with whiteout as it’s hard to see the original numbered marks with the black paint and strobing light.
 

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I see you’ve got that temperature controlled distributor vacuum thingy on your thermostat housing, those can go bad and get stuck ( yours looks new though). If you aren’t getting manifold vacuum to the distributor vacuum advance while cruising (light load), your gas mileage will suffer. Check that your vacuum hoses connect to the vacuum thingy (I really should look up what it’s called 😂) correctly. Also check your vacuum advance diaphragm can hold vacuum; suck on the end of it and it should move, if it feels all your doing is suck air through a straw it’s probably torn.
 

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Above, you did mention ethanol gas. That definitely affects fuel mileage. Non-ethanol gas definitely gives these cars better mileage and longer life to fuel components.
I didn't see a mention of float level, or what type of air cleaner you have but the plug definitely looks like you're running lean, so you may be working the engine harder to get the same amount of work done.
I run my tires at 30 PSI cold, so they may heat to 32 when driving. How wide are your tires? Pizza cutters will get you mucho MPG, but handle poorly. Gorilla-width tires handle great but really create a lot of rolling friction.
I also didn't see what rear gear ratio you're running. You know what those old 2.80s can do for mileage vs. 3.89s etc.
I'm running a .030 over 289 on a 25-year-old rebuild with a stock A / K intake, a 4100, a .460/.472 cam, 10 degrees timing, iron manifolds into 2 &1/4 exhaust with Flowmasters and valance tails, a C4 and 2.80 gears...got 16.5 mpg overall on a 7-hour round trip last week that was a mix of highway, two-lane stop and go in Birmingham and some hill climbing in the country. Of course, overall driving habits have a big role in economy. Maybe put a heavier spring on the throttle haha! Good luck !
 

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The idle mixture screws don't effect the jetting, so if you put in some different main or secondary jets you shouldn't have to mess with the idle screws-especially if you have all the low speed idle/boggs/ect all worked out. It sounds like you need to know more and you can't do it without a dyno with an air fuel gauge or getting your own. Your plugs look lean but you think you are rich. You could always try the smell test?
 

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Ditch the temperature switch. That was purely for emission control for no vacuum advance at idle until the engine was fully warmed up. Set initial timing to 12°-14° with vacuum hose off. After that run manifold vacuum. You'd be surprised at how much fuel economy takes a hit sitting at a stop sign or red light. With manifold vacuum it will help during these periods.

With my 66 I know on the times I have met up with @2nd 66 and @dobrostang for xcrossing a couple times I have averaged 17 mpg driving 100 miles each way including beating the daylights out of the car at the events the whole day
 

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Ditch the temperature switch. That was purely for emission control for no vacuum advance at idle until the engine was fully warmed up. Set initial timing to 12°-14° with vacuum hose off. After that run manifold vacuum. You'd be surprised at how much fuel economy takes a hit sitting at a stop sign or red light. With manifold vacuum it will help during these periods.

With my 66 I know on the times I have met up with @2nd 66 and @dobrostang for xcrossing a couple times I have averaged 17 mpg driving 100 miles each way including beating the daylights out of the car at the events the whole day
Temperature switch is just a decoration. Not even hooked up. It was originally for A/C. Even though I have it I’m running a fan clutch. Switch would bump idle up for A/C to increase fan speed. I’m planning to remove that thing and put a more accurate under hood coolant temp senor in its place.

Timing is 12 deg but I’ll recheck with the advance blocked off. I’ve been running manifold vacuum. Hose is pulling lots of vacuum from the tee at the back of the manifold.

One thing I’ve noticed is that the timing seems to advance the same amount according to the light and the marks under the timing light when you rev at idle with or without the vacuum hose hooked up. Should I not see much more advancing with it hooked up?
 

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The idle mixture screws don't effect the jetting, so if you put in some different main or secondary jets you shouldn't have to mess with the idle screws-especially if you have all the low speed idle/boggs/ect all worked out. It sounds like you need to know more and you can't do it without a dyno with an air fuel gauge or getting your own. Your plugs look lean but you think you are rich. You could always try the smell test?
Exhaust does not smell uncharacteristically rich after she’s warmed up. But it’s hard to tell owing to how noxious these non car cars are! I’m very sensitive to it. Used to get massive headaches from being near the exhaust. But no longer probably owing to it running better.

The plug tip is medium charcoal gray but the plug seat is quite dark. I need to restart with fresh plugs. I’ll gap to .38
 

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Above, you did mention ethanol gas. That definitely affects fuel mileage. Non-ethanol gas definitely gives these cars better mileage and longer life to fuel components.
I didn't see a mention of float level, or what type of air cleaner you have but the plug definitely looks like you're running lean, so you may be working the engine harder to get the same amount of work done.
I run my tires at 30 PSI cold, so they may heat to 32 when driving. How wide are your tires? Pizza cutters will get you mucho MPG, but handle poorly. Gorilla-width tires handle great but really create a lot of rolling friction.
I also didn't see what rear gear ratio you're running. You know what those old 2.80s can do for mileage vs. 3.89s etc.
I'm running a .030 over 289 on a 25-year-old rebuild with a stock A / K intake, a 4100, a .460/.472 cam, 10 degrees timing, iron manifolds into 2 &1/4 exhaust with Flowmasters and valance tails, a C4 and 2.80 gears...got 16.5 mpg overall on a 7-hour round trip last week that was a mix of highway, two-lane stop and go in Birmingham and some hill climbing in the country. Of course, overall driving habits have a big role in economy. Maybe put a heavier spring on the throttle haha! Good luck !
Rear is 225/60/15. Fronts b are 215/60/15. They handle quite nicely! Gears are the stock standard. 2.79’s according to the data plate decode.
It’s a new carb and float level was set by Holley. I didn’t see the need and nor did the shop foreman see any need to mess with the settings. Single exhaust though b upgrading to dual this summer, and a standard type fuel pump.

Unfortunately no more ethanol free gas available near me. Unless I pay 25$ per gal for the racing fuel 🤣 I’ve seen videos about how to make your own by using water to separate it. But that’s impractical even if it works.
 

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I see you’ve got that temperature controlled distributor vacuum thingy on your thermostat housing, those can go bad and get stuck ( yours looks new though). If you aren’t getting manifold vacuum to the distributor vacuum advance while cruising (light load), your gas mileage will suffer. Check that your vacuum hoses connect to the vacuum thingy (I really should look up what it’s called 😂) correctly. Also check your vacuum advance diaphragm can hold vacuum; suck on the end of it and it should move, if it feels all your doing is suck air through a straw it’s probably torn.
I’m anxious to try this ‘suck test’ with a piece of unvarnished clean tubing. Definitely plenty of vacuum coming from the existing line. Dizzy and diaphragm are new. Still have the old one sitting on a shelf.

Another way to test this is to drive car without the advancing hooked up and there should be a huge difference.

As in mentioned in another reply at idle the timing advances when I rev up at idle. But doesn’t seem to be much different with or without vacuum adv diaphragm hooked up. If the difference is only a few degrees when it’s on I’ll probably not be able to see that under the light.
 

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Timing is 12 deg but I’ll recheck with the advance blocked off. I’ve been running manifold vacuum. Hose is pulling lots of vacuum from the tee at the back of the manifold.
Oop, this could be your problem. You want to set your base timing with no vacuum on the distributor (hose pulled and plugged). You’ll get another 10+ degrees of advance with the vac advance hooked to manifold vacuum... this means your base timing is near TDC! Fix this first, then hook up your vacuum advance and readjust idle speed accordingly.

One thing I’ve noticed is that the timing seems to advance the same amount according to the light and the marks under the timing light when you rev at idle with or without the vacuum hose hooked up. Should I not see much more advancing with it hooked up?
As you rev it with no load, you will see the timing advance from the distributor mechanical advance. You already have a lot of manifold vacuum at idle, so the vacuum advance doesn’t have much more to give, even if you rev it.
 

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Oop, this could be your problem. You want to set your base timing with no vacuum on the distributor (hose pulled and plugged). You’ll get another 10+ degrees of advance with the vac advance hooked to manifold vacuum... this means your base timing is near TDC! Fix this first, then hook up your vacuum advance and readjust idle speed accordingly.



As you rev it with no load, you will see the timing advance from the distributor mechanical advance. You already have a lot of manifold vacuum at idle, so the vacuum advance doesn’t have much more to give, even if you rev it.
Noted, and thx for the clear explanation! This will be the first thing I’ll check when I get out to the car this afternoon 😉
 
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