turr'ble MPG?

[66benchcoupe]

A few comments.

Don't rely on initial timing, set the timing with all the advance dialed in. I suggest 34 degrees total timing as a safe starting point. Read the instructions for the Pertronix dizzy you bought, that should tell you at what RPM the timing is fully advanced. Rev the engine to that RPM and set with your timing light. BE SAFE, stand back or to the side in case the fan or a fan belt comes apart. Wear safety glasses.

The carb. Set the parking brake and put the axle on stands, chock the front wheels. Lightly seat the idle mixture screws and then turn out 2 turns. Attach a vaccum gauge to an unported vaccum source (manifold vaccum). Start the car and put the trans in D. Set the idle adjustment screw as low as it will idle in gear and start turning the idle mixture screw in at 1\8th turn increments until you get the highest reading on the vaccum gauge. At that point back out the adjusting screws 1/8 to 1/4 turn, vaccum will drop slightly. Your idle mixture should be set properly at that point. If you still have issues with the carb then the problem lies elsewhere (float level, the secondary system, etc.). The instructions assume your ignition system is functioning properly and you have no vaccum leaks. There's lots of variables here so take it a step at a time. If you get angry or frustrated, just walk away and cool off.

I suggest reading "How to Build Horsepower Vol.2 - Carburetors and Intake Manifolds" by David Vizard. I've read several books on carburetors and this one explains how they work very well. A book is no substitute for experience but a little knowledge goes a long way when you're trying to diagnose a problem Unfortunately, this book is out of print but is still offered by some sellers on the internet.

And if you can find a Barry Grant carburetor and technical info catalog there's some really good tips in it.

Good Luck,
Robert

 

[hbar]

Thanks for all that info. I had actually ordered that book off Amazon before you posted. I ordered a few Mustang books...that might've been the one I got for $0.41 (plus 3.99 s/h). I'm looking forward to reading that puppy.

I worked with the vacuum gauge tonight, but I didn't really see much change as I turned the screws. It seemed to be right around 15mmHg in gear, and around 18ish in park. I did lower the curb idle quite a bit. Should changing the idle mix screws 1/8 - 1/4 turn produce a noticeable change in the vacuum reading?

The biggest difference I noticed after taking it out is that the car is again running hotter than it was earlier today. My laser said it was ~175. According to my notes from 3 years ago, I put in a 190 t-stat. A few times while I was idling, the temp. needle would pretty much peg out, and then settle back down. I assume that was the t-stat opening up. I'm trying to get my head around how these carb tweaks are affecting the temp so much.

 

[66benchcoupe]

Sounds like the carb is idling on the main circuit. Rebuild it. And, unless there's some wiring issue with the temp gauge or the sender and wiring , replace the thermostat.

Laser temp gauges are usually calibrated for a black non-reflective surface, the 'emissivity' of other surfaces will vary. Consult the manual that came with your gauge, you may be getting an inaccurate reading.

Robert

 

[AzPete]

The mixture screws should show a change on the vacuum gauge. As stated, it sounds like you are idling in the run circuit. What that means is you used the curb idle adjustment to get the car to run. Doing this opens the throttle plates to far and does not allow the fuel to travel thru the idle circuit. The easy way to tell if this is true is if the car idles at all when both screws are turned in gently until the bottom out. If this is happening, then the carb is either out of adjustment or to dirty internally to function properly. The throttle plates should just be cracked open.

 

[Starfury]

I actually just checked the voltage regulator last week, and it was giving a strong 2Hz pulse. For the life of me I can't figure out why the gauges weren't just engineered to use a constant 12V.

Because you don't have a constant 12V source. The battery supplies 12.4-12.8V when charged and not running, but the alternator supplies 13-14.5V depending on engine speed and electrical load. You don't want your gauges to sway every time you step on the gas, do you?

It's much easier to supply a constant reference voltage to everything that needs it.

 

[AzPete]

Then how is it that after market gauges do not use the constant voltage regulator.....they run 12 volts.....

 

[hbar]

The mixture screws should show a change on the vacuum gauge. As stated, it sounds like you are idling in the run circuit. What that means is you used the curb idle adjustment to get the car to run. Doing this opens the throttle plates to far and does not allow the fuel to travel thru the idle circuit. The easy way to tell if this is true is if the car idles at all when both screws are turned in gently until the bottom out. If this is happening, then the carb is either out of adjustment or to dirty internally to function properly. The throttle plates should just be cracked open.

I put in both mix screws and the car died, so I don't think that's the case.

 

[AzPete]

Good to know. Now, back them out 1 & 1/2 turns and leave them there. Car should start but may have idle issues where you need to adjust the idle screw only. Make sure the choke opens after the car is warmed up.

 

[Starfury]

Then how is it that after market gauges do not use the constant voltage regulator.....they run 12 volts.....

Not sure. Could be they just alter the resistances to minimize the affect of source voltage changes. Could be they have their own little solid state 5V voltage regulator chip. They're plenty small enough to fit inside the housing of any gauge.

 

[hbar]

I haven't touched the carb any further, but after driving the car for a few days w/o tweaking anything, the temperature gauge seems to have inexplicably settled down to right in the center of the gauge. Looking forward to that Holley book coming in so I can start to understand more about the carb than just "turn this screw." Even so, it's running great now, aside from the lousy mileage.