340 Gets Terrible Gas Mileage
@Sport340
A lot of good info and suggestions here.
That said, I would work on the tuning and improved mpg will be one of the dividends.
Ideally you or a buddy would have the time and tools to take the valve cover off and measure lift and duration with a degree wheel.
I'm going to assume that's not in the cards at this time.
So
Continue on the path you were discussing with RustyRatRod.
That is figure out the current timing and advance curve. Then make a change, reset the idle mix and throttle position if needed.
See how well it goes when put in gear.
This will be an iterative process. Very different than tuning an engine to factory specs. There will be changes that are steps backword and you'll have to return to your previous baseline. Write everything down. Expect to burn lots of fuel in the process. Gas milage will be terrible as much of it will be gallons per zero miles. You will learn a lot, and if you stick with it, it will be satisfying.
Now lets get to what you have.
At 900 rpm, I would have expected at least some vacuum on the hose that attaches to the distributor.
However, your car has this really nice looking example of an OSAC valve.
It may be the reason you didn't feel the any vacuum on the hose when you disconnected it from the distributor.
Even if this engine still has some of the other emissions control systems, the OSAC really will not play well with the performance cam. Honestly, the OSAC didn't play well with drivability on the factory engines. The spark delay was no help to mpg either.
OSAC discssed in the 1973 Master Tech Sessions; 1 on page 11, in #7 on page 9
or pdf versions here Browse MTSC by Model Year – 1973 – MyMopar
For now, I would leave the vacuum hose plugged. Work only on the mechanical timing.
Once the mechanical timing is ballpark, then route a vacuum hose directly from the carburetor's timing vacuum port to the distributor.
Take the second vacuum hose and put it storage.
What might be ballpark timing?
Lets start with initial.
24* at 900 is what might be necessary on a radical cam because there is a lot of exhaust dilution and not much real compression and heat at low rpm.
Much more typically on a cam that most people might install, something around 16 - 18* BTC is all that is needed.
Getting from 24 to 18 in one try is probably not going to work out. Better to get to take it down in 2 or 4* increments.
Then adjust fuel mixture. Turn the screw in (leaner) until it begins to stumble, then turn back (richer) either an 1/8 or a 1/4 turn. If its an automatic, put it in gear and see how much rpm it loses. The goal is to lose the least amount of rpm going in gear. Try it at 20 and then 18 and see which is better, then maybe 16 if 18 had better power (less rpm drop) in gear.
You may find that at less than 900 rpm, the timing is lower.
In which case 24* at 900 wasn't initial. It was initial plus some advance.
I think maybe before tuning, first try to assess the current setup.
Is there any other emmissions equipment? '73 would have had EGR. There may be some temperature control valves for either the EGR or the OSAC. Make sure all that is either connected up as it should be, or fully disconected and plugged.
Also what distributor is on the engine.
@Sport340
A lot of good info and suggestions here.
That said, I would work on the tuning and improved mpg will be one of the dividends.
Ideally you or a buddy would have the time and tools to take the valve cover off and measure lift and duration with a degree wheel.
I'm going to assume that's not in the cards at this time.
So
Continue on the path you were discussing with RustyRatRod.
That is figure out the current timing and advance curve. Then make a change, reset the idle mix and throttle position if needed.
See how well it goes when put in gear.
This will be an iterative process. Very different than tuning an engine to factory specs. There will be changes that are steps backword and you'll have to return to your previous baseline. Write everything down. Expect to burn lots of fuel in the process. Gas milage will be terrible as much of it will be gallons per zero miles. You will learn a lot, and if you stick with it, it will be satisfying.
Now lets get to what you have.
At 900 rpm, I would have expected at least some vacuum on the hose that attaches to the distributor.
However, your car has this really nice looking example of an OSAC valve.
It may be the reason you didn't feel the any vacuum on the hose when you disconnected it from the distributor.
Even if this engine still has some of the other emissions control systems, the OSAC really will not play well with the performance cam. Honestly, the OSAC didn't play well with drivability on the factory engines. The spark delay was no help to mpg either.
OSAC discssed in the 1973 Master Tech Sessions; 1 on page 11, in #7 on page 9
or pdf versions here Browse MTSC by Model Year – 1973 – MyMopar
For now, I would leave the vacuum hose plugged. Work only on the mechanical timing.
Once the mechanical timing is ballpark, then route a vacuum hose directly from the carburetor's timing vacuum port to the distributor.
Take the second vacuum hose and put it storage.
What might be ballpark timing?
Lets start with initial.
24* at 900 is what might be necessary on a radical cam because there is a lot of exhaust dilution and not much real compression and heat at low rpm.
Much more typically on a cam that most people might install, something around 16 - 18* BTC is all that is needed.
Getting from 24 to 18 in one try is probably not going to work out. Better to get to take it down in 2 or 4* increments.
Then adjust fuel mixture. Turn the screw in (leaner) until it begins to stumble, then turn back (richer) either an 1/8 or a 1/4 turn. If its an automatic, put it in gear and see how much rpm it loses. The goal is to lose the least amount of rpm going in gear. Try it at 20 and then 18 and see which is better, then maybe 16 if 18 had better power (less rpm drop) in gear.
You may find that at less than 900 rpm, the timing is lower.
In which case 24* at 900 wasn't initial. It was initial plus some advance.
I think maybe before tuning, first try to assess the current setup.
Is there any other emmissions equipment? '73 would have had EGR. There may be some temperature control valves for either the EGR or the OSAC. Make sure all that is either connected up as it should be, or fully disconected and plugged.
Also what distributor is on the engine.
