Sounds like it needs more initial timing. You posted this Thursday. This is Saturday. Have you even checked initial timing yet?
340 Gets Terrible Gas Mileage
- Thread starter Sport340
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That valve is a delay valve for the vacuum advance on the distributor. I would bypass it and run the vacuum line straight to the vacuum advance from the carburetor. It is an emissions device designed to lower tailpipe emissions and is not conducive to performance.
But that valve is not your problem. Check the initial timing and report back with where it is.
Hey Rusty I tried to check the timing. Car idling at 850 - 900 rpm with the vac gauge hose plugged. #1 cylinder is on driver's side first cylinder closest to front of car, right? The timing mark does not show up on the timing tab. The timing tab ranges from 10 before and 10 after with 10 before at the top of the timing tab as I look down on it with my timing light. I can see the crank pulley a fair bit below the 10 after mark and I do not see the mark on the crank. I cannot see much above the 10 before mark due to water pump in way. I must then assume that the initial timing is in excess of 10 before. My timing light is old (40 years old) and has not been used in awhile but does this tell you anything?
Thanks
Dave
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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You will need to install a timing tape to use that light. But if there is stuff in the way that you can't read much past 15 degrees, then you will need to get a dial-back type timing light.
And you will need to prove that your mark on the balancer is accurately synchronized to TDC #1 piston.
And you will need to prove that your mark on the balancer is accurately synchronized to TDC #1 piston.
70 Buster 340
Carbon Footprint
That's not right.
Soon after I got my 72 340 Duster, I thought the gas mileage was very bad. So I was checking the tune up and when I checked the timing, I happened to see the crimp around the factory fuel pump was leaking fuel bad. So double check for fuel leaks
Hillbilly timing tape the balancer...
Cranking and having to floor the pedal to start is a common symptom of not enough initial timing.
As others have noted, a timing tape will help considerably. I would then also confirm the damper has not spun (using a piston stop) so you know whether it is reading properly.
Off the topic, but my the 460 in my old '77 Ford F250 got 8-10 mpg. It didn't matter if the truck was empty, loaded, carrying a 9.5 cabover camper or pulling a 32' 5th wheel.
I think it would have gotten 8-10 mpg if I'd pushed it off a cliff!
Ok would like to resurrect this post. To refresh, I am getting 6 mpg on 1973 Dart Sport 340 that has a Edelbrock 650 with some sort of performance cam in it (do not know what cam is in it). I finally got a digital timing light and was able to determine initial and total timing. Initial timing is 24 at 900 rpm. Total timing looks to be 42 at 2800 rpm. I plugged the vacuum advance line that runs to the distributor when I checked the timing but it doesn't seem like there is any vacuum on the hose (put my finger over it). Trying to figure out if the way the timing is set is responsible for the bad gas mileage. If anyone has any thoughts I would be interested. Thanks!
MOPAROFFICIAL
Oogliboogli
Vac source could be ported, meaning it only pulls vacuum while in the mains / or at RPM and not while in the idle circuit
MOPAROFFICIAL
Oogliboogli
Poor mileage I think I'd want to know what cam is in it and where it's degree to. I'd also like to see what a spark plug looks like.
You 42 or something full advance but mention its with vac advance hooked up, so you need to verify all timing, initial ,mechanical, and with vac
You 42 or something full advance but mention its with vac advance hooked up, so you need to verify all timing, initial ,mechanical, and with vac
sccachallenger
Well-Known Member
24 degrees at idle, with vac plugged? Cant believe it starts easily, make sure balancer hasnt slipped. Verify tdc lines up with mark on balancer.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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For fuel economy;
Yur ignition timing is all wrong
Yur cam is all wrong,
Yur bore to stroke ratio is all wrong.
Yur combo could also be all wrong.
OOPs you are the OP, my bad, let me get my foot out of my mouth.
You said;
because you said
Before I put my foot in my mouth again; I would like to know;
1) your cranking cylinder pressure, 2) your rear gear, 3) your Convertor stall.
BTW; that device on the firewall, you asked about is a spark delay device. For fuel economy you sorta need to bypass it, so long as so doing does not cause detonation.
Your cruise timing should be about as high as possible so long as the car does not slow down with additional timing. It could be something like; 52/56 degrees at 2400 to 2600. Without a dash-mounted, dial-back, timing device, this would be hard to determine.
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Rat Bastid
Dunamis Metron
He could very well have that much timing. A SBM will start with 36 in a locked out distributor. Did it for years.
The OP needs to VERIFY his timing.
Then he needs to post some pictures of his plugs.
Bore/stroke ratio has almost zero affect on fuel mileage.
That thing should easily pull 10 MPG all around driving IF the carb and timing is right.
The OP needs to VERIFY his timing.
Then he needs to post some pictures of his plugs.
Bore/stroke ratio has almost zero affect on fuel mileage.
That thing should easily pull 10 MPG all around driving IF the carb and timing is right.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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By itself you might be correct.
But if a an engine is set up with a longer stroke, to yield a higher Dcr with a same cam, the greater cylinder pressure can result in fantastic fuel economy.
Whereas a low-compression 340 with a big cam, will have very low cylinder pressure, and so to make a given cruise-power, will require a larger throttle-opening, thus killing fuel economy.
@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.
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.
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Dan the man
Well-Known Member
A lot of good ideas here, I didn't read every word, but if it has champion plugs in it replace those worthless hole fillers. Also the fastest way to check for a vacuum leak is to spray wd40 along where the intake manifold bolts to the cylinder heads same thing for the carburetor, do this with the engine running. I'd say that it's probably the big cam causing the problem. And too, the 340's are not known for gas mileage
lemondana
BlackDart
Put a Thermoquad back on it!
Dan the man
Well-Known Member
That may help somewhat. I think that a full tune up would help, make sure that there's no vacuum leaks, vacuum advance is working properly and correct timing and like I said earlier if it has champion spark plugs, get rid of them. A true but sad thing is that the sbm's are not known for mileage. I had 3 different trucks all of which were 2 wheel drives with 2.73 gear ratio and none of them got more than 13 mpg on the highway, that's just how mopars are, gas hogs.
Dan the man
Well-Known Member
What part do you disagree on
MOPAROFFICIAL
Oogliboogli
I figure I'll take a red x with the rest of you...by echoing that I will never use champion plugs in my motors.
I had the insulators come loose back in 1997, they slid up n down on the electrode. Poor quality control back then BUT because of that experience 'one I've never had with any other brand' I wont use them to this day.
I'll add..
Some people are such sensitive assholes...that without even questioning why... 'to possibly learn something' they just hit the x.
Volaredon who comes here to...learn? Share?..but instead comes off like a know it all. Not one person appreciates that approach...I can tell ya that.
I had the insulators come loose back in 1997, they slid up n down on the electrode. Poor quality control back then BUT because of that experience 'one I've never had with any other brand' I wont use them to this day.
I'll add..
Some people are such sensitive assholes...that without even questioning why... 'to possibly learn something' they just hit the x.
Volaredon who comes here to...learn? Share?..but instead comes off like a know it all. Not one person appreciates that approach...I can tell ya that.
MOPAROFFICIAL
Oogliboogli
Vulare Don also doesn't know that spread bore carburetors have poor fuel distribution compared to square...
Another case of somebody's preference shoved up their azz so far that it's clouding their common sense...
Another case of somebody's preference shoved up their azz so far that it's clouding their common sense...
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