I used to get about 17 +/- mpg a few months ago, but now its A LOT worse. It started this problem when it lost almost all power and was extremely sluggish, and would randomly stall and not start for a few min (later figured out it was the coil + not getting juice sometimes) I calculated it up over several weeks (I fill my tank once a week, and drive less than 200 mi a week) and figured I was getting about 12 mpg! I recently got around to upgrading to HEI, it has regained its power, and idles a heck of a lot smoother (but still at 900 neutral, 600-650 in gear), but, im only getting 13 mpg with HEI. The carb is adjusted, it doesnt run rich, it has a very new (within 5000 mi) tune up, new fuel pump/filter/lines, new 2 1/4 exhaust with a glasspak, and the valves were adjusted late last year. Also keep in mind, the 17 +/- mpg was with the 2in exhaust, and I figured a less restrictive exh would help with mpg with the super six

P.S, I do not have a lead foot...

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'74 Duster 225 Super Six w/904
14" Cragar SS wheels

If it's not carb related, and the ignition system is in good shape, I'd say your distributor isn't advancing correctly or more than likely you have lost your vacuum advance can (diaphragm leak)...Use a manual hand pump and see if the vacc. advance still works. If you pump and it loses it's vacuum easily....

Good Luck,

-D.idiot

There isnt a diaphragm leak, so I hooked a vac gauge up to the carb and it has no vac at idle, if give it a little throttle it gets vac, but as soon as you rev it, it goes back down to 0and bounces back up again. Also there is no difference in idle quality, or power with the advance line hooked up or un hooked.

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'74 Duster 225 Super Six w/904
14" Cragar SS wheels

if you don't have a hand pump for the vacc. can on the distributor, you can use a new "clean" hose and suck on it...you'll know if the can has a leak....The problem here is two fold: 1) vacc can has a leak you get no vacc. advance off idle+ which equals mileage loss 2) it also induces a vaccuum leak at the carb which if lean enough may also hurt mileage...

Seconda case scenario, is looking at your timing curve with a dial back timing light.... hows the mechanical/initial look? What is the initial set to?

???

-D.Idiot

after documenting your initial advance, what is your total showing?

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

This is what I did, no leak

Also, im not an expert on timing, so I have no idea how to check mech/initial timing When looking at it with a timing light, its really jumpy so its kinda hard to tell anyways

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'74 Duster 225 Super Six w/904
14" Cragar SS wheels

dusterguy225,

If you don’t have a dial-back timing light mark 10 degree increments with white-out starting from the factory timing mark. As your timing advances you will be able to see & easily estimate 2 1/4 degree increments between your white marks.

What DI & Agg Ted are looking for is a list of rpm points and timing advance reading at each of the rpm points, and if you have a vacuum gage reading manifold vacuum at the same time that information can be added to the list also. I did this very drill a few months ago when recurving my distributor, and discovered a lot of interesting trends. You will need a tachometer for this project.

Get out a clip board, pull on your white lab coat, stock your pocket protector with colored pencils, warm up your engine, and establish a timing base line. I have several loose leaf notebooks holding every start point & change I have made to my car sectioned off by engine, transmission, suspension, electrical etc. This information, when organized this way, is handy to have at your finger tips, saving a lot of duplication of effort.

Start at Idle in “Nâ€￾ with vacuum advance vacuum line disconnected from the carburetor & its carburetor port plugged off; record rpm, degrees of advance, and vacuum inches HG for this, and for each 100 rpm increments, until the timing dose not advance any higher. Just turn the idle adjustment screw on the throttle in to raise the rpm. You now have established your mechanical advance curve. Back off the idle screw to bring engine to curb idle.

Second step:
Connect the vacuum advance canister to carburetor, and repeat the 100 rpm data recording points until timing dose not advance any more, and back off the screw to reset curb idle. Now you can see how the vacuum advance interacts with the mechanical advance. You will be able to see when the vacuum advance kicks in, and how it adds to the mechanical advance to give a much higher total advance.

You may recall as the throttle is opened under acceleration the manifold vacuum drops, at WOT it goes to zero. The vacuum advance reads these changing vacuum levels and automatically retards timing under acceleration, and advances it when throttle is held steady or constant as when cruising on flat ground or down hill. When timing is advanced to its highest level that is when you are getting maximum fuel economy. Engines don’t like a lot of timing under acceleration, if too much advance is present, the engine will ping indicating poor burning dynamics in the combustion chamber.

Just list all the data points here, and we will be able to see if all is well, or not.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

dusterguy225,

Try to do your best at collecting the data. First, do it with the vacuum line unplugged to the can, but plug the line to carb so there is no leak. Then do a plot with it plugged in.

Wjajr's instructions are good because he just did it to tune his engine for optimum drive-ability and mpg.

If your timing is jumpy and the marks are moving around allot, that is caused from excessive timing chain slop.......you must have quite a few miles on your motor because mine at 365,000 was pretty steady, maybe 2 or 3 degrees of slop at most......

My engine right now is only running on 5 cylinders (40 pounds of compression on #6) and it still gets 18.8 mpg. So, I hope you get the data collected because it will help you with the next steps. Careful tuning makes a big difference in mileage.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

I have a feeling getting him back to 17mpg isn't nearly this complicated.

Was the timing mark jumping around when you were getting better mileage? Why are you idling in Neutral at 900rpm? Do you just like it idling that high or will it not go any lower without running rough? When warmed up, you should be running 700-750rpm at idle with it dropping down to 550-650rpm in gear. Whats your overall Vac at idle?

Thanks guys for all the help, I will have to do all this hopefully on sat. or sun.

Yes, the timing mark has been jumping since I bought the thing. Im idling at 900 rpm because when its in gear, it drops all the way down to 600 (650 on a GOOD day) It idles rough and is extremely shakey any lower than that.

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'74 Duster 225 Super Six w/904
14" Cragar SS wheels

Hmmm.

(1) valve adjustment
(2) check your timing chain for stretch. Rotate motor by hand until the timing mark lines up with TDC. Pop the distributor cap and slowly turn the motor counter-clockwise. Watch the rotor and stop turning the motor as soon as the rotor starts to move. Now look where the timing mark is. If the mark is beyond 5 BTDC, then you need to replace your timing chain.
(3) check if your vibration dampener has slipped
(4) check the weights and springs in your distributor. They should be lubed and move easily
(5) make sure you are firing at TDC on the compression stroke and not TDC on the exhaust stroke

Pull-off the valve cover and recheck the valve lash adjustment.
Run the lash loose, .012 & .022 at first to see how the engine sounds and runs.
DD

You may want to replace the phenolic distributor drive gear if you're going to replace the chain and timing gears. When both are done together you'll get the most accurate reading with the light because you've eliminated potential "slop" in the system. Run it with the oil filler off-if she's puffing when running you've got a lower end (broken ring or burned piston problem)-don't ask how I know! Good luck

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Good gun control is hitting your target repeatedly

Ok, I will check the timing, chain slop etc. But I probably wont change the dist gear or the chain, im going to hold off untill I get the 86 hydro motor rebuilt, cuz its getting a new dist and double roller chain. The one thats in it is a reman (looks like 20 years ago) motor, so it probably has never run right

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'74 Duster 225 Super Six w/904
14" Cragar SS wheels

Have you done a compression check? Have you pulled each plug wire while idling to make sure each cylinder is firing?