In our 73 Duster with everything stock, 225 /6 and Holley 1920, the best mileage I seem to get is around 17mpg. I slightly retarded the timing to about 8 and the idle mixture is as lean as I can get. So far, performance has not been compromised but I never seem to get a mileage change. I would still like to get better mileage. Are there any other things I might try? Thanks as always for the great help.

Mike

Two easy ones are numerically lower rear end gearing and an overdrive transmission

Try tweaking the entire advance curve, not just the inital timing. I know the feather duster got special distributors and carbs with smaller jets, but were dismal performance wise (or so I read).

An MSD or similar box with a higher power coil may add a little more as well.

Idle mix is part of it, but how is your jetting in the mains?
The usual suggestions apply: Weight, tire pressure, alignment ...
I've seen your Duster, but I forgot what you have in it. 904? I remember rally wheels, but which bolt pattern? Differential?
My '64 Valiant gets 17 around town, and 21 at 70 MPH carrying two adults and a loaded trunk. I think there's more there.

I have the 904 automatic with the stock 7 1/2 rear end. I put #51 main metering jets in it. I consistently get 17 mpg, it doesn't seem to matter if it is in town or on the highway. I am not sure about the other timing issues. Is there an article that can explain it to a basic person? Thanks.

Mike

I would check the distributor's advance curve. You need to know how much total advance (initial + mechanical) the engine is seeing and how much vacuum advance is added-in at light throttle. A good working vacuum advance "pod" that pulls-in lots of additional advance at light engine loads is a big help for better MPG.

But, I suspect that the Holley 1920 may be the real problem.
What caught my eye is the #51 main jet size, that is really small for a 1920. When I go this small on a good working carb you know it, power is off and flat-spots and high speed lean surge start to occur. (usually around # 54-56 jet size)

Has this carb ever been rebuilt? It could have a high float level, a loose main jet / metering block or a bad metering block / gasket that allows extra fuel to get into the main metering circuit. The "economizer valve" (power valve) setting may also be set to rich. What is the engine vacuum reading at idle?
DD

I'll check again when I get home but as I recall it pulls about 19 vacuum at idle. I also think I may have quoted the jet size wrong. I inquired here before and used a middle of the range jet to what was suggested so I bet it's a #55. I'll check that too. The carb has been rebuilt and the float was set right. I can always try that again as well. I am not sure how to determine the mechanical advance and how to make all that work. I have checked for vacuum leaks numerous times all around the carb and intakes (using water and also using the propane method) and never found any leak. I have the vacuum advance pot on the distributor set light so it should be responsive. I could maybe get another partial turn out of that. The economizer is set to specs. I just feel like I should be getting better mileage. I'd love to get it to about 20mpg without too much adverse effect on the performance. Thanks.

Mike

I'd even be happy with 15!!! My d-150 /6 gets about 13 mpg. There are known problems with the carb and timing curve,(no vacuum advance at all) so I hope to get it improved, but I know it will never get 17 mpg in my wildest dreams.

I actually do have some on-topic things to mention. While I don't know the answers, I do have some other items you may consider. You may ask the guys getting 21 mpg where they stand on these items.

What grade of gas do you run?
What is your compression ratio?
Are your plugs at a hot enough tempurature range?
Are you running points or Electronic Ignition? (I'm guessing EI, but you're on the border age-wise)
What size exhaust pipe?

A steady 17 from a 1920 sounds pretty good, from the posts I've read. You're near the point where the cost of improvements might outweigh the savings in gas useage, but it's worth a try. Good Luck

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1980 Aspen 225 super six

I use the cheapest gas I can get (regular unleaded). I checked my compression after having the heads redone and I had consistent levels across all cylinders within normal ranges. I have electronic ignition and just use the plugs specified for the engine (I think I have Autolites in there now). All the ignition and wiring components are new. My exhaust is supposedly standard which I think is like 1 1/2 inch diameter (I can measure when I get home). Exhaust is new with a turbo muffler in it. What I think it sounds like is I need more info on the timing curve and how to work all of that. Thanks.

Mike

hello sl6 sages- sad stories here on the poor state of sl6 mileage. good answer from the doctor. my 76 feather duster with small 1 barrel carb 2.9 axle and od trans gets real world miles of 22 in town and almost 30 on the highway but is sluggish as hell. have had a number of sl6s since 64 but never saw one with an automatic which got 20+ in town. the DI has a feather and i guess he would agree. unless carb is shot tohell and leaking gas into the cylinders dont think much will help there except leaning down the mixture screw. weekly tire checks at 35 psi and a clean air element make a difference. sages say oil makes a difference but i dont think it does as i have experimented with 20/50, 10/30 and 30 and have seen no difference except, oil consumption is lower with 20/50. too bad they ended the sl6 before it could have been fuel injected or made it into a slant 4. bet none of you remember the pontiac tempest of 62 which had a slant 4. keep up the good work contributors .

you might try going to a slightly bigger tire. be careful though, too big and you kill the bottom end. how much throttle does it take to maintain freeway speeds? on my dart when it was an automatic it would take 1/3 to 1/2 throttle to maintain 65-70. turned out i had a seized stator in the torque converter. shifted like it had a shift kit, was kinda neat. i like the four speed better:)

zedpapa

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1970 dodge dart w/225 /6 bored .040" over, holley 390cfm w/vac. sec., compcams 252s, clifford shorty headers w/2.5" exhaust w/flowmaster, f-body 11" front discs, aluminum A-833OD, 8 1/4 w/3.21 SG
soon to have 5 gears!!!