Just curious if my MPG could be an indicator of how my motor is running. I have a FSM and other manuals but haven't gotten a timing light and such (had a water heater and oven bust all within a week).

The guy I bought the car from this year was pretty meticulous and did compression checks pretty often, the last one he took had all cylinders at 100-110 which was a few years ago but less than 1000 miles.

Engine idles fine but is missing the fast idle on the carb (I found the parts in the trunk in a plastic container). Carb is leaking a tiny bit so I will rebuild it pretty soon.

I plan on rebuilding the carb and adjusting it and the timing after I buy a light. Is there any timing light purchasing advice I should take? I haven't had one since I sold my old Duster to my brother.

Anyways I have been getting a steady 20mpg mixed driving with an auto trans, does that sound like everything is in good working order? I only really drive it on weekends and I try to stay out of busy traffic times.

Thanks
Brandon

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It's more of a subjective indicator of how the engine is running... if you are keeping track of mpg per tank (and your speedometer/odometer
is geared correctly to the tire and rear ratio combination...or you are using a gps unit that can be determined to be on target), it will tell you
when something isn't right especially if you were getting an average of 2x mpg all highway driving (and you didn't make any changes to the
components of the engine), then on the last fill up you only got 14 mpg for the same route and driving pace...then you know you've got an
issue (then it would be time to pin down the need for a tune up, or if it's missing and the ignition or quality of gas needs attention).

Doing a cylinder pressure test will tell more about whether the rings are sealing correctly, etc...(good compression and numbers that are reasonably
equal in each cylinder)..

If you had a buddy with an old Engine Analyzer Oscilliscope you could plug it into the ignition system and see if the ignition is firing correctly, etc...
or even just pulling a plug wire to find a miss in the ignition if needed...

Mechanic-ing is like being a doctor... if an mpg drop is indicated as a 'symptom' then you will have to sleuth the cause(s) of the illness.

Consider also adding vacuum gauge, tachometer, and air fuel mixture gauge under your hood for closer/fine measurements of the engine health.

Lifting the hood and seeing all within their tight operating range inspires confidence.

Or if you see something changing from the norm they help you adjust the state of tune.

In the end you could see an end result of maintained MPG for similar driving conditions.

I call MPG a "soft" measurement since it contains so many variables. Above other type measurements are "hard" measurements leading you to a fine tune & improved knowledge.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Thanks for mentioning the vacuum gauge, planned on most of the other stuff but I haven't worked on carbed cars in twenty years or so and forgot about that one. I moved away from my dads place a while ago so I can't borrow his equipment anymore.

Brandon

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20 MPG in mixed driving on a stock engine is really quite good. Not too many thing can be wrong for it to be that good. Did you calibrate/measure your odometer?

Lou

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Home of Slant6-powered fun machines

Lightweight 64 Dart might do better on MPG than 20......is it a 904 auto or upgraded to OD?

My 65 Dart with 4 speed OD running on flats trends at 24 mpg in cool weather and 28 mpg in warm weather if you keep your foot light on the pedal and at speed limits.

So place your expectation higher than 20 and keep dialing the slant for best gauge readings.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

It's a 904. I will admit though that I don't ever drive it during peak driving times, and never to work (wife won't let me haha).

I do need to check the odometer for accuracy though, should be good I think as the car is still on crappy 13" tires that I think match original size.

Brandon

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Quote:

I do need to check the odometer for accuracy though, should be good I think as the car is still on crappy 13" tires that I think match original size.

I used my GPS as well as road signs marking mileage in tenths to decide how much % change was needed to get the correct mph/mileage and ultimately mpg calculation.

Once I determined how much it was off the wheel and tire size/diameter was changed to dial it in.....beats looking for correct transmission speedo pinion gear or messing with rear axle ratio......especially if you were going to alter wheels or tires anyway.
I suspect you might have a 2.9 rear axle ratio with the 904 trans?

In my case I went from 13 to 15 diameter rear wheels to dial in the speedometer. (the wider 15 inch wheels also got a bit more rubber on the road for a rear that originally tended to slide easily when braking hard).

Bigger diameter wheel/tire combo diameter tends to lower mph displayed on Speedometer and record less miles driven on Odometer.

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Pretty sure the axle is a 2.9

Downloaded a speedo app and it tracks exact to my car speedometer, I'll still need to verify on some road with mile markers though.

Brandon

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My '70 valiant 225 SL6 averaged 23 mpg, totally stock. My '67 dart SL6 was averaging about 21 mpg but it was built a bit more than a stock engine, double 2 barrel carbs, dual exhaust, etc, so it turned out that 21 mpg translated to way too lean fuel condition and it burned through a head gasket. So, mpg is relative to build and demands put on the engine.

Best to check, as said above, spark plugs to see if it's burning rich, poor or OK, is it running smoothly, does it operate in the normal temperature range, is the timing set properly, etc. More advanced diagnostics are cylinder compression, air to fuel ratio, vacuum, oil pressure, voltage output and the like. Start with the basics and if you're inclined/interested, then you can do some more advanced diagnostics if you believe it's not running OK. If it is a stock engine, and the main stuff is OK, just enjoy your wonderfully engineered SL6.

It's funny, back in the day if I had a slant 6 in my car, people would scoff, "why don't you pull that and put a 340 in there!?" nowadays, they see my SL6 and say, "wow, what a nice engine!" How times have changed.

brian

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Motor is running great and still getting right about 20mpg. The car is pretty much 100% original but I'm planning on the HEI conversion later in the summer/fall. The previous owner was pretty meticulous and kept all the service records, I found the receipt for when the head was rebuilt with hardened seats about 17k miles ago just the other night.

Pretty sure the carb is a touch rich but I'm not going to mess with it till I get the ignition done in a bit, I've been having fun just driving it around as is. Sure feels like a dog compared to my 70 duster that I used to have with a super six setup though. Gonna do a compression check when I get to the HEI.

Brandon

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Do you get the timing light yet and check timing?

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1965 Dart 110k, 225, Carter BBD Super Six, 2 1/4 single exh., sbp manual scarebird front disc, 7 1/4 rear 2.94 sure grip, 14 x 4.5 OEM wheels, 833 OD with hyd. throwout bearing, HEI, electric fan, ram air/heated air, Accusump. http://plymouthcarclub.com/

Sounds like the previous owner took good care of you SL6. As mentioned, check the timing, that might be the "dog" in it. If you're getting 20mpg, I wouldn't worry about the carb being a tad too rich, way better than a lean mixture. If the plugs look white or a light tan color, you're in good shape. Carbon black is a sign of way too rich and greenish tints can mean a lean condition. It sounds like you're in good shape, just check the timing and see where you are. I had a SL6 that ran great but had a bad lag when I punched it down. I never fixed that, because the car ran so darn good otherwise, dependable, easy starts, like a Timex, it took a lickin' and kept on tickin'.

brian

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