Topic: Brutal Fuel Economy Rebuilt "A" Code with Autolite 4100

[rocket289k]

Hi Guys,

I'm getting brutal mileage on a rebuilt 289 "A" with a rebuilt Pony Carbs 1.08 Venturi Autolite 4100 carb (standard jetting per C5ZF-D carb - 48f Primary / 58f Secondary jets).  I had the carb rebuilt in 2010 before Pony Carbs closed up shop when Jon passed away.  I installed an aftermarket cam as part of the rebuilt but other than that she's stock (cam - Comp Cams XE256H also bought in 2010 - noted since I know there are tons of recent issues with Comp Cams). 

I recently drove 50 miles and burned a 1/2 tank of fuel ~ gallons of gas while driving around 55 - 65 MPH.  That's roughly 6 MPG.  I could understand if I was driving around with a 428 but this is a 289.  I'm completely confused.   The has a C4 automatic transmission and a conventional 3.00 to 1 rear end gear ratio.

I've pulled the plugs and they look great - no signs of fouling etc.  The car isn't blowing any black smoke at idle or when running down the road.  The engine has about 300 miles on it now so I know it's still breaking in but this mileage is crazy bad.  Any thoughts?


Regards,

Ron

[CharlesTurner]

If it was running ultra-rich, the plugs would be fouled out.  Are you going by the gauge to determine how much gas was used?  Do you have a Ford or aftermarket sending unit in the tank?  I suppose you have checked for fuel leaks?

Be sure to note mileage before and after a fill-up and write down the gallons pumped.  Do that a few times to get an average mpg over a few fill-ups.

A stock 289, driven conservatively should be getting at least 15mpg around town and a little over 20 on the highway.

[rocket289k]

If it was running ultra-rich, the plugs would be fouled out.  Are you going by the gauge to determine how much gas was used?  Do you have a Ford or aftermarket sending unit in the tank?  I suppose you have checked for fuel leaks?

Be sure to note mileage before and after a fill-up and write down the gallons pumped.  Do that a few times to get an average mpg over a few fill-ups.

A stock 289, driven conservatively should be getting at least 15mpg around town and a little over 20 on the highway.

Hi Charles,

I'll answer your questions in order.

1 - I'm going by the gauge but I'm also tracking the amount of fuel that I add to the tank when I fill up and the mileage.  I know that the gauges can be inaccurate.

2 - I'm using an aftermarket fuel sending unit (I changed the float from a cheap plastic one (that leaked) to a brass unit)

3 - I have checked for leaks and haven't found anything

Regarding the mileage 15 MPG in town and around 20 MPG is what I was expecting too.  The mileage that I'm getting is from primarily highway miles too.  Hence my immense confusion.

Regards,

Ron

[CharlesTurner]

The only explanation would be a leak somewhere then, especially if the plugs are not fouled and the tailpipe(s) are not black and sooty.

Go through a few fill-ups to get the exact mpg... of course, assuming your odometer is working properly!

[jwc66k]

To determine reasonably accurate mileage requires a higher level of discipline than most drivers are willing to put up with. The first item on the list is to check the odometer. Some new cell phones have built-in GPS, I use a Garmin. Use either. It will take about 10 miles to get a descent mileage deviation figures out (9 on the odometer is 10 GPS, multiply odometer by 1.1). The next one is simple. ALWAYS fill your gas tank full. Then do the math - odometer miles (corrected) divided by gallons. I can tell you the mileage on every my cars I've ever owned, currently determined on a spreadsheet (they go back to 1991), formerly on a circular slide rule. It's a trick my father taught me. He was an enlisted pilot in the US Navy, ADC/AP.
The best mileage I got for a long distance was 18.1 mpg over 3180 miles from Massachusetts to California, with a 66.5 mph average, in a 66 Fastback, 4v A code, 4sp, car weighed 3900 lbs at a New York Thruway (I-90) truck scale near Syracuse. I wish I had that car again.
Jim

[ruppstang]

It should not be a issue on a rebuilt carb but one time I had a power valve go out on a Autolite 2100 and I got the kind of mileage you are getting. Just a thought.

[rocket289k]

Hi Guys,

Thanks for all of the input.  We inspected the carb today and everything is in good shape so the issue isn't anything obvious.  As a next step we going to change the jets on the Primaries and take them down a couple of sizes and leave the Secondaries alone.  As I mentioned it had the proper jet installed in the carb (47 Primary and 58 Secondary).  I'm operating the engine at altitude.  The elevation is 2200 ft above sea level where I'm at so maybe leaning out the jets on the primaries will help a bit.  Any additional suggestions are welcome. 

PS - I can find absolutely no evidence of a fuel leak.

Regards,


Ron

[CharlesTurner]

Think you ought to replace the power valve as Marty suggested, just to be sure.

[rocket289k]

Think you ought to replace the power valve as Marty suggested, just to be sure.

Yup - I'm going to give that a shot.  A power valve isn't expensive to swap out to see if that helps.

Ron

[66kcoupe]

Another question. how accurate is the speedometer, i had a problem with that once throwing of my calculations. How's the timing?. BTW my 66 Hipo will get 15 to 18 mpg on the highway, and my old 66 289 2v with 2:80 gears got 19-20 mpg on the highway.

[rocket289k]

UPDATE - Power valve appears to be AOK.  I'm going to look at the timing next. 

PS - The mileage traveled was calculated using either a GPS or known distances from town to town and then filling up to fill to determine the amount of gasoline used.  Everyone that I've pinged seems to be pointing their fingers at the timing being too retarded and needing to be advanced.  I'll keep the group posted but it will likely be about a week until my next mileage update.

Regards,


Ron

[CharlesTurner]

You're going to think you have a new engine if the timing is retarded way off!

[tomtri66]

Try tuning like this:
Connect a good quality external tach
Set idle speed, then set idle fuel mixture in until engine starts to bog.  Then turn out 1/8 turn at a time, waiting until idle settles each time, until you get the highest rpm on tach.
Re- set idle speed.
Set timing using the tach to the highest rpm you can achieve at idle.
Re-set idle speed.

Give that a try.  I hardly ever use a timing light anymore just use this method. Woke my 289 right up.  Of course you want to check total timing with your light to make sure you are not exceeding the max on your cam card. And do some wide open throttle launches to ensure you are not detonating.  If you are, retard until detonation stops.

[rocket289k]

UPDATE

Dialed in the timing by ear (since there is no way to pull vacuum on the 65 manifold).  I went through all of the timing exercises (advanced the distributor as much as possible until it pinged under hard acceleration and then just slightly retarded the timing).  Unfortunately, the car still runs extremely rich right across the power band (at idle, moderate load and secondaries open). 
 
So I think I’ve ruled out retarded ignition timing being the culprit.  As a result I ended up downsizing the jets to see if I can finally lean this engine out.  Although I validated that the carb is running the factory correct jets I have a hunch that the 0.030 clean up bore + more aggressive and efficient aftermarket cam have significantly altered the efficiency of my engine and therefore the amount of vacuum it draws and fuel it will leverage with the stock jets.

Net Result

Adjusting the jetting helped a ton.  The engine no longer running rich, throttle response is crisp and the plug color looks good.  The primaries went from the stock 48f to 45f and the secondaries went from the stock 58f down to a 52f.  Given the number of jet sizes that I had to go down I would suspect something else is flawed with my carb.  I'm going to test drive the car over the weekend to obtain some mileage readings.  I'll provide another update at that time.


Regards,

Ron

[rocket289k]

UPDATE - it appears a ton of the problems that I ran into were as a result of my engine being starved for airflow. The stock air cleaner thematic actuated air flap was stuck closed. As a result, the engine was only receiving constricted and HOT exhaust airflow from the riser pipe.

While I set about repairing the air flap I removed the snorkel from the air cleaner. As a result, the engine the began to run quite lean with the new jets. So I started again with the stock jets. I then drove ~500 miles on a nice long road trip. I got 16 MPG on the trip. Upon pulling the plugs the engine was running just a little bit rich. As a result, I jetted down 2 sizes to 46 on the primaries and 56 on the secondaries. I'll see how I make out on the trip back home and report back in.

Regards,


Ron