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Old 03-16-2014, 12:09 PM   #1 (permalink)
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1955 Pontiac (advice for setting efficient but driveable mixture?)

In 3 weeks my wife and I fly to Alabama to pick up our "new" 1955 stock Pontiac Star Chief, V8, Auto with all the 55 goodies. I have not been able to find an mpg gauge for cars before obd II.

As this vehicle has a Carter 4 barrel on it I remembered from long ago that the idle circuits affect quite a bit of the power range I will use (I expect to drive 50 all the way back to Arizona, with some visits in Texas.) I checked the service manual and found I was correct. In the days of my youth we set the idle for smooth idle and takeoff (gas was +-.20 cents.)

I know I will not achieve 96 mpg, but 1 or 2mpg improvement over 2500 miles can help. I can get an oxygen sensor and adapter for the pipe, plus a air fuel gauge for about $30 on eBay and install them before leaving AL, and adjust the carb until I get a good lean, drivable vehicle.

OK, here is where I need your help! I know manufacturers set the mixture at 14.7 for the catalytic converter, which I don't have. I know leaner ratios are better for mileage, but without running too lean, what ratio should T strive for?


Last edited by dtmerritt; 03-16-2014 at 12:10 PM.. Reason: missing space
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Old 03-16-2014, 04:21 PM   #2 (permalink)
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Not much you can do in the time available. Tire pressure to sidewall maximum and grille block is about it. Unless it has bias ply tires, in which case set the tire pressure a little above factory recommended.

Vacuum or mechanical secondaries? Drive so as to not open the secondary barrels. Gas mileage probably has more to do with getting an even mixture distribution between cylinders than the overall mixture. Possibly disconnect/block the secondaries closed? Make sure the accelerator pump is set correctly and floats set for correct fuel level.

If you really want better mileage, that car is begging for a wide ratio 5 or 6 speed manual transmission.
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Old 03-16-2014, 04:47 PM   #3 (permalink)
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A little off topic but 'carburetor' related.....
My 66 mustang 6 cylinder 1bble ran lean on the highway because I had instlled headers on the straight six. (3-2-1) ran 1 3/4" pipe into a muffler that then dualed uot to gt tips.
I got 27-28mpg all day long at 60-65.
I never adjust the the factory carb except to set idle.
I 'believe' that over exhausting helped create less pressure that lead to lean burn that lead to great mpg.
I realize you don't have time to rework the exhaust.
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Old 03-16-2014, 05:11 PM   #4 (permalink)
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To set a carb you seat the mixture screw, then unscrew 1.5 turns and adjust from there. Of course for cars you got jet size too and accelerator pumps. Depending on the previous owner you may have some vacuum leaks and those lean out your mixture.

Id use a vacuum gauge to adjust for highest vacuum and maybe use a squirt bottle to check for vacuum leaks. Lean is mean and I think once unleaded fuel was invented or used they had to richen up the mixture to make up for the difference. If its the original engine you may need to use a leaded addative.
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Old 03-16-2014, 05:28 PM   #5 (permalink)
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Set the timing and make sure the ignition poins are clean and properly gapped, if it still has points. Inspect the points for metal transfer and if present polish with emery cloth or replace points and condenser and gap properly before you set timing. Bad condenser causes the metal transfer.

Now adjust the mixture screws so you have the highest RPM, go both lean and rich to find it. Then lean them out until your peak RPM drops by 50. For example it's 750 then adjust the mixture screws leaner until it is 700. In most cases you can adjust the accelerator pump volume but I would not worry about that unless you find flat spots or stumbling on acceleration.

Enjoy the ride.

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Old 03-16-2014, 05:33 PM   #6 (permalink)
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Cobb reminded me that I did have a vacuum gauge at some point.....
I also had installed on of the first mpg electronic devices to measure mpg...

even back then I realized that your have to be able to measure results!
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http://ecomodder.com/forum/showthrea...tml#post247938
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Old 03-17-2014, 12:30 AM   #7 (permalink)
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Maybe I missed it somewhere in the reading of this post nobody ever answered your question about cruising AFR. I find that a ratio of about 16.5 to 18.5 to 1 yields good economy. I don't think you can reach that ratio by just adjusting the idle jets though.
Good luck
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Old 03-17-2014, 09:48 AM   #8 (permalink)
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If you have the capacity to do so before setting out to drive your new ride home, turn up the cruising AFR to as high as you feel comfortable doing so without overheating the heads. You'll need a pretty good infrared thermometer to do this. It won't be a perfect indication but just shoot it at the head-to-header connection on the exhaust header to get an idea of how hot the head is getting. With better instrumentation and some experimenting, you can maybe lean it past peak temp for some ersatz "lean burn" performance, but it's hard to do on the fly with a carb, obviously.

Also worth considering: did the PO update the valve seats? If not, get some lead substitute so you don't pit the valve seats. Man, that stinks when that happens.

You might want to fill your pockets with a few bits to ensure the carb's fitness for a long trip. These guys carry a bunch of Carter repair parts: Carter Carburetor Parts

When you get home you might want to consider swapping carbs to something considerably more modern and tunable.
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Old 03-17-2014, 10:09 AM   #9 (permalink)
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Thanks for actually reading and answering the question.
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Old 03-17-2014, 10:14 AM   #10 (permalink)
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That is a good idea. I sent my infared thermometer to be there, thinking of cooling system. I thought of hardened seats after the heads were done. There is a case of Redline in the trunk now.

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