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?

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.

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.

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.

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.

regards
Mech

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!

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

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.

Thanks for actually reading and answering the question.

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.

I'm fairly confident there was no cat on a car built in 1955

but a wideband O2 sensor could be helpful, and or a Pyrometer
you may also want to look into a vacuum gauge :thumbup:

Just remember running lean and cranking up the timing will raise your NOX levels if you are subject to any emissions testing. :thumbup:

Those older rides are usually grandfathered out of emissions testing. I can imagine some nanny state looking over your shoulder but I don't realistically expect it, not with a '55.

The O2 sensor won't help his car but it could go a long way to helping with the tuning, hooked up to an AFR gauge on the dash or even just a handheld.

I wonder what it would take to retrofit an on-the-fly mix control like in light planes? They have to be adjustable while running to compensate for air density changes as the plane gains altitude. I know we have some light plane pilots on this forum, anyone want to weigh in on that idea?

My truck has a carb. Leaning it out for highway cruising would be pretty sweet.

Personally I would adjust carb, points, ignition timing to stock for the trip. I wouldn't risk doing damage.

However when you get it home then I would have some fun. I'm on the same page I'm building a 68 rambler American and I can't wait to start playing around with the carb for fuel economy.

Couple notes-
Don't waste money on a 30 dollar afr gauge. They only come with the gauge and are made to wire into the signal wire of a narrow band o2 sensor. Get a good wideband afr gauge that comes with the o2 sensor.

Invest in a vaccum gauge first!

On my car not only am I planning on messing with the carb but the entire fuel system. I ditched the mechanical pump and I'm running an electric with a relay system grounded by an oil pressure safety switch (much like gms of the late 80s early 90s) if I shut the engine off for engine off coasting the pump will shut off. Im also running a fuel pressure regulator. I'm going to set the float to factory spec but I'm going to play with pressure at the carb. Im not counting on that making a difference at all but who knows. It's a carter yf 1 barrel.

Edit: also your only going to see gains from playing with jets rather than idle mixture screws. the idle mixture circuit isnt even used at cruise speed which is where you want your lean condition. the main jet circuit is what you want to modify. only problem with that is your timing curve is set at a certain spot and guess what, if you jet it wayyy lean your going to deal with detonation and chance melting a piston or 8. msd makes an adjustable timing knob where you can retard your timing up to 15 degrees on the fly. it requires driver feedback and constant monitoring but i think it would be fun as heck to have lean burn in a carbed car!! i should pm pgfpro about it he could give some insight on timing a lean burn car.




Also I'm ditching points. I'm 25 and I know how to set these cars up but I really want to take advantage of a stronger spark if I can get it, so I opted for the pertronix conversion. Oh yea and reliability and not having to change them after 20k will be nice too :)

Take that mechanical fan off and toss an electric fan on via a switch or thermostatically controlled switch. I'm running an adjustable thermo switch.

I'm glad you asked this as well. Often I see tuning related to modern cars but these pre-emission cars get to benefit from tuning best for pure economy. Plymouth made a feather Duster in 1976 that with the slant 6 and overdrive manual got great mileage. I wonder how much better a car like that could do with some modern fuel injection and aero mods.

Planes use exhaust temp as the guide for leaning.

http://flighttraining.aopa.org/stude...ls/engine.html

I would like a wide band in a few months when my wallet recovers, but for now I purchased a narrow band and the mechanic there will install it for me. I'd rather, but both me and my wife are crippled and have to use scooters to get around. Because of an auto accident I have almost no cartilage in my right hip, add in a stroke on the left side, well I can walk into the bathroom by hanging on (from my electric wheelchair). The stroke made my left hand pretty useless however, so I do what I can, and pay someone to do what I used to.

In AZ no emission test is required, IF. You have to belong to an organization and have classic car insurance, it must only be used for car shows and kept in a locked garage.

I am thinking EFI.

that seems aweful lean to me
but all my AFR tuning was done at WOT

thats why I mentioned a EGT (Pyrometer) could be a good safety gauge

I rarely go over 550 degree Celsius but that is under load and afr closer to
15 to 1.

fuel injected lean burn cars go into the lower 20s but the neat thing about that is the exhaust temp actually drops once you cross a certain threshold.

they explain it a little in the article frank posted. the engine will run hotter closer up to about 18:1. but once you cross a threshold the exhaust temp drops, the downside is power loss. losing 20 horsepwer in a car on the ground while entering lean burn-not a big deal, many of us on this site drive lean burn cars many miles every day. losing horsepower in an airplane mid flight- not so fun. thats why he explains how to lean the engine out mid flight until the engine starts to sputter then richen it back up so it still has power but is at its leanest possible setting.

of course we are talking about carbureted cars and this is more difficult to achieve as compared to injection. with a carb you are going to be starting at the factory base point and dropping jet sizes, retarding timing, driving, repeat. would take a half a day to do or more where as with fuel injection and a laptop it could take just a couple hours. an even more involved route would be putting the distributor on a curve machine and custom weighting it. and/or installing the timing knob that msd makes.

i like this thread. got me thinking alot

As to engine tuning stick to the GM recommendations of the period, IMO. "Best lean roll" is what I went for on my old Chryslers. The final carb adjustment was done in gear, at idle, and resting bumper against tree. (I wrote up an explanation an eon ago, here: Idle Speed Question")

I'd be more concerned with wheel bearing and brake adjustment all around. As well as looking for steering gear slop. Alignment needs be spot on.

If they can be found, articles on the old MOBIL Economy Runs of the era (one of the MoPar magazines; 1990's, IIRC) had quite a bit of info on how those drivers could achieve extraordinary FE.

A tach and a vacuum gauge on dashtop will provide enough info for consistency. Something this age undergoing a really long drive needs be more concerned with actually completing the trip versus a small fuel savings. FE is a measure of soundness, not savings, per se.

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