Automotive Air Conditioning Information Forum (Archives)

Provided by www.ACkits.com

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

how lean can an engine go?

mhamilton on Wed February 08, 2006 8:26 PM User is offlineView users profile

I'm sure you are all tired of the questions about my Chevy, but you guys always give the best answers!

In the process of troubleshooting my idle problems, I rebuilt the carb and adjusted the mixture. Since then I have been searching for the reason GM wanted the idle set leaner than Stoich. The factory instructions are to set the mixture to get the max rpm, then lean the mixture to drop about 30 rpm. From what I found, it looks like GM tuned the mixture to get the least HC output.

Looking at last year's emissions report for my car (before rebuilding the carb), I was at 31 ppm HC (standard 250ppm), 0.10% CO (standard 2%), and 15.3% CO2. Talk about low emissions! Today I found an emissions chart http://www.autoshop101.com/forms/h56.pdf that puts the a/f ratio between 17 and 18 to 1. How safe is that for the longevity of my engine?

Right now I have the idle mixture set the usual way at max rpm/vac. I do have to go for emissions testing come April, so I may have to set it back. Also, if it would help city mileage without burning the valves I wouldn't mind leaning it back out. What do you all think?

clueless on Wed February 08, 2006 8:29 PM User is offline

only way to know for sure is to mesure the exaust gas temprature. Since that is a bit involved, if it doesn't overheat or have preignition problems I wouldn't worry about it.

CorvairGeek on Wed February 08, 2006 8:57 PM User is offline

As you are measuring emissions after an operating catalytic converter (I assume) and air injection (I assume again, though it may be in divert mode), your air/fuel ratio cannot be accurately calculated with the formula. Jerry

-------------------------
Jerry

mhamilton on Wed February 08, 2006 9:35 PM User is offlineView users profile

So the lean mixture doesn't actually put the a/f ratio that low, but rather just lowers it a bit and the AIR and cat do the rest of the work?

Any idea how much the a/f ratio changes when the idle is lowered 30rpm by leaning the mixture? I have absolutely no idea what the changes would be, but i know the drop in rpm is rapid , less than 1/4 turn. Would that even make a noticeable change?

brickmason on Wed February 08, 2006 10:09 PM User is offline

In my opinion, its best to turn the air up just a little than to not have enough.

Increasing the fuel intake too much will foul a new set of spark plugs in a hurry, not to mention as the air filter gums up it will make it run even leaner.

-------------------------
Just another brick in the wall

Had a little mule I fed him castor oil and every time he jumped the fence he fertilized the soil

NickD on Fri February 10, 2006 6:57 AM User is offline

When an idle problem occurred in these cars back, first thing guys would fool with was the idle mixture controls when that should have been the last thing. Idle mixture is adjusted last after everything else is check including timing, with an AF ratio meter installed after bypassing the cat. for testing, adjusting the mixture to maximum rpm would give about a 12:1 AF ratio, the power ratio, but backing it off lean to drop that 50 rpm or so would move the ratio closer to 14:1, but never 14.7.

Don't have to be concerned about EGR, or EGT at idle, or even too lean a mixture, engine ain't doing anything at these speeds, but making it too rich to compensate for a fouled plug does make it run smoother, but also carbons up the engine like crazy leading to more problems. The carb was susceptible to both power piston and accelerator pump leakage that should be fixed first before fooling with the mixture controls.

mhamilton on Fri February 10, 2006 1:19 PM User is offlineView users profile

Nick, I agree 100% with you about not using the idle mixture as a quick fix. From the factory these carbs had the idle screws sealed. The last owner had those plugs removed and I don't know what was done. There was all kinds of shoddy work to compensate for no maintenance (like massive vac leaks from melted hoses, faulty timing advance, etc). Since I rebuilt the carb and everything else, I wanted to reset the idle to specifications. Now that I understand how that mixture works with the emissions equipment, I'll set it per the factory manual.

Jerry, speaking of the air injector, do you know if the diverter valve should hold vacuum? I have the model that has one signal line that connects to manifold vac. Book says "a sharp increase in vac diverts the air." I can understand a bleed orifice to make it work, but mine won't hold any vac at the signal port.

JJM on Fri February 10, 2006 1:42 PM User is offline

I never saw any reason to remove the mixture plugs, or previously, the limiter caps -- even when rebuilding. In fact, new GM replacement carburetors came from the factory with the limiter caps plugged. Obviously, they had it down to a science how many turns to back the mixture screws out for the desired RPM drop.

One other way you can quickly check for RPM drop is using artificial enrichment, injecting propane directly into the intake manifold. Ford SPECIFICALLY recommended the artificial enrichment method for setting carbs on late 1970's and early 1980's vehicles.

The "Electronic Lean Burn" system in 1970's Chrysler vehicles supposedly allowed for burning 18:1 A/F ratios. I'm sure those ratios were obtained, because those Chryslers idled so rough you could see the front seats shake while standing in front of the hood. No wonder Chrysler almost went extinct -- all in the name of emissions BS.

My feeling has always been to use factory specs; set 'em and forget 'em.

Joe

CorvairGeek on Fri February 10, 2006 7:28 PM User is offline

I saw my first propane enrichment information on a (light duty emissions) 1978 GM car. Don't recall that they specified it too many years. Saw the staked-in stainless plugs on the 1979 models, so that may have taken care of the problem.
I agree whole heartedly on limited adjustments. I still have all the plugs in place on my '84 305 Caprice (206K), still passes emissions every year at 0.0% CO and

-------------------------
Jerry

NickD on Fri February 10, 2006 11:32 PM User is offline

I played with propane back then, must really be missing the boat big time for peak mixture adjustment. Would be a fine way to adjust as long as the propane bottle remained with some gas in it and was permanently installed. By not the case, remove the propane and the idling was crap. Removing the propane left the engine running way to lean as proven by my air-fuel ratio meter.

Would depend more on this meter, and accurate tach more as a guide, but the predominately parameter was a ripple free wine glass sitting on top of the air cleaner.

Another point about idle, can burn 2-6 gallons of gas per hour and get you nowhere, so always limit the time my vehicle is idling, but in this process, developed an extreme hatred for traffic lights, not a vehicle on the cross road in sight, but that stupid light stays red, sometimes for months or it seems that way.

Can run my air fuel ratio meter on the highway braze the main jets shut and rebore to compensate for a 14.7:1 fuel air ratio and set the power ratio to 12.5:1, also adjust the distributor to the point just below pre-ignition for both maximum power and fuel economy. Was a lot of fun and looking forward to doing that with my 454 CID with dual exhaust and a four barrel carburetor. Ha, screw the silly-con diode for having to do the thinking for me.

JJM on Sat February 11, 2006 2:18 PM User is offline

CorvairGeek,

I believe your timeline is correct, 1979 was the first year GM staked in those stainless steel idle mixture plugs.

There's even less of a reason to remove the mixture plugs your 1984 Caprice: You've got the E4MC carb CCC. As long as the idle dwell is 30, fluctuating back and forth between 25-35, your idle mixture is properly set. Never understood why folks insisted upon removing the mixture plugs on these vehicles.

Anytime I see the mixtures plugs removed with the telltale cuts in the throttle body, the idle air valve cover removed, TPS and M/C plugs out, choke rivets replaced with sheet metal screws... hack job immediately comes to mind. Whenever I rebuilt those carbs, I always restore all the tamper resistant items for that new, professional factory look. Okay, I'm a bit extreme, but you would never even think the carb was rebuilt, looks like a brand new AC Delco unit (they come with all the tamper resistant items in place too).

Joe

CorvairGeek on Sat February 11, 2006 2:59 PM User is offline

I don't understand the obsession either Joe. I saw a friends '85 1/2 ton 4.3 carburated that the original selling dealer (FL - I've got lots of other stories same dealer) did a hack job on the non-CCC Quadrajet. When he complained about how poorly it ran and smoked black, they told him it was tampered with he would have to pay to have it rebuilt. Since he had the paperwork that they did the tampering and no one else had ever touched it (less than 6 months old-original owner), they had no response and he never pursued it. He was happy just to have it adjusted correctly, but I think I would have tried to get a new replacement for such a stunt (much younger and energetic).
I've never even had a reason to hook up to the green dangling dwell pigtail connector on my Caprice, and I'm the original owner. The reliability of the CCC system (except for the NOx control/performance, but that was the era) and MPG (as well as the rest of the car) has been quite impressive to me. The original paint and fit and finish is another story.

-------------------------
Jerry

mhamilton on Sat February 11, 2006 6:16 PM User is offlineView users profile

Quote
Originally posted by: JJM
CorvairGeek,

Anytime I see the mixtures plugs removed with the telltale cuts in the throttle body, the idle air valve cover removed, TPS and M/C plugs out, choke rivets replaced with sheet metal screws... hack job immediately comes to mind. Whenever I rebuilt those carbs, I always restore all the tamper resistant items for that new, professional factory look. Okay, I'm a bit extreme, but you would never even think the carb was rebuilt, looks like a brand new AC Delco unit (they come with all the tamper resistant items in place too).

Joe

Can you get replacement mixture plugs? Maybe I will call GM and see if they can get me the entire throttle body with everything preset and sealed. That would be ideal. I have no idea why anyone took the plugs out of this carb. I know the first owner was not inclined to do that, he always took it somewhere to be serviced.

I think even more annoying than tampered mixture screws is when vacuum lines are removed and plugged with screws. I tore all that garbage "repair" work out and started with fresh hoses following the original schematic. I never understood what people were thinking by randomly plugging things off, when all of the vacuum accessories make the engine start and run better.

I know how you feel about those choke coil screws, I couldn't bring myself to using those from the rebuild kit. I had to go with the correct size rivets.

NickD on Mon February 13, 2006 5:59 AM User is offline

Wow, by plugging off the EGR valve you can replace 30% of recycled exhaust gases with pure air fuel and get all kinds of HP! So lots of guys were doing this, but were mystified why they burnt out their exhaust valves or even put holes in the piston tops a few miles down the road.

But the early EGR valve circuits were so inadequate with no warning to the driver, they would plug themselves up and leave the driver stranded with a burnt out engine. Was a boom for engine rebuilders that died with improvements.

Back to Off Topic Chat

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.