View Full Version : adjustment CO%

hawk gt 62 rhd
01-23-2007, 04:42 PM
Hello to you all,my guestion is about the right CO% of a 289 with Carter WCFB,double separate exhausts and [1]50000 miles.AT this moment I meassured 8% is this normal or should this be less for a cleaner engine.

01-23-2007, 09:07 PM
are you taking the measurement at idle? bring the engine up to cruising rpm slowly, give the machine a chance to react and look for a very low reading. that shows you efficiency at cruise. idle co is mainly a function of the mixture screws ( assuming clean air filter, etc. )and won't necessarily impact cruise co.


R2 R5388

01-24-2007, 12:02 AM
I have had much experience with checking exhaust emissions on both old cars and new. First I will say, on older cars, especially with today's fuels, it is best to er on the side of slightly too rich than too lean. Fuel is part of what keeps an engine cool, and an engine with too lean a mixture will run hotter than one with a richer mixture. Lean mixtures lead to detonation which can cause engine damage. That being said, If it is too rich, the fuel mileage will be poor, and the excess gasoline will eventually contaminate the oil. Excessive fuel can cause the rings to unseal in the cylinder walls. So as you can see mixture is pretty critical.

What I like to see when I set up an older car's carb is a cruise mixture of no more than 4% co, preferably 1-2% for best mileage, and an idle mix of 2-5% co. If you have an analyzer that does HC, and CO2 as well as CO, that is even better. Try to set the idle mixture with the lowest CO and HC reading you can get along with the highest CO2. That will usually be 2-5% CO and 200- 400 parts per million HC and 11 - 14 % CO2. If the valves on your engine are too tight, then HC's will be higher and engine will not idle smoothly at the lower end of the CO scale. (lean mixture) Likewise, if compression is down in one cylinder, or there is a vacuum leak into one cylinder, Idle will be rough and HC's will be higher.

Your reading of 8% I would say is a bit too high. If it is 8 % at idle and then goes lower at cruise, try turning in the mixture screw(s) a bit at a time to lower the reading. If it is 8% at cruise - i.e. 2000 rpm, Either the float level is set to high, the metering rods are set too high for the throttle opening or there are the wrong jets in the carb, or some combination! I believe the metering rods are easily adjusted on a wcfb, and that would be an easy place to start because all you have to do is remove the front upper dust cover on the carb to see them. They are held in place by a set screw that tightens the bracket they are suspended on to a rod that is connected to the throttle linkage. If the metering rods are lifted up too high in relation to the throttle opening, then the car will be too rich. First thing is to check the position of where things are set very carefully. When you loosen the set screw, things get loose and it is a good idea to know where you were at to start with. I'm assuming the car is too rich at cruise, so that means the rods may be too high, so try loosening the set screw just enough to slightly turn the bracket so the metering rods are just a bit lower. then recheck the CO reading. If you are not sure of the float level, then it also would not hurt to check it. While you are doing that, gently shake the floats to see if there is any gas in them. That will cause the float to sink and raise the float level. It also wouldn't hurt to write down the numbers on the jets themselves for later reference. Idle mixture will be affected by float level so that may need to be re-adjusted too if you change the float level. If there is still no change in the cruise co readings, recheck the level of fuel in the carb to make certain it is actually lower now than it was the first time, and if it is, it may need to be lowered again, or it is time to check the jet numbers with a chart and see what is supposed to be in the carb, or do more work with the metering rod adjustment. Then there is always the wild card, that the jet numbers may be correct, but at some point, some well meaning soul drilled out the jets! Unfortunately, there are lots of variables when it comes to carbs and fuel mixtures.
I've thrown out a lot of info on this post, hope some of it helps.
Al Goodkarmechanic

John Kirchhoff
01-24-2007, 10:13 AM
Excellent info Al, I learn something new every day. Thanks!

Dick Steinkamp
01-24-2007, 10:27 AM
Good stuff, Al. :). Thanks.


hawk gt 62 rhd
01-24-2007, 03:57 PM
Thanks for your answers Ernier and Al.
There is something more I like to know,that is about the two idle scews.
Is it so that the right one is for the right side of the engine,and the left one for the left part ,so that I can measure into the right and left side exhaust?

01-24-2007, 11:12 PM
The mixture screws each control 1/2 of the engine, but it is not like you would think - left screw does not mean left exhaust, and vice versa. The intake manifold is a split level and one side of the carb feeds the upper level and the other the lower level. The cylinders are grouped where the 2 end cylinders are paired with the 2 middle cylinders on the other side of the V. The same is true on the other plane of the intake with the remaining 4 cylinders but this time it is the end cylinders on the other side of the V and the corresponding other middle cylinders. Each pipe on a dual exhaust car carries exhaust for the corresponding side of the V so each pipe has exhaust from 2 cylinders controlled by the left idle screw, and 2 controlled by the right idle screw.
The best way to begin an idle mixture adjustment is to warm up the engine to operating temp, and then turn the engine off. Turn in one mixture screw while counting the turns as you go. Turn it until it lightly bottoms out, do not force it tight. Write down the number of turns, including half turns quarter turns whatever it it. etc. Then repeat with the other side. Now pick a number based on how many turns you had to turn them in, and unscrew each screw the SAME number of turns out. Older carbs adjust mixture rather quickly, so I'm guessing on a WCFB, you will probably want to start with 2 tuns out or 1 3/4 turns out. If you got 3 or more turns each when you were counting the first time, that could explain your 8% CO reading at idle. If you don't turn them out far enough, the engine may not idle at all, and you should turn them each out another 1/2 turn and try to restart. Remember to turn each screw the same number of turns out because each side of the carb is controlling 1/2 of the engine.
If you don't have a CO meter, You can set the mixture by ear. To set the idle mixture by ear, you start as before, setting both screws the same number of turns out after you have counted how far out they were previously set.
Remember, it is good to know your starting point, so if all else fails, you can put things back where they were previously set.
I Can Not Stress This ENOUGH!!
It applies to many things on a car, not just mixture screws.
If the engine is idling pretty smoothly, then one, at a time, turn a mixture screw in slowly 1/8 tun at a time, until the idle speed starts to drop and get rough. Then back the screw out just enough to get the smooth idle again, and repeat with the other screw. My experience has been that this method usually results in slightly rich idle mixtures, but slightly rich is O.K., and the engine will be smooth and steady.

quote:Originally posted by hawk gt 62 rhd

Thanks for your answers Ernier and Al.
There is something more I like to know,that is about the two idle scews.
Is it so that the right one is for the right side of the engine,and the left one for the left part ,so that I can measure into the right and left side exhaust?

01-24-2007, 11:36 PM
This thread is a KEEPER ;)

http://im1.shutterfly.com/procserv/47b6ce20b3127cce8d0e3b50356c00000000400CcNWTlozYsb http://im1.shutterfly.com/procserv/47b6dc03b3127cce970d7aafd15f00000010100CcNWTlozYsb
Studebaker Fever
60 Lark
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Arnold, Missouri

hawk gt 62 rhd
01-28-2007, 11:12 AM
Thanks Al for your nice answers .I did all things you mentioned and the engine is runnig smooth and reacts very nice on the trottle pedal.
about the risk of too low Co% you are very right.
I had it 4,5 at idle 600 revs and 1,5 at 2000 revs my sparkplugs were really nearly white!!
Now im driving 5,5 and will check soon for the color of the plugs.

01-29-2007, 10:21 PM
Glad I could help, they are so much more fun to drive when the carb is set up right.

01-31-2007, 10:04 AM
Great thread! Thanks especially to goodkarmamech for the detailed explanation. I have the same task to do on about 3 hawks with old WCFBs.

I have a question about the instrumentation used by hawk gt 62 rhd. I know from my days in airborne instrumentation engineering that most automotive instrumentation is less than accurate even though it is what we have to work with. Were you doing these measurements at home with some kind of a hand-held unit or in a shop with a garage-quality tester? I just want this info as a point of reference, as I plan on using an old Sears hand-held tester as a starting point.

I agree with and have long held the opinion that spark plug readings are the best indicators. Again, thanks.

hawk gt 62 rhd
02-10-2007, 05:50 PM
Here is my answer about thte equipment on measuring CO.
I used aproffesional SUN tester whitch must be leak tested an callibrated every time.
I measured in both the exhaustpipes,no difference

Its so that the turning of just 1/8 makes differences.And keeping the screws equal .
At this moment the engine is very smooth wiht 6 o/o CO,6 degr adv ign and an Impco air filter with Bosch W 6 sparkplugs and break point gap of 0,45 mm all this at 550 revs/min.
I am using euro 95 petrol with potassium lead substitute and 10W 40 ESSO uniflo oil for low temp after that Shell 15W40 X 100.