lean!!!

[Marx3]

Hi some time agoI rebuild a late 70´s APT-carb for a friend. The carb sits on a chevy 383 with 195 cc trickflow heads, a 230/236@050 cam, dualplane intake and 2200 stall. Engine is in a ´73 Nova.

The carb was built per recipe 2, 73 jets 44 rods and DA sec rods. The carb is a Single mainairbleed, and drilled the bleed to 070 ( maybe too large??? )

Problem is that nomatter what the heck I do, the car runs lean... I use Innovate's 3837 Wideband O2 sensor. Exhaust should be completely tight, no leaks.

At first run it was about 20 afr @ cruise and about the same at WOT.
Today I installed a pair of 72 jets with a set of 35 ( ! ) rods, to see if it did anything. Still about 20 @ cruise, and now 18.4 at wot...

I have the converter under suspicion for slipping, would that have any "leaning" effect on the mixture? I mean, 18.4 at wot with 72/35 and DA rods just dont make sense right???

:-)

[Marx3]

carb number is 17080215

[1983zsled]

I have a AEM UEGO wideband on my sbc 355, vortec heads ect
I used recipie number two as my camshaft was the same duration.
It sounds like a air leak into the headers before the wideband sensor....
I have heard/read that a small pin hole will give you very high Air readings...
I guess that is why they call it a $250 light show.
I used muffler cement on the header connections and welded everything else.
My sensor goes as high as 16:1 at cruise and goes to 11.5:1 at wot ?
I gave up on using the sensor and used the methods described in the book.
What do the plugs look like ? How does it idle? What is the a/f at idle...
Dial it in using those methods and I'm sure you'll be happy.
If it was that lean, the car should be lurching, bucking, and trying to kill itself, especially when in high gear and at slight throttle.
Good Luck !

[Marx3]

Yeah those sensors you stick in the tailpipe are pretty sensitive to leaks! The exhaust-system is 3" from headers and exit just behind the rearwheels, the tailpipes DO have two tiny holes drilled into them ( like .150" or so ) about an inch from the tail... except from that there is only the collector to suspect, and it should be bolted pretty tight etc... but I managed to cover them with severeal layers of ducttape, just for the test-drive.

I Guess I just need to look at the collectors, cause the engine runs pretty smooth at all levels.

Only problem is that the car does not seem to be moving according to the engine-rpm :-)
I guess yours is pretty lean at cruise, and preeetty rich at wot...

[Cliff Ruggles]

If it was that lean, the car should be lurching, bucking, and trying to kill itself, especially when in high gear and at slight throttle.

+1

In any and all cases, you have to tune for best performance in all areas, regardless of what a meter tells you.  I don't use or trust them.  Our "tip-in" procedure will tell the tuner if the jet/rod relationship is close for part throttle A/F.  Driving the vehicle will tell the operator if it needs help in any particular area.

I use the dragstrip for full throttle fuel tuning, best ET and MPH, etc.....Cliff

[Marx3]

I usually only use the meter on "stock" cars that the owner is not going to race at any time. Just to check.

But the car must have an airleak in the exhaust, it just does not make sense otherwise :-) I mean withy 72 jets and 35 rods, the carb should be richer than a 850 Holley from the factory :-)

btw I cant dragrace the car for test, since something in the driveline is holding it back, I thought converter, but was told that either the converter transfers power, or it doesn't... Car smelled burned, like clutch on a manual tranny car... I guess the transmission ( th400 ) is gone :-)

[Marx3]

btw plugs looked allright/ on the sooty side, but guess that makes sense with the 35 rods.

Found today that the collector on one of the headers was not tight! Back to scratch!