MelWff

1. Why did you remove the power valve in the secondary block, I assume you installed a plug?
2. What does your air cleaner look like, is it a dropped base with a smooth approach to the carburetor bores and is the top of the air cleaner a solid piece of metal?
3. Just for reference, the 780 used by Chevy on the 70 to 72 LT-1 used a yellow spring, lighter than your silver.

n8dogg

Seems to be the only way to reduce fuel through the PV circuit is to reduce the PVCR size. This is achieved by drilling, tapping and installing air bleeds.

My air cleaner is an aftermarket Moroso, relatively straight base, solid metal top, exposed filter is approx 2.5" ( I have not measured).

The previous owner gave me what looks to be an original air cleaner. But it does not fit right on the carb. It has the square "key" notch in front to fit on the original 780cfm Holley. MY 750cfm Holley does not have this key/notch on top of the carb.

I will try the lighter spring and see what happens!

chevymans 77

I looked through the thread and didn't see where you stated if you have rear PV's, you don't need the rear PV's, put a plug in it and see if that fixes the problem then work form there.

Neal

cagotzmann

I think I may have found an answer to this problem. I will verify tonight when I do some more testing.

First changing the secondary Jets doesn't have the effect you would think in WOT. I went from 75-72-68-65 with very little change. So I am now thinking the O2 sensors may not show what is really happening.

While speaking with a fellow drag racer he said have you tried changing the timing to see the effect of the O2 sesors readings. He said unless you are using a 5 gas analyzer you may be getting a false reading from the O2 sensor. His claim is if you make any changes to the fuel delievery system (jets etc) the actual AFR will change. Just because you cannot read the changes with the WB O2 Sensor doesn't mean it has not changed. WB O2 sensors dont read AFR but measure O2 with the use of additional sensors such as EGT(exhaust gas temp) you can get an approximation. Most EFI Systems on modern cars have many sensors to help get a very good indication of AFR which we are not using.

Because I have a 6530 programable MSD box I can easily change timing in seconds. Hopfully this will shed some light on using WB 02 sensors with carbureted engines.

n8dogg

No PV in the rear, but thanks for the suggestion.

I'm not sure if what you're saying makes much sense. The whole purpose behind a wideband 02 sensor is for highly accurate readings of lambda in the exhaust stream of a gas powered internal combustion engine. Carbureted or fuel injected does not effect WB readings. WB's also do not work with other sensors, many tuners simply clamp a WB into an exhaust tailpipe.

A wideband sensor actually measures the stoich value of the fuel being used. It achieves this by trying to regulate how much oxygen and combustion products enter and exit the sensor. If there is too much oxygen it knows it's lean, if there are too many combustible products it knows it is rich. If you work off Lambda, the sensor/gauge will show you if it is stoich/rich/lean no matter the fuel you are using.

Again, it does not matter if your engine is fuel injected or carbureted. Rich is rich, lean is lean, the sensor doesn't lie!

What can effect the readings are exhaust leaks, uncalibrated sensor, catalytic converters, hardware/software problems.

Here is a good read:
http://www.innovatemotorsports.com/resources/news3.php

cagotzmann

Here is a simply test that should demonstrates the possible error.
Remove a plug wire from one of the cylinders and watch the change on your gauge. It will most likely register as a more lean condition while the AFR has not changed meaning the carb will still deliever the same amount of fuel for the air flow, just some on the fuel cannot be burned and this is picked up by the O2 sensor ?

But I am going to test the change in timing to see if it actually changes the O2 sensor to read differently. I hope it doesn't.

chevymans 77

what size are the primary/secondary high speed air bleeds,

timing should not effect AFR until it is moved enough to cause misfires then it will,

n8dogg

Of course AFR reading will change if you have a misfire. You have unburned fuel and air traveling through the engine and exhaust stream.

chevymans 77

n8dogg, the research I've done along with my personal experience with an LM1 wide-band sensor shows that a misfire will show up as a lean AFR due to the excess O2 in the exhaust gas stream. The wide-band senses the excess O2 in the exhaust stream to determine what the AFR is.

What style secondary metering block did you use? Was it a kit made for that purpose?

Try disconnecting the secondaries and make a run to see what the AFR does on the primaries. This should show if the issue is with the secondaries only or if the primaries are contributing also.

Where do you have the fuel level set on the secondaries?

Neal

cagotzmann

I got the same result result tonight with my zeitronix goes lean with a misfire.
But changing the timing + or - 3 degrees did not create enough of a misfire to make any changes to note in the AFR of the sensor reading.

n8dogg

You both are right, a google search confirms that a misfire will create a lean spike in 02 sensor readings.

cagotzmann

Well I found the source of my problem why changing the secondary jets has no effect on my AFR Readings. Simple because the secondaries don't open period. I put a video camera on them with no movement at all.

WOT 1st through 4th gear and nothing.

Still it works pretty good for only 2 barrels. Can't wait to see the difference with all 4 running and tunned.

I thought they were working because I put a clip on the arm to see if it moved but I guess it must have just slipped down from vibration or something. The camera didn't lie.

Now to figure out why it doesn't move is the next step.

REELAV8R

good find! I'm willing to bet that at WOT you still have a bucket load of vacuum. Lots and lots of pull on the primary jets.

cagotzmann

Here is what I found with the vacuum secondary unit.
The diaphragm is torn in 2 spots.




So for those that cannot get the AFR for WOT to change after moving 6jet sizes lower please confirm the secondaries actually work. I will fix this and retest my results when done.

cagotzmann

Update. After replacement of the diaphragm and now running 69-68 jets and the yellow spring the AFR for WOT in now well above the 10-11 range.

With the secondaries not working range was 10.9-11.2 now the range is 13.1 - 13.7.

The test was using 2nd starting @ 2000 RPM - 5000 RPM

I first get a little leam then drops to ~ 11.8 for about 1/2 sec then climbs to 13.7 you can see when the secondaries open as the AFR climbs from 12.3 to 13.7 steady @ ~ 13.2 around 3000 RPM.

So it was confirmed my problem with low AFR (Rich) for WOT and changing secondary jets had no effect in AFR was due to secondaries not opening due to failed diaphragm.

Haggisbash

Good to see you had a win:-)

OMF

How much did the seat of the pants dyno notice?

cagotzmann

My Seat of the Pants Dyno doesn't work.

But I use a GTech to measure G's.

Before the fix from 2000 RPM

1st Gear 0.53 G's now 0.66 G's

2nd Gear 0.42 G's Max Now Reached .49 G's

and from 3000 RPM reached 0.53 G's

3rd Gear now 0.41 G's

Now 3rd feals like 2nd gear did previously.

Pop Chevy

I believe your air bleeds are too small. That's why the jet changes are not effective. Also what is your rear PV ?

n8dogg

Update for those following the over rich condition at WOT as described in my OP. I replaced the secondary spring from natural color to the next lightest, purple, and I can tell that my secondaries are now opening more than before. The engine grunt is louder and more pronounced than before. I still have yet to get my WB02 sensor hooked up again to check the AFR. Hopefully this week sometime, it's been really busy.