Posted By: Big Squeeze
The benefits of an Oxygen Sensor.......... - 01/14/12 04:10 PM
I didn't want to hijack another thread, so I'm starting this post to help people see the benefits of using an O2 sensor to tune for driveability...........
When tuning at the track without an O2 (or even with an O2), most everyone knows to start with the timing conservative, jet for maximum trap speed and then creep timing in until trap speed stops going up......
From past experience, I roughly know what WOT numbers to to shoot for so that's where I get it to before I ever go to the track..........
I was tuning a 6 pack last week.....actual 10.5 to 1 compression.....stock stroke 440.....exhaust manifolds........Dwayne Porter small hydraulic cam........brand new carbs that came with 62 jets in the center carb and a 6.5 power valve........
The VERY FIRST THING to do when doing any carb tuning is to find out what your motor wants for initial timing and curve the distributor.........This motor wanted 27* at idle and I set it up to have 36 total, all in by 3,000 and the timing doesn't start advancing until 1,000.......
Initially, I'd set all 6 idle mixture screws at 3/4 turn out.......which put the idle mixture in the low 12's..........
At cruise, it showed the A/F in the high 12's, which is a little richer than I thought this motor needed to be.........
At WOT A/F was in the low 11's with the stock metering plates............which is about 1.5 points richer than I where I thought it should be...........
While cruising.......when I slightly push on the pedal to speed up, the A/F went to mid 15's for a split second and then the Power Valve would open and richen it up into the low 13 to 1 range...........
When the end carbs would open while going WOT at low RPM, it had a pretty large bog.........
So, I busted all the carbs apart.........I put 60 jets in the center carb to lean it out a little at cruise......installed 85 jets into the end carbs for a starting point........installed a 10.5 Power Valve to get it to richen up sooner when slightly accelerating.........and went two spring steps stiffer on the secondaries to keep it from bogging.......
I put it all together and when I started it, fuel was gushing out of the vent of the front carb......so I pulled the needle and seat to check for trash and there wasn't any.....I put it back in and eye-balled about how much it had been sticking up past the top of the float bowl and locked it down.......I pulled the float bowl off and found that there were little marks on the floats where it had hit the jets I'd installed into the metering plates I'd modified to hold jets, so the jets had been holding the float down.......
I pulled the flat washers out from between the jets and plates to move the jets away from the float (I'd put those washers between the jet and metering plate because when the jet is screwed in, there isn't much room between the jet and the metering plate gasket)........
I button it all up and go for a test drive......Cruise A/F was 10.5 to 1.........the WOT bog was gone........it still had a small lean spike to about 14.0 to 1 when slightly accelerating but I couldn't feel it.......and WOT was now in the low 12's/high 11's.............
OK........I'd gone 2 jets smaller on the primaries and it's WAY richer now........and I've put the highest PV they make in it and it's still got a slight lean spike.........
I get back to the shop and check the float level on that front carb, which I should have done before I left
......I had to lower it a 1/4 turn to get it to where it needed to be........
I drive it again and the cruise was in the low 13's and the stumble from slightly accelerating is worse............
SO A 1/4 TURN ON THE FRONT FLOAT BOWL CHANGED THE CRUISE A/F FROM MID 10'S TO LOW 13'S
..........Think about that 
.........How would you guys that tune without one know that???? 
To fix the lean spike while slightly accelerating, I know it needs the end carbs leaned out and the center carb richened up so the cruise A/F stays the same.......and as some people know, the idle mixture screws have a significant effect on cruise A/F, especially when you have 6 idle mixture screws.......
So, I set all 6 idle mixture screws to about 5/8 of a turn out........change the primary jets from 60's to 65's........and go for a test drive.......Cruise is in the low 13's......WOT went from high 11's/low 12's to mid to high 12's and I didn't even change the secondary jets!!!.....and that's because I'd screwed all 6 idle mixture screws in about an 1/8 of a turn....
The slight lean spike is gone because the center carb is jetted up......and that's my stopping point........The car runs as smooth as glass......starts and idles with just a bump of the key........will blow the tires off from an idle while going WOT...no lean surges......no stumbles........you can't even feel when the end carbs open because it's so smooth...... 
Now, to tune it to it's maximum, I need to go to the track and jet the end carbs for maximum MPH.....or go to a chassis dyno and jet the end carbs for maximum HP..... 
There is absolutely NO WAY you could figure all that out without an O2 sensor........I hope this helps guys understand the importance of using one.......and I do sell a video that covers this
BTW, I've got to give credit to Dwayne Porter....He's helped me a TON over the last 5-6 years when it comes to carb tuning and he's WAY ahead of me.......
When tuning at the track without an O2 (or even with an O2), most everyone knows to start with the timing conservative, jet for maximum trap speed and then creep timing in until trap speed stops going up......
From past experience, I roughly know what WOT numbers to to shoot for so that's where I get it to before I ever go to the track..........
I was tuning a 6 pack last week.....actual 10.5 to 1 compression.....stock stroke 440.....exhaust manifolds........Dwayne Porter small hydraulic cam........brand new carbs that came with 62 jets in the center carb and a 6.5 power valve........
The VERY FIRST THING to do when doing any carb tuning is to find out what your motor wants for initial timing and curve the distributor.........This motor wanted 27* at idle and I set it up to have 36 total, all in by 3,000 and the timing doesn't start advancing until 1,000.......
Initially, I'd set all 6 idle mixture screws at 3/4 turn out.......which put the idle mixture in the low 12's..........
At cruise, it showed the A/F in the high 12's, which is a little richer than I thought this motor needed to be.........
At WOT A/F was in the low 11's with the stock metering plates............which is about 1.5 points richer than I where I thought it should be...........
While cruising.......when I slightly push on the pedal to speed up, the A/F went to mid 15's for a split second and then the Power Valve would open and richen it up into the low 13 to 1 range...........
When the end carbs would open while going WOT at low RPM, it had a pretty large bog.........
So, I busted all the carbs apart.........I put 60 jets in the center carb to lean it out a little at cruise......installed 85 jets into the end carbs for a starting point........installed a 10.5 Power Valve to get it to richen up sooner when slightly accelerating.........and went two spring steps stiffer on the secondaries to keep it from bogging.......
I put it all together and when I started it, fuel was gushing out of the vent of the front carb......so I pulled the needle and seat to check for trash and there wasn't any.....I put it back in and eye-balled about how much it had been sticking up past the top of the float bowl and locked it down.......I pulled the float bowl off and found that there were little marks on the floats where it had hit the jets I'd installed into the metering plates I'd modified to hold jets, so the jets had been holding the float down.......
I pulled the flat washers out from between the jets and plates to move the jets away from the float (I'd put those washers between the jet and metering plate because when the jet is screwed in, there isn't much room between the jet and the metering plate gasket)........
I button it all up and go for a test drive......Cruise A/F was 10.5 to 1.........the WOT bog was gone........it still had a small lean spike to about 14.0 to 1 when slightly accelerating but I couldn't feel it.......and WOT was now in the low 12's/high 11's.............
OK........I'd gone 2 jets smaller on the primaries and it's WAY richer now........and I've put the highest PV they make in it and it's still got a slight lean spike.........
I get back to the shop and check the float level on that front carb, which I should have done before I left
......I had to lower it a 1/4 turn to get it to where it needed to be........I drive it again and the cruise was in the low 13's and the stumble from slightly accelerating is worse............
SO A 1/4 TURN ON THE FRONT FLOAT BOWL CHANGED THE CRUISE A/F FROM MID 10'S TO LOW 13'S
..........Think about that .........How would you guys that tune without one know that???? To fix the lean spike while slightly accelerating, I know it needs the end carbs leaned out and the center carb richened up so the cruise A/F stays the same.......and as some people know, the idle mixture screws have a significant effect on cruise A/F, especially when you have 6 idle mixture screws.......
So, I set all 6 idle mixture screws to about 5/8 of a turn out........change the primary jets from 60's to 65's........and go for a test drive.......Cruise is in the low 13's......WOT went from high 11's/low 12's to mid to high 12's and I didn't even change the secondary jets!!!.....and that's because I'd screwed all 6 idle mixture screws in about an 1/8 of a turn....
The slight lean spike is gone because the center carb is jetted up......and that's my stopping point........The car runs as smooth as glass......starts and idles with just a bump of the key........will blow the tires off from an idle while going WOT...no lean surges......no stumbles........you can't even feel when the end carbs open because it's so smooth...... Now, to tune it to it's maximum, I need to go to the track and jet the end carbs for maximum MPH.....or go to a chassis dyno and jet the end carbs for maximum HP.....
There is absolutely NO WAY you could figure all that out without an O2 sensor........I hope this helps guys understand the importance of using one.......and I do sell a video that covers this
BTW, I've got to give credit to Dwayne Porter....He's helped me a TON over the last 5-6 years when it comes to carb tuning and he's WAY ahead of me.......
Try setting the outboard mixture screws at 1/8 to 1/2 turn out and then use the center mixrture screws to clean the idle up so it will idle at 13.8 AFR or leaner 
Good read
after we got serious with SILVERFISH...My wife has been on my a$$ to get it back running as we've had this car before we had kids(Scott's 31)and hauled them everywhere in this car...Lots of memories related to this car like Scott driving it to his high school prom etc.etc...
and you can only hope 
they smarten up.
