Got a quick query. How much effect does timing have on oxygen content?
I’m trying to dial in an engine and am using an O2 sensor. It’s a small
block Chevy V8 and the sensor reads rich, which is very much what I’d
expected re: rich smell, fouled plugs, atrocious mileage. The engine’s new
and the carb’s used but in excellent (visual) condition.
The carb’s a 650cfm double pump Holley that’s in very new condition (used
but still in original box) but got it from my machinist who drag races. The
jets indicate stock but who knows if somebody drilled them. The
distributor’s a Mallory Comp 9000 and at this point who knows what the spark
curve’s set to.
So, as I try to dial in the spark curve and fuel, I’m wondering, "Can the
timing curve significantly change O2? Am I trying to dial in the wrong
system first?"
Best Regards,
Drink
25
Jan
Timing Effect on Oxygen Content
On Sat, 27 Jan 2007 18:57:02 -0600, "James Drinkwater"
- Hide quoted text — Show quoted text -
<jdrinkwa…@satx.rr.com> wrote:
> Got a quick query. How much effect does timing have on oxygen content?
>I’m trying to dial in an engine and am using an O2 sensor. It’s a small
>block Chevy V8 and the sensor reads rich, which is very much what I’d
>expected re: rich smell, fouled plugs, atrocious mileage. The engine’s new
>and the carb’s used but in excellent (visual) condition.
> The carb’s a 650cfm double pump Holley that’s in very new condition (used
>but still in original box) but got it from my machinist who drag races. The
>jets indicate stock but who knows if somebody drilled them. The
>distributor’s a Mallory Comp 9000 and at this point who knows what the spark
>curve’s set to.
> So, as I try to dial in the spark curve and fuel, I’m wondering, "Can the
>timing curve significantly change O2? Am I trying to dial in the wrong
>system first?"
> Best Regards,
> Drink
Timing should have little effect on oxygen content. The oxygen
content is determined by the fuel – air ratio going into the cylinders
and that is controlled by the carb. Maybe re-jet it to be sure you
have stock jets, then start adjusting the mixture screws.
Tom S.
James Drinkwater wrote:
> Got a quick query. How much effect does timing have on oxygen content?
> I’m trying to dial in an engine and am using an O2 sensor. It’s a small
> block Chevy V8 and the sensor reads rich, which is very much what I’d
> expected re: rich smell, fouled plugs, atrocious mileage. The engine’s new
> and the carb’s used but in excellent (visual) condition.
> The carb’s a 650cfm double pump Holley that’s in very new condition (used
> but still in original box) but got it from my machinist who drag races. The
> jets indicate stock but who knows if somebody drilled them. The
> distributor’s a Mallory Comp 9000 and at this point who knows what the spark
> curve’s set to.
> So, as I try to dial in the spark curve and fuel, I’m wondering, "Can the
> timing curve significantly change O2? Am I trying to dial in the wrong
> system first?"
It’s lot more complicated then you make it sound. You mention fouled
plugs so misfiring cylinders seems likely, which is going to mean you
could have a rich exhaust even with the carb and distributor OK.
Distributer could affect mixture quite a bit if the rich condition is
due to float level set to high, but modifying the timing is only going
to make the engine run worse even if you succed in making the exhaust a
little leaner.
How good is the compression?
-jim
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If the carb was used on any sort of a "drag" motor… then it probably
has the wrong power valve in it (or the power valve is damaged); and the
power valve is opening during part throttle cruise.
There’s a description of how the power valve works here:
http://www.chevyhiperformance.com/techarticles/74058_carburetor_valve...
"See if this description sounds familiar: Your Holley-carbureted street
car idles terribly, has taken to fouling spark plugs every week or so, the
gas mileage absolutely sucks, and there’s an ominous black cloud swirling
from your exhaust."
I think you’re supposed to select a power valve that’s rated about 2" HG
lower than whatever your part throttle cruising vacuum reading is:
http://www.chevyhiperformance.com/techarticles/74058_carburetor_valve...
http://pstuning.com/ "Tom S" <tscal…@cox.net> wrote in message
news:ufapr29fvv65ppsitrqhhmbm7jjf0nqrec@4ax.com…
- Hide quoted text — Show quoted text -
> On Sat, 27 Jan 2007 18:57:02 -0600, "James Drinkwater"
> <jdrinkwa…@satx.rr.com> wrote:
>> Got a quick query. How much effect does timing have on oxygen content?
>>I’m trying to dial in an engine and am using an O2 sensor. It’s a small
>>block Chevy V8 and the sensor reads rich, which is very much what I’d
>>expected re: rich smell, fouled plugs, atrocious mileage. The engine’s
>>new
>>and the carb’s used but in excellent (visual) condition.
>> The carb’s a 650cfm double pump Holley that’s in very new condition
>> (used
>>but still in original box) but got it from my machinist who drag races.
>>The
>>jets indicate stock but who knows if somebody drilled them. The
>>distributor’s a Mallory Comp 9000 and at this point who knows what the
>>spark
>>curve’s set to.
>> So, as I try to dial in the spark curve and fuel, I’m wondering, "Can
>> the
>>timing curve significantly change O2? Am I trying to dial in the wrong
>>system first?"
>> Best Regards,
>> Drink
> Timing should have little effect on oxygen content. The oxygen
> content is determined by the fuel – air ratio going into the cylinders
> and that is controlled by the carb. Maybe re-jet it to be sure you
> have stock jets, then start adjusting the mixture screws.
> Tom S.
- Hide quoted text — Show quoted text -
James Drinkwater wrote:
> Got a quick query. How much effect does timing have on oxygen content?
> I’m trying to dial in an engine and am using an O2 sensor. It’s a small
> block Chevy V8 and the sensor reads rich, which is very much what I’d
> expected re: rich smell, fouled plugs, atrocious mileage. The engine’s new
> and the carb’s used but in excellent (visual) condition.
> The carb’s a 650cfm double pump Holley that’s in very new condition (used
> but still in original box) but got it from my machinist who drag races. The
> jets indicate stock but who knows if somebody drilled them. The
> distributor’s a Mallory Comp 9000 and at this point who knows what the spark
> curve’s set to.
> So, as I try to dial in the spark curve and fuel, I’m wondering, "Can the
> timing curve significantly change O2? Am I trying to dial in the wrong
> system first?"
> Best Regards,
> Drink
Oxygen levels in combustion gases are as much a
function of combustion efficiency as they are applied
fuel mixture.
I use an LM-1 to tune my car. Lower numbers
indicate a richer mixture, and high numbers mean a lean
mixture. The magic 14.7:1 will provide a good balance
of power and economy for a near stock engine. If
you’re using a radical cam, big heads, etc, you’ll need
to run a little richer for smooth operation. Best
power production is generally 13-13.5:1 or so.
I’ve set mine up to have a mixture of about
13.8-14.0 at light throttle and cruise, and around
13.0-13.2 under power.
Here are two things I’ve noticed:
1. Yes, timing curve does affect sensor readings.
2. If the mixture is very rich, the reading
actually goes up – looking lean. If you add fuel, the
readings drop (as you would expect). If you add even
more fuel, the numbers start coming back up. Why does
that happen? I don’t know, but it does.
If you’re indicating a near normal reading (13:1
or higher), but it still smells rich and fouls the
plugs, use a smaller jet.
–
.boB
2006 FXDI hot rod
2001 Dodge Dakota QC 5.9/4×4/3.92
1966 Mustang Coupe – Daily Driver
1965 FFR Cobra – 427W EFI, Damn Fast.