I am trying to figure out why my A/F guage starts off in the normal range, but starts to creep up to the rich range (especially after I start driving). And it isn't because it is warming up...

I've noticed that I appear to be sucking a little oil through the PCV into the base of the carb... will this mimick a rich condition? Plugs look OK... not overly fouled. The guage reads on the border of rich/normal after a few seconds... any thoughts?

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

wait a sec, you're sucking oil through the base of your carb? that's not good! First, you have a vacuum leak if you're sucking ANYTHING through the carb base. Install a new gasket, this will fix the rich condition. Second, replace the pcv, change the hose. make sure everything is hooked up right.

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'86 IROC 5-speed
305 LG4
edelbrock performer rpm intake
edelbrock 600 cfm carb
msd pro billet hei distributer
'95 3.23 rearend
hotchkis rear suspension
PST front polygraphite suspension
"speed kills. buy a f@&d, live forever."

 

bwahahaha, see what happens when you ask a tech question here.

I have an idea, plug off the PCV port and see if it still does it. To answer your question though, yes PCV can have the effect of richening the mixture. In a worn engine it's a good trick to tape over the PCV hole and put it back in the VC if it is failing for HC as the excess blowby/vaporization will richen things up.


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Ed Maher - Moderator @ The TPI & Carb Boards
92 Z28 Convertible - Quasar blue / Tan top
305 TPI A4 2.73 - 14.8 @ 93.1
Stock except ported plenum and dual cats
-=ICON Motorsports=-

- Definitely prototypes, high powered mutants of some kind. Too weird to live, too cool to die

 

Yeah Ed, if I can only find that damn cap that fits the PCV port! I've seen it a dozen times, just never when I need it

Iroc, I don't have baffles in my valve covers (to clear the stud girdle), so the PCV can suck up some oil sometimes... I really do need to come up with something better, but it works fine. I'm mainly trying to nail down this "am I really rich or what" question for the day I spray!

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

So I disconnected my PVC valve, and these are the results...

My vacuum is a little stronger... like 13"-->14" now. This allowed me to screw in the two idle air bleeds on the primary metering block a little. The car also idles a little smoother now and lower (about 750rpms in drive). I am happy about that.

But, I have to hook up the PCV for emmissions, and with the stud girdle bar, the PCV doesn't fit into the grommet completely. I guess I'll have to glue it in or something to keep the vacuum high.

And with all this said above, I still have the same problem where the A/F guage reads "normal" for a few seconds (though it did appear to take slightly longer this time) and then slowly creep up into the "rich" range.

I'm going to drive around a few days like this to see if it was oil or something getting sucked into the base, that it burns off or something and the problem goes away. I'll hit it with some carb cleaner tonight too.

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

How do you go about testing vaccuum in your car and whats the right reading you should be at?

 

Without a computer controlled carb taking signals from an ECM that references an O2 sensor you'll never nail down a perfect fuel curve. That's why the manufacturers went to computerized A/F management in the first place.

Going from OK to rich after a few seconds of runtime on a hot engine is normal (not sure if that's what you are describing)- the liquid fuel in the intake takes a little while to work it's way down the intake runners into the cylinders. Your only choce is to run it leaner and then have it work it's way up to normal.

 

Transam... normal for a performance cam is about 12"-15" and stock is more like 18" or so...

Damon, so you think either/or:

1) This is normal and I should tune it somehow to lean, so that it will read normal when I drive? How can I lean it out? I don't think going to a smaller jet size would have any influence on idle A/F... and messing with the float levels isn't seeming to make a difference really. Got any suggstions?

2) That maybe this A/F (Autometer) gauge is worthless and I shouldn't pay too much attention to it? Short of reading plugs (a real PITA), how can I be sure or monitor the A/F ratio?



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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

C'mon chief, you're just being lazy now.

-set your idle mixture for best vacuum. You use the idle mixture screws. If the idle mixture screws don't have the desired effect, make sure the throttle plates aren't open too far to maintain the idle speed. If they are, drill small holes in the primaries and close 'em up a bit. Yes you will set the idle when warm under normal conditions.

-You adjust your WOT A/F however you damn well please, plug cuts, track tuning, etc.

And of course timing comes into this as well.
...ed

 

Well, if that is the suggestion Ed... then I'm at a loss! I am going to have to think about this some more I guess as to what may be going on why I'm reading (though maybe not running) rich on the A/F gauge. Might involve a call to Autometer too...

 

You can 'verify' your AF guage by hooking a DVM up to the same wire as the O2 signal wire. 0.45V is stoich. By 0.9-1.0V it should prolly be pegged.
If you want to make a quick adjustable voltage source to correlate voltage w/ your AF guage, head to radio shack and get yourself a potentiometer (adjustable resistor, and a regular resistor that is 11x bigger.
Hook up the 11x resistor to your battery, then to one side of the pot, the other side of the pot to ground. The middle pot wire is your new signal wire (btw, you don't really need the 11x resistor, it just idiot proofs the circuit from you giving full 12volts to the guage and blowing it, now full voltage on the high side of the pot will be 1/12 total voltage or ~1 volt)

As for emmisions, i guess you missed the part in my first post where i said that on old worn engines taping over the PCV valve hole and reinstalling so it looks present.
Also, if it isn't going in all the way, why not cut the body of the PCV valve down and epoxy over it for the same effect when you take it to get tested.

 

I wil probably go this route considering I all ready have a volt meter.

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Also, if it isn't going in all the way, why not cut the body of the PCV valve down and epoxy over it for the same effect when you take it to get tested.

I was thinking about doing this too... but does it need to create a vacuum on one side and not on the other... something about helping with blowby and stuff

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-