CamarosRUS

A buddy of mine has a stand alone wideband setup that we played around with a little today. Really had me scratching my head but was a real eye opener.


I'm thinking the entire time my idle is pig rich because of the nose burning exhaust smell and probably lean up top and at part throttle cruise. When we hooked up the O2 sensor it was showing something crazy like way over 16:1 A/F at idle and pretty much spot on during cruise and at WOT.

I adjusted the idle mixture screws to produce about 14.4:1 A/F at idle and they are sitting almost 2 1/2 turns out each. The kicker is the fact I'm idling at over 1500 RPM with the primary butterflies COMPLETELY CLOSED! I know its getting air from the PCV and the secondaries need to be closed slightly but I can't believe the thing idles like it does.



Does it make sense it would smell so rich but be so lean? cam is 242/248 @ .050.

travis401

iTrader: (1)

Whats your timing at? Too much initial advance will make it idle high.

CamarosRUS

Initial is about 16* but vac advace is hooked up to a full vac source so it it idles at about 32*. I know is what is keeping it idling so high and need to play with the advance curve but just found it interesting the thing idled with the primary idle screw completely removed.

pancherj

I have a 236/242@.050 hydraulic roller cam in my car. It is on a 109LSA, so there is a good bit of overlap. I run a very lean idle, but the exhaust smell is REAL strong. I don't think you will get away from that. Because of the overlap, some fuel/air is going to be pulled right out the exhaust port.

I would be looking to try more initial advance. My car seems to like 25* best...I just need to limit my HEI so it only supplies 11* more. With vacuum advance, I idle at 45* advance at 950RPM.

try unhooking the PCV and see if that makes a difference with your 1500RPM idle. I would suspect it is a combination of PCV and throttle blades/shafts supplying the idle air...even with the blades closed.

Twin_Turbo

iTrader: (25)

Why would you want that?

Without vac. adv. set it so full centrifugal is in by 2500 rpm (lighter springs), then set it so max timing is @ 34-36. See what it comes down to @ idle. You'll need a 18 or 16deg vac can that coems in at the proper vac reading (depends on how much manif. vac yours makes) This so it won't go over 52 @ cruise/light load.
If idle timing is on the high side you'll need to extend the curve with one of the kits out there.

skinny z

You'll find the afr gauge one of best investments you can make especially when it comes to carb tuning.
Never understood why some tuners hook up vacuum advance to full manifold vacuum. If you think about it, what happens when you crack the throttle a bit? Your vacuum drops to near zero and there goes a big chunck of your spark lead. The engine will not respond very well. Conventional adjustment puts the vacuum advance on a ported source so it's not being applied at idle. Mechanical advance typicaly is all in by 3000 or so. In my case (as in others) the engine likes quite a bit of initial advance. 14-16 isn't uncommon. Total is determined by what the engine wants. Effecient combustion chambers don't need as much total advance (not including vacuum) as other heads. 34 degrees is common with 36-38 for heads with poor chamber designs.
Try disconnecting the vacuum advance, setting your initail to 10-12 for starters and see how the engine responds. As has been mentioned above, it may be necessary to limit the total mechanical advance available in the distributor so you can run more initial.
All of this you may already kow, it's just seeing vacuum advance hooked up to full manifold vacuum usually spells trouble.
Others may argue.
And yes, a very lean condition may a cause strong fuel smell. That would produce a lean misfire and it's unburnt fuel leaving the combustion chamber.

five7kid

iTrader: (14)

If that's happening, you've either got a problem with the vacuum port you're using, or something seriously wrong with your engine.

skinny z

Not at all. Hook up a vacuum gauge to manifold vacuum (not ported vacuum) and hit the throttle. The instant the throttle blades move, vacuum drops. The vacuum will then climb when the throttle is released, even causing a spike as the rpm's subside.
Is that not the case?
Just to satisfy myself, I just tried it in the driveway. Idle manifold vacuum at 18"(stock 305), crack the throttle and my vac gauge almost drops to zero. It rebounds to over the idle value and then comes back to 18.
Am I missing something?
Rereading my previous post, I can see you might interpret "crack the throttle" with SLOWLY opening it a small amount.
I'm refering to an ABRUPT opening.

Orr89RocZ

iTrader: (20)

When I free rev, my vacuum gets higher. Not sure how much of a throttle change you are using. I'm just using light throttle rev. On a heavy throttle rev I still dont think it drops near zero. Thats a BIG change to go from idle vacuum to WOT atmospheric pressure.

But I to find it funny that you can get rich idle smell when running lean. With alot of overlap on a cam you can fool the WB alittle to thinking its lean, so you need to double check the plugs as well.

skinny z

Still think I need to clarify.
I'm talking about a quick but brief opening of the throttle from idle.
At WOT, manifold vacuum should reach near zero (1" maybe 2")or the volumetric effeciency isn't as it should be.
This is NA stuff I'm talking about. In particular, carbed apps. EFI works on the same principle.

CamarosRUS

My PCV and high idle timing advance are the main culprits. Found it interesting, needs some serious tuning but I'm learning so much lately.

My transition circuits are terrible, there is a serious bog taking off normally right off idle.



Because the motor isn't perfectly tuned and likes a ton of timing at idle as well.

Three ways to get the crazy high idle timing it likes:
1. Lock out dizzy (not so good)
2. Run 30-32* initial and limit the mech advance to 2*...(locked out basically)
3. Run what base timing it wants (16-18* for me), hook up the vac advance to fully ported and tune the can to read 32* timing at idle, run a more feasible timing curve with about 20* mechanical advance.


Fo sho!!! That and a vacuum gauge visible while under load.


It's what the motor likes best. Regardless if you are using the 'full' or 'ported' source the available manifold vacuum because completely equal between those two when the throttle blades are opened, like in your example.


This is the coolest thing I've learned in a long time about large cams. That rich smell is, indeed, extra fuel but the combustion process itself may very well be lean (or rich for that matter).

O2 sensors only measure the amount of available oxygen, having unburned fuel in its path actually exposes the sensor to increases levels of O2 causing a leaner reading that what may be inside the combustion chamber.

This is why lean spikes are evidence of misfires.

This means tuning a cam with substantial overlap becomes much less black and white with a wideband setup and more subjective. This is where vacuum gauges come in handy to find a good median between measured A/F and manifold vacuum.

...sorry just talking it out to myself.

five7kid

iTrader: (14)

Yes. It happens with ported vacuum as well, it just doesn't have vacuum at all until the throttle opens.

pancherj

A bog right off of idle sounds like either the pump shot OR not enough timing advance. Hard to say which one it could be. You just need to experiment.

As for the ported versus manifold vacuum debate...I was always under the impression that ported vacuum came about due to emissions concerns (in the mid-1970's). This was to reduce NOx level at idle I believe. The few cars I have owned have always run better off of the manifold vacuum port on the carb. I can't say this would hold true for every motor.

skinny z

Correct me if I'm wrong.
If you hook up your vacuum advance to full manifold vacuum then you would be adding spark advance at idle through the vacuum advance as well as whatever initial you have.
So at idle, total timing might be 12 initial PLUS whatever the vacuum advance will give at high vacuum, say 10 degrees. That's 22 total. I can see how some engines might like that much lead. (Not far off the 16 my engine likes (liked)).
However, if you blip the throttle, instantly the manifold vacuum drops (as it should) and so will the advance that was provided by the vacuum advance. That would cause a serious bog or flat spot. You've dropped from 22 to 12 and don't have the benefit of any increase in rpm to provide mechanical advance.
It's easily demonstrated with a vac gauge, tach and timing light.
That's why I question why "some" engines will run better with full manifold vacuum applied to the vacuum advance.

pancherj

I think you have it right. At idle, my car seems to like 25* advance PLUS vacuum advance (right around 45* at idle). However, I need to find a way to limit my HEI so I can run that type of curve. Under part throttle cruise, I have more vacuum than idle, so the can will add a little more vacuum advance plus some added mechanical advance (now I am up to 52* at a light throttle cruise). I haven't monitored to see what happens when I blip the throttle. The vacuum should drop some (not sure if it will be a lot). The pump shot would probably cover up that hole...but maybe not. I tried to set my non-computer advance curve to match the computer controlled curve that TRAXION had programmed into the car. Unfortunately, an HEI will never be as adjustable as the computer

five7kid

iTrader: (14)

Skinny, think about it - if you have the vacuum advance hooked to ported vacuum and have the initial timing set at 12 degrees advance, when you open the throttle the "ported" vacuum will be EXACTLY the same as the manifold vacuum, giving the same total advance as you would have off-idle as if you had the vacuum advance hooked to full manifold vacuum.

The ONLY difference is how much advance you have at idle.

You don't get more vacuum out of the ported connection than you do from the manifold vacuum. "Ported" or "timed" vacuum is simply isolated from the manifold when the throttle blades are closed at idle.

Further case in point: With the CC carb system, you set the base timing with the ECM wire disconnected (no conditions-based timing adjustments by the computer) to 0 to 6 degrees, depending upon which engine you have. With the wire hooked back up, at idle you will have around 20-25 degrees of advance at idle.

Hmmm, sounds a lot like using full manifold vacuum, doesn't it?

skinny z

That's my point exactly.
From plus 20 degrees with full manifold vacuum advance at idle, to around 10 when you blip the throttle.
Don't you think that the sudden drop in vacuum advance will affect throttle response and engine performance?
I can't see how it wouldn't.

Orr89RocZ

iTrader: (20)

Also would depend on pump shot when you blip the throttle because once you blip the throttle and hold, the vacuum will go back up and may even exceed idle's vacuum. Thus your timing will increase again. If the fueling is right, the timing change at the slight blip may not be noticeable.

On EFI cars, i've seen timing ramp up fairly quickly from idle to midrange with no change in response really since fuel delivery was smooth.

five7kid

iTrader: (14)

If it has 10" Hg manifold vacuum with the throttle open, it can't have any more than 10" Hg ported vacuum, right?

Why would a "drop" from 20" Hg to 10" Hg manifold vacuum affect throttle response if a "rise" from zero to 10" Hg using ported vacuum wouldn't? During the time it takes for either change to happen, you are going to have more total advance with manifold vacuum than you will with ported vacuum, and in general, more advance will produce better throttle response (the "right" timing advance always produces the best throttle response, of course).

skinny z

Question 1: Right

Question2: Ported vacuum will not be zero to 10" but will stay at zero. Zero vacuum is zero vacuum regardless if it's ported or otherwise once you've uncovered the ported vacuum source (from an idle position).

Getting closer.
My point is that when you open the throttle abruptly, manifold vacuum drops to nearly ZERO. At least that's what my gauges tell me.
That's where the timing goes. Ported vacuum will be (at or near) zero as well. (from zero to zero so no net effect)
I see what you're saying in that if hooked to a ported vacuum source, then there's a potential rise in vacuum from a closed throttle position to slightly open. That is true when only considering the relative position of the throttle blades and it's effect on idle speed (higher engine speed equals higher vacuum).
However my facts hinge on the vacuum dropping to near zero values and it's the ACTION of the changing throttle position that causes this. Once you've advanced to the new throttle postion (and not returned to idle) vacuum will stabilize. If the engine in question doesn't do that, then there are other issues at hand and the method of providing some spark advance is a band-aid for an otherwise ailing engine.

JaBoT

Don't trust a wideband at idle on a bigger camed engine. The overlap causes fresh air to pass strait through to the exhaust at low rpm making the wideband read lean when it's really not.
Just use the old max vac way for setting up idle.

onebad82z

iTrader: (7)

Well said 57. If I had read that years ago I would have understood it much, much better.

Damon

Oh, hey, guys! <Damon walks in late to the conversation, as usual, puffing a smoke>

Dead-lean at idle and high idle speed even with the throttle plates completely closed? C'mon guys, I'm diappointed in you! This has "vacuum leak" tattooed on it.

Big cams typically require MORE throttle opening to hold a given idle speed becuase of the low manifold vacuum at idle. If you got 'em closed down and it's still idling high and dead-lean there is air getting in somewhere other than past the throttle plates, regardless of how much timing you've got at idle. i.e. vacuum leak.

Carry on, gentlemen.

five7kid

iTrader: (14)

Oh, were we still talking about that???

CamarosRUS

My understanding was the excess liquid fuel that passes straight from intake to exhaust completely bypassing the combustion process causes the lean reading.





You're correct but there are more possibilities. When I close those secondaries the car won't idle. Honestly seems like the car's secondaries affect the idle more so than the primaries.

Along with high(er) timing than normal and a possible vacuum leak under the manifold its hard to get it to settle down.


My #1 plug is black, the rest are okay so I either have a vacuum leak under the manifold at #1, wasted valve guides, washed walls or a combo of all of them.

My idle varies +/- 300RPM at any given time (1500RPM to about 1200RPM) and the vacuum reading flutters +/- 2 or 3 Hg as well...

JaBoT

An o2 sensor reads oxygen levels not fuel.

CamarosRUS

Correct, O2 sensors read oxygen levels only and dumping all that fuel past the O2 sensor creates a lean reading. This happens because the O2 sensor is picking up all the oxygen in the raw fuel.

This is also why cams with a large duration show to be lean at idle; the wide overlap pulls unburnt fuel straight past the combustion process at lower speeds.

JaBoT

No dumping all that "fresh air" past the o2 is what causes the lean reading. An 02 sensor reads gas not liquid fuel.

CamarosRUS

"reads gas not liquid fuel"...that's the same thing there. Or are you referring to the gaseous state of gasoline?

Anyone else care to chime in? O2 sensors read OXYGEN only, no matter the physical state.

ZimGTA

Who's on first?