Harney's diatribe - rocker geometry to install & beyond

mharney · Post by **mharney** » Sun Dec 19, 2004 3:08 pm

So...

anyone know how many points a single jet size (Idle, IDF) can affect the Air/Fuel Ratio?

I just got my WBO2 working properly, and it looks like i was running about 17.5 to 18 through the whole breakin. Sure didn't sound like it except for the popping. Just didn't think it ran that lean. I guess the fact that I cannot conjure so much as a speck out of the tailpipe with a rag is a good clue.

Is going from 55 to 60 gonna be enough?

rcb78 · Post by **rcb78** » Sun Dec 19, 2004 9:16 pm

I would imagine that under a no load condition, the AF will not read accurately. Under load, the throttle plates will be in different position for a given vacuum signal. You should have more progression ports exposed beneathe the throttle plates when under a load, meaning more fuel under seemingly identical conditions. Maybe just conjecture, but it makes sense to me. --Ryan

mharney · Post by **mharney** » Sun Dec 19, 2004 9:40 pm

Yeah, but 4 points? Idle should be close to stoichiometric, and these WBO2 sensors are supposed to be pretty accurate.

rcb78 · Post by **rcb78** » Sun Dec 19, 2004 10:04 pm

I'm sure the sensor is reading accurately, but I think the difference between a loaded condition and a free rev condition would make quite a difference. Under no load, you can get away with alot more 'iffy' tuning. Kinda like how you can use extra advance under light load condistion (i.e. vacuum advance). --Ryan

mschilling · Post by **mschilling** » Mon Dec 20, 2004 6:47 am

According to what I've been told, the idle mixture is controlled by the idle screw and not the idle jet. The holes in the side of the venturi control the transition to the idle jet circuit, and then the idle jet comes into play. That could explain your readings at idle.

mharney · Post by **mharney** » Mon Dec 20, 2004 6:54 am

I'm operating the carbs out of the idle position, so the mixture screws are not affecting me right now, so much. I've got the plates cracked open enough for about 2000 rpm. When they ARE at idle, I can open up the mixture screws about 1 and 1/2 turns before the engine starts to pick up and then I can open them all the way without dropoff.

17 to 18 on the afm is pretty lean.

rcb78 · Post by **rcb78** » Mon Dec 20, 2004 7:24 am

Looks, like you still have two slip joints on the 2 & 4 cylinders,,, possible leaks? --Ryan

mharney · Post by **mharney** » Mon Dec 20, 2004 7:32 am

Oh yeah, they leak a little. I'm sure that could be a contributor, but I've got a lot of exhaust pressure.. Hard for me to believe that anything is getting IN at idle or up a little.

These clamps suck mightily. I will probably have to just grind them off and put real exhaust clamps on.

VW Grommet · Post by **VW Grommet** » Mon Dec 20, 2004 7:43 am

hiya mharney, regarding the slip joint exhaust leaks..I had a pesky one that I even tried slightly flaring one end to fit tighter into the joint (which did not work BTW). I'll tell you what "seemed" to work was some curil-T and slip the two together. It's still holding and didn't burn off..the "T" is for hi temp use. We will see how long it lasts...

rcb78 · Post by **rcb78** » Mon Dec 20, 2004 11:33 am

You may be mistaking pressure with flow. High flow usually means lower than atmospheric pressure, same way how a crackcase scavenging/evacuation port in the collector work. All I'm saying is that I've heard of other people having their AF readings off by a few points from something as simple as slipjoint leaks. So it's at least worth the effort to be sure. --Ryan

yeahmag · Post by **yeahmag** » Mon Dec 20, 2004 2:52 pm

mschilling wrote:According to what I've been told, the idle mixture is controlled by the idle screw and not the idle jet. The holes in the side of the venturi control the transition to the idle jet circuit, and then the idle jet comes into play. That could explain your readings at idle.

My understanding of the idle circuit is a little different. The idle jet controls the mixture (ratio of fuel to emulsified air) and the idle "mixture screw" controls the volume of mixture delivered. With a combination of the two is how you get it where you want it...

-Aaron

mschilling · Post by **mschilling** » Mon Dec 20, 2004 3:06 pm

I guess you mean the idle mixture when you said "get it where you want it"? That's what I had thought too. But it makes sense (what I wrote earlier). Try changing the idle jet. Does the idle change? For me the answer is not really.

rcb78 · Post by **rcb78** » Mon Dec 20, 2004 3:34 pm

The idle jet affects the mixture withing the idle circuitry, the mixture screw meters that mixture into the main airstream. There's two different AF mixtures at work when you think of it like that. --Ryan

yeahmag · Post by **yeahmag** » Mon Dec 20, 2004 3:53 pm

Changing the idle jet alone should not signifigantly change the idle speed if you are anywhere near the correct mixture. Changing the mixture screw should also not radically change the idle speed until you "put the fire out" in one direction or another. Maybe a few hundred RPM at the most, but not anything like you working idle speed/throttle stop adjustment screw - minor variences in RPM only.

The idle jet will change your A/F Ratio both at idle, transition, and low speed cruising. This isn't to say that changing the mixture screw -won't- change the A/F ratio, but it's a different parameter to fiddle with...

*Mixture Screw = Volume of mixed air and fuel. You are changing the volume of fuel being delivered to the cylinder from the idle jet.

*Idle jet = Control of the pre mixed (emulsified) air and fuel delivered to the mixture screw. Controls A/F Ratio's in transition and light/low cruising speed/RPM.

It's a balance.

-Aaron

mschilling · Post by **mschilling** » Tue Dec 21, 2004 4:36 am

Hmmm, seems to me this is a popular misconception.