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Sun Nov 01, 2009 4:59 pm

We recently installed a Innovate LC-1 with a dash readout, this in preperation for my MegaSquirt install.

Anyway, we we're just getting the the L-jet in tune first, but we noticed something funky. If we were cruising on the highway with an AFR of somewhere around 13 and you let the throttle close, the AFR would enrich, going to about 10:1. After slighly touching the throttle it jumps back to around 13.

This seemed a little weird, why would the mixture have to enrich when you close the throttle? I didn't understand why this happened in the first place. I was making some measurement for my TPS adapter (again for the MegaSquirt) on a throttle body. I noticed the vacuum connection is exactly in front of the throttle valve.

So this explains the closed throttle enrichment, since there is little vacuum upstream of the throttle valve. Which in turn means little vacuum at the pressure regulator and that means high fuel pressure and that means rich.
Now when the throttle valve opens ever so slightly it will look like in the image below. The vacuum connection is now downstream of the throttle valve, so in the higher vacuum region. High vacuum means low fuel pressure and that means lean(er)

(Pic with throttle valve slightly open, you can see the where the throttle valve is when it's closed. Pic taken with phone, sorry for blurriness)

So, that explains the behaviour of the mixture, but why is it designed like this? Or did I miss something here? Or does this have something to do with getting a correct idle mixture?

Why would you want to enrich a mixture at completely closed throttle?

I'm sure someone here can enlighten me 8)

Sun Nov 01, 2009 5:18 pm

Do you still have your decel valve hooked up?

I think you're barking up the wrong tree. The vacuum connection at the throttle is for the distributor. The pressure regulator gets the vacuum from the manifold directly.

Sun Nov 01, 2009 5:37 pm

:oops: :oops: :oops: :oops: :oops:

Guess that means I've got this messed up

Hmm... I'll take a look in my engine bay this weekend. (I don't live where my bus lives)

Sun Nov 01, 2009 8:29 pm

our bus goes to about 15:1 or slightly more when I lift off. That is when the decel valve opens - also the throttle body closes which allows the AFM vane to close so you have essentially idle mixture14.05:1 plus decel valve air.

We spent the day mapping a like new AFM board that FIC provided me to replace a board that was worn more than I was comfortable with.

Here is a trace of what our Bosch AFM trace looks like now that it is adjusted with the new board- freeway speed taken today. I deliberately set it to run between 14.0:1 and 14.3:1. It is so steady it looks like it is running an O2 sensor except that if it was it would be at 14.7:1 which is too lean for me.

Next is the spreadsheet showing the progression of trial and error of adjusting the AFM. I finally got the hang of it. You have to map it in 250 RPM increments or smaller on a spread sheet. The red line is the final adjustment. Notice how much flatter the curve is compared to when I started. I am not sure how much better a non-O2 FI can get on a street bus. The mixture starts at idle CO then drops as the RPMs come up - perfect because one is coming off the throttle then holds steady all the way to high RPM's where it goes rich again to cool and protect the engine.

Sun Nov 01, 2009 8:40 pm

that port goes to the distributor and carbon canister valve on the air cleaner. That way the distributor only gets vacuum when the throttle is open and the carbon canister only purges when the throttle is open.