'78 Bus Idle Rises When Brakes Pumped

[dtrumbo]

Hi all,

I know I've read about this phenomenon in other threads but because the search function is... well... anyway. When the bus is nice and warmed up and idling and I pump the brakes rapidly the idle speed will rise maybe a couple hundred RPM and then settle back to where it was. Is this normal? Does this indicate a rich mixture condition (I'm hoping since my mileage sucks)? I've tried to wrap my pea-brain around how the vacuum (absence of air) going to the brake booster alters the mixture and it's just not clicking. Can anyone help remove some of the vacuum from my cranium? Thanks.

[Westy78]

Yep that's a sign of a rich mixture. At least at idle.

[dtrumbo]

Thank you sir, I'd rather be rich than lean... although at $4.20/gallon I should probably go through Colin's AFM procedure now that I have a new motor with a lot fewer vacuum leaks.

[vdubyah73]

somewhat normal, maybe not a few hundred rpm though.
power brakes work by having a vacuum supply on one side of a diaphragm and atmospheric pressure on the other. somehow, i'm fuzzy here, when you step on the pedal the vacuum is allowed into one side, while the atmosphere is still on the other. the atmosphere pushes against the diaphragm trying to fill the void of vacuum introduced to the other side. engine vacuum has to replenish the supply every time you release the brake pedal, you could call it a regulated vacuum leak. regulated by the brake pedal.

[Amskeptic]

somewhat normal, maybe not a few hundred rpm though.
power brakes work by having a vacuum supply on one side of a diaphragm and atmospheric pressure on the other. somehow, i'm fuzzy here, when you step on the pedal the vacuum is allowed into one side, while the atmosphere is still on the other. the atmosphere pushes against the diaphragm trying to fill the void of vacuum introduced to the other side. engine vacuum has to replenish the supply every time you release the brake pedal, you could call it a regulated vacuum leak. regulated by the brake pedal.

Dick, lean out the mixture until the brake pump test gives you a slight decrease in idle when you pump the brakes.

VWs do brake boosters right. They evacuate both sides of the booster diaphragm at rest so the whole thing is a vacuum chamber. Applying the brakes allows atmosphere on the *front* chamber (through the hose that comes from the left front wheel well) to help you push the brakes, while the rear chamber is still under engine vacuum. When you release the brakes, engine vacuum evacuates the front chamber. This is the air that we IACers use to test the fuel mixture, the only people on the planet to do so.
Colin

[dtrumbo]

So it's this un-metered atmosphere in the brake booster that the engine vacuum is evacuating into the intake plenum that is un-richening my mixture. NOW I get it! Thank you sir! I'm scared to monkey with my beautiful new engine just before the big trip to Maupin, but my wallet and my cylinder heads are saying please do. Too much $$$ lost and too much potential carbon build up in the balance.

[dtrumbo]

O.k. so yesterday I put my big boy pants on and did the AFM adjustment procedure that is a 'sticky' in the very forum. I "centered" my mixture screw at 4 1/2 turns which ended up being about 1/2 turn CCW (leaner) from where it had been. I then ended up moving the static adjustment 1 tooth CW and the dynamic adjustment 3 teeth CCW. If I read the procedure right, this leaned out the idle and richened (or kept the same) the high speed setting. Now at idle a touch of the wiper CW reduces RPM and a touch of the wiper CCW does nothing. Before performing this procedure it was the other way around. At 3000 RPM a touch of the wiper CW reduces RPM but a touch of the wiper CCW raises RPM. This is how it was before so hopefully it's "a little lean" like we want it according to Colin's instructions. To confirm, the brake pedal pump does cause the idle RPM to drop a tiny bit whereas before it caused the idle RPM to rise noticeably.

On the way to work this morning the bus ran great. Noticeably more power going up hills on the way to the freeway and seemingly less pedal required to maintain 60 MPH once on the freeway. We'll see if the gas mileage improves and after it runs this way a bit I'll check the plugs for how they're feeling.

[Amskeptic]

O.k. so yesterday I put my big boy pants on and did the AFM adjustment procedure that is a 'sticky' in the very forum. I "centered" my mixture screw at 4 1/2 turns which ended up being about 1/2 turn CCW (leaner) from where it had been. I then ended up moving the static adjustment 1 tooth CW and the dynamic adjustment 3 teeth CCW. If I read the procedure right, this leaned out the idle and richened (or kept the same) the high speed setting. Now at idle a touch of the wiper CW reduces RPM and a touch of the wiper CCW does nothing. Before performing this procedure it was the other way around. At 3000 RPM a touch of the wiper CW reduces RPM but a touch of the wiper CCW raises RPM. This is how it was before so hopefully it's "a little lean" like we want it according to Colin's instructions. To confirm, the brake pedal pump does cause the idle RPM to drop a tiny bit whereas before it caused the idle RPM to rise noticeably.

On the way to work this morning the bus ran great. Noticeably more power going up hills on the way to the freeway and seemingly less pedal required to maintain 60 MPH once on the freeway. We'll see if the gas mileage improves and after it runs this way a bit I'll check the plugs for how they're feeling.

Let us know how it works out. And pull an easy plug, like #3 after a couple weeks to check for any unintended bleaching of the ground electrode

[dtrumbo]

Let us know how it works out. And pull an easy plug, like #3 after a couple weeks to check for any unintended bleaching of the ground electrode

Ouch!

So with that I moved the cog six more teeth CCW and turned to mixture screw out one turn to get the idle back to where I wanted it. On the bright side, my MPG went from 11'ish to 16.1 after a tank and half. I'll run it with this new, slightly richer setting to Maupin and back and see where I'm at after that.