Spark plug color

[rusted nuts]

With the vacuum pump test I did this morning, it is airtight and holds steady.

Good job.

 

[ChuckN]

Vacuum advance has nothing to do with acceleration. That's why you block it off when setting the timing. A stumble is 99% of the time a lean condition. If it bogs and catches up, it's a rich condition. I played with the 1406 for years before ditching it all for EFI. You need to play with the jet size, rods and vacuum spring in the secondary's. Regarding the plugs, reading the color don't mean much anymore with todays gasoline. My EFI is tuned at a perfect 14.7 AFR and they run white as a ghost. Tp properly read plugs, you normally run wide open throttle, shut the engine off and coast to a stop before the idle AFR contaminates them. Not very practical in a non-race car.

Cool- as a last ditch effort, I’ll try the strongest spring and see if that helps. If not, I’ll go up the next jet per the algorithm. I already have the accelerator pump on the most aggressive setting.

 

[Matt69olds]

Give the engine what it wants, not what you think it needs!!

To set cruise AFR, pick a long stretch of smooth road, ideally on a typical weather day. Drive the truck, pay attention to how it drives. Then go one step leaner on the jet/metering rod or (if you’re smart and have a Q-Jet!!!) turn in the APT screw. Then drive the truck again on your test road. Keep leaning it out until you get a lean miss, that’s best described as a “fish bite” sensation. Once you have found the lean limit, go a couple steps richer.

As for your part throttle stumble, you probably need more accelerator pump shot. Temporarily cap off the vacuum advance, just to eliminate that from the test. If no vacuum advance solves the issue, you need a canister that takes more vaccum before it start to advance.


Read this:

Timing & Vacuum Advance 101 -good article by GM engineer

www.460ford.com

 

[Rusty Nail]

Looks like a hint of ash deposits to me.
Preignition?
What kind of fuel does it burn-87 gas or ethanol?
No tellin since you're jacking with the timing..does that "burn off" or will it need cleaned? How did it get there initially IF it will burn off? Hmmmm.
So you'll need to clean em then? With a wire toothbrush or somethin.
What effect do those deposits have on the flame? Certainly some effect?

no?


Ah! Aftermarket fuel additives maybe.

 

[rusted nuts]

Vacuum advance has nothing to do with acceleration. That's why you block it off when setting the timing. A stumble is 99% of the time a lean condition. If it bogs and catches up, it's a rich condition. I played with the 1406 for years before ditching it all for EFI. You need to play with the jet size, rods and vacuum spring in the secondary's. Regarding the plugs, reading the color don't mean much anymore with todays gasoline. My EFI is tuned at a perfect 14.7 AFR and they run white as a ghost. Tp properly read plugs, you normally run wide open throttle, shut the engine off and coast to a stop before the idle AFR contaminates them. Not very practical in a non-race car.

Never had a bad vacuum advance have You.

 

[ChuckN]

Give the engine what it wants, not what you think it needs!!

To set cruise AFR, pick a long stretch of smooth road, ideally on a typical weather day. Drive the truck, pay attention to how it drives. Then go one step leaner on the jet/metering rod or (if you’re smart and have a Q-Jet!!!) turn in the APT screw. Then drive the truck again on your test road. Keep leaning it out until you get a lean miss, that’s best described as a “fish bite” sensation. Once you have found the lean limit, go a couple steps richer.

As for your part throttle stumble, you probably need more accelerator pump shot. Temporarily cap off the vacuum advance, just to eliminate that from the test. If no vacuum advance solves the issue, you need a canister that takes more vaccum before it start to advance.


Read this:

Timing & Vacuum Advance 101 -good article by GM engineer

www.460ford.com

Excellent read. Still digesting and going to reread again. Have some homework to do tomorrow.

 

[ChuckN]

Looks like a hint of ash deposits to me.
Preignition?
What kind of fuel does it burn-87 gas or ethanol?
No tellin since you're jacking with the timing..does that "burn off" or will it need cleaned? How did it get there initially IF it will burn off? Hmmmm.
So you'll need to clean em then? With a wire toothbrush or somethin.
What effect do those deposits have on the flame? Certainly some effect?

no?


Ah! Aftermarket fuel additives maybe.

Oh, and before I forget, 90 grade ethanol free, all fresh, with royal purple octane boost.

 

[SquareRoot]

Never had a bad vacuum advance have You.

Yup. Didn't cause the issue you're describing.

 

[ChuckN]

@Matt69olds that is very informative. I had no idea that the timed/ ported vacuum port was a emissions era idea.

I could have misread it- but out here in the garage…if a stock vacuum advance needs that much vacuum to activate, how would that ever get that much vacuum above the throttle butterflies to begin with?

And if there is little vacuum above the butterflies (ported or timed advance) at idle to a “braap” moment, when the vacuum drops all around, what difference would that make to disconnect the vacuum advance to test for a difference? Not going against what you’re saying, just trying to understand.

This has been a good lesson for me. My dad taught me everything about cars, or at least a lot- but either he didn’t understand the rationale either or he never brought it up.

 

[ChuckN]

Ok, so I was wrong- apparently there is a very good amount of vacuum that is introduced above the throttle plates when they are opened.

So, I have to test the vacuum a little later after the wife is done with her work at home (too noisy). I know the 355 has a lumpy cam but have no idea what vacuum it’s putting out- it may be a cam that just sounds aggressive, or it may truly be a bigger cam.

Not throwing out the carb adjustment though- I still think that needs to be addressed.

 

[ChuckN]

Has about 13-14 of vacuum at idle. Disconnected vacuum advance, no improvement. Onto the carb next!

 

[Dutch Rutter]

Something to keep in mind, Edelbrocks pretty much require a fuel pressure regulator to be installed between the carb and the fuel pump. If you don't have one get a good adjustable one with a pressure gauge, 4-5psi is all they want.

If this was your problem you'd be having a rich condition and other problems tho.

 

[Matt69olds]

Has about 13-14 of vacuum at idle. Disconnected vacuum advance, no improvement. Onto the carb next!

I suggested disconnecting the advance for 2 reasons. Number one: to make sure a broken wire on the pickup coil wasn’t causing your stumble, and just to eliminate the advance calibration from the variables.

Let’s say you have an advance canister that added a lot of timing with little vacuum. You have the idle speed set with it connected. You give the engine a little throttle, now the advance pulls a bunch of timing.

Since the problem is there either way, both my theories are useless.

I’m not familiar with Edelbrock carbs, is there an adjustment for pump shot? You should get accelerator pump shot as soon as the throttle moves, if not you will have a stumble.

 

[ChuckN]

I suggested disconnecting the advance for 2 reasons. Number one: to make sure a broken wire on the pickup coil wasn’t causing your stumble, and just to eliminate the advance calibration from the variables.

Let’s say you have an advance canister that added a lot of timing with little vacuum. You have the idle speed set with it connected. You give the engine a little throttle, now the advance pulls a bunch of timing.

Since the problem is there either way, both my theories are useless.

I’m not familiar with Edelbrock carbs, is there an adjustment for pump shot? You should get accelerator pump shot as soon as the throttle moves, if not you will have a stumble.

Hey, thanks for the reply. As it is, one of the first things I did was set the accelerator pump on the most aggressive (biggest shot). And the carb is freshly rebuilt, all new gaskets and seals.

Since you’re not familiar with Edelbrock carbs, the jets are in the base of the carb, and the metering rods (like needles at the end) go through the jets and limit the fuel upon full vacuum. They are oriented vertically and have little trap door that they are accessible with the air cleaner removed, one on each side, for the primary throttle. The vacuum pulls the rods down and limits the fuel, “needle through the hole” so to speak. The full vacuum fights against the “step up springs “ that cause the metering rods to lift the rods out of the jets when the throttle plates open and vacuum drops, in turn releasing fuel. What the varying strength of the springs do is regulate how fast the metering rods open the fuel flow when vacuum drops.

I appreciate your help. And I might not be done with the accelerator pump- apparently there’s a stiffer spring for that for the plunger that creates even a stronger shot for the bigger and more aggressive carbs- but I think (since I have a needle and jet tuning kit) I’ll start with that first.

 

[ChuckN]

Something to keep in mind, Edelbrocks pretty much require a fuel pressure regulator to be installed between the carb and the fuel pump. If you don't have one get a good adjustable one with a pressure gauge, 4-5psi is all they want.

If this was your problem you'd be having a rich condition and other problems tho.

I have the Edelbrock fuel pump and it is in spec per my measurements. Thanks for the help tho! I welcome all responses.