[OrigOwner77]

Hi all, need a little help figuring this one out.

I installed a Davis Unified distributor in the '68 400 that's powering my '80 T/A.

With the vacuum advance can connected to the ported port on my Holley 3310, if I lightly "bump" or "nudge" the throttle at idle in Park, the idle speed stays a hundred or so RPMs higher. Then if I rev the motor, it drops right back to the normal idle speed. Basically I can place light pressure on the throttle and get the RPMs to stay 100ish RPMs increased. Throttle springs are tight, throttle cable isn't binding, etc. Ruled all that out.

Distributor was shipped with DUI's factory "generic" Pontiac config (ordered via Summit). Per DUI, it has 24 degrees mechanical advance by 3000 RPMs, and a vacuum can providing 14-15 degrees. The following behavior was present out of the box, and I did try swapping the weights/springs from my previous combo which did not make a difference (was thinking the springs were letting the initial bounce around a bit). I don't recall offhand what's in the distributor now, but the mechanical curve is where I want it (I think I either used DUI springs and original GM weights/center plate, or just tweaked the springs and kept the DUI weights/plate).

Confirmed the vacuum advance is what is in play here, as disconnecting the vacuum line (plugging carb) and repeating this method will NOT cause the behavior to reoccur.

I didn't have this issue with my original distributor (which had an aftermarket adjustable vacuum can).

While the issue doesn't affect driveability at all and is merely something I have to actually try to cause, I'm a bit confused (and intrigued) as to what would cause this. It's more of an annoying quirk than anything.

The last time the carb was off the car, the idle transfer slots were verified square (e.g. throttle blades not cracked too far). Haven't really played with the idle adjustment since so I don't see the idle transfer circuit causing this, but I defer to the carb/distributor/tuning gurus here for their insight.

Could it be the vacuum can reacting to the ported signal from my carb differently from the prior vacuum can / distributor I had?

Thanks!

[Tom Vaught]

Ok, so the carb is the same, the ported vacuum signal is the same. The difference is the two vacuum cans calibration. Assuming that both distributors are HEI units, why not just swap vacuum cans?

Tom V.

[OrigOwner77]

Originally Posted by Tom Vaught

Ok, so the carb is the same, the ported vacuum signal is the same. The difference is the two vacuum cans calibration. Assuming that both distributors are HEI units, why not just swap vacuum cans?

Tom V.

Thanks Tom. I figured it was the vacuum can by process of elimination. I'm actually planning to swap an adjustable can back on the car not only to eliminate this behavior but also because I'm becoming obsessed with tinkering / tuning.

I think I'm moreso interested in the "why" / "what's happening behind the scenes" rather than preventing / fixing the issue (since I've planned to ditch this can in favor of an adjustable one anyways).

At/near idle, in park with no load, I didn't expect vacuum advance to come into play since the ported port should have minimal/no vacuum. Guessing it's just a matter of a tiny bit of vacuum causing the advance unit to advance just enough that idle speed creeps up, and hangs there until there's a bit more sharp spike to return it to baseline?

[JLMounce]

I'm far from an expert here, but ported vacuum works based off of throttle position on the carb. it is possible to have the butterflies open enough at idle that you're no longer on the idle circuit and thus the ported vacuum can come into play there.

The one can you have is likely more susceptible to small changes in vacuum than your owner can.

Or I could be completely off base and the experts will let you know.

["QUICK-SILVER"]

First thing I would do is try closing the throttle lever by hand. May need to change the angle or direction of your return springs. Throttle shaft may be getting a little lose in the base plate causing throttle blades to bind. A quick blib of the throttle can make them slam shut. Where light throttle pressure lets um hang up a bit.

[Tom Vaught]

He had no issues with the other distributor and no changes to the carb Throttle Blade Position, Return Springs, No Binding shafts, etc, so explain why you think that changing the vacuum can on the distributor caused those assumed carb hardware changes?

Tom V.

[flat-bill]

What is your idle rpm? I your centrifugal advance is a hair off starting at your idle rpm you may bring in a couple degrees of advancce just off idle. Billk

[Chris65LeMans]

For those considering DUI distributors - if you order directly from them in Memphis, they will custom curve it to your car's specs.

["QUICK-SILVER"]

Originally Posted by OrigOwner77

.

With the vacuum advance can connected to the ported port on my Holley 3310, if I lightly "bump" or "nudge" the throttle at idle in Park, the idle speed stays a hundred or so RPMs higher. Then if I rev the motor, it drops right back to the normal idle speed. Basically I can place light pressure on the throttle and get the RPMs to stay 100ish RPMs increased.!

Originally Posted by Tom Vaught

He had no issues with the other distributor and no changes to the carb Throttle Blade Position, Return Springs, No Binding shafts, etc, so explain why you think that changing the vacuum can on the distributor caused those assumed carb hardware changes?

Tom V.

Because what he said was pretty much a text book example of throttle blades not shutting all the way.
And I did say it was the first thing I would try. And how hard would that be just to rule out a possible problem.

If it doesn't work, swap out the can. No one said not to.
Before swapping, I would at least put a timming light on it and see what was happening.

100 rpm's makes me think sticky throttle more than sticky vacuum can.

Just a thought trying to help the OP.
Not even trying to start a debate and in no way disagreeing with your post.

Everything good
Clay

[Tom Vaught]

Originally Posted by "QUICK-SILVER"

Because what he said was pretty much a text book example of throttle blades not shutting all the way.
And I did say it was the first thing I would try. And how hard would that be just to rule out a possible problem.

If it doesn't work, swap out the can. No one said not to.
Before swapping, I would at least put a timming light on it and see what was happening.

100 rpm's makes me think sticky throttle more than sticky vacuum can.

Just a thought trying to help the OP.
Not even trying to start a debate and in no way disagreeing with your post.

Everything good
Clay

I was going by this statement:

"The last time the carb was off the car, the idle transfer slots were verified square (e.g. throttle blades not cracked too far). Haven't really played with the idle adjustment since so I don't see the idle transfer circuit causing this, but I defer to the carb/distributor/tuning gurus here for their insight.

If the throttle blades were in the wrong position, I agree with you post.
The OP said (above) that he has not screwed with the carb. And the carb was idling at the correct speed before and the blades were set in the proper position.

I have seen more "carb problems" that turned out to be something else (like the calibration on the vacuum can being different) than you can imaging in 50 years of playing with carbs as a hobby and later as a Holley employee. I had a very similar issue with a 66 W-30 Old 442 TRI_POWER guys car in Florida, even the slightest vacuum change caused the timing to increase due to a soft initial spring choice.

But thanks for the post because everyone wants to help at the end of the day.

Tom V.

[Matt Meaney]

I think I would put a vacuum gauge on the carb port(s) and track what it reads at 100 rpm intervals. then put a vacuum pump (with gauge) on the canister and track what amount of vacuum does what for timing change. compare the two.

maybe the vacuum canister or what it's attached to is "sticky"

[Cliff R]

Take a vacuum reading at the ported vacuum source on the carb at idle speed. If the carb tune is correct then it should read zero or very close to it. Certainly no where near enough vacuum to even think about applying any timing from any advance unit adjustable or not.

If you see vacuum at the ported source at idle speed, then you need either more initial timing or more idle bypass air, or a little of both. This assumes the carb has plenty of idle fuel for the application and that the source location for the ported vacuum supply is correctly located in the baseplate.


Problems like you are seeing are in most cases a little timing being added by the mechanical advance, either from springs just a tad too light, or miss-matched center cam/weights, or even a little "slop" in the mechanism someplace such as loose weights on the pins, etc......Cliff

[OrigOwner77]

Wow thanks, all, for the responses. Some great info & knowledge here!

Re: the throttle return spring, I did confirm that it wasn't the issue. Probably should have mentioned it, as it was one of the first things that came to mind for me also.

Cliff - I'll definitely take a vacuum reading at the ported port. I didn't think the ported port, even if the throttle blades were cracked further than they should be, would have enough vacuum to trigger the vacuum advance can. However, since the issue will not reproduce if I disconnect vacuum advance, that's why I pointed my finger at the vacuum can.

I'll also double-check what springs are in the HEI when I look at swapping back to the adjustable can. Maybe they indeed are borderline triggering mechanical advance by being a hair too light. I don't feel this could be though, since again the problem won't occur with vacuum advance disconnected. Also don't anticipate wear or slop in the mechanism / incorrect cam/weights, since the distributor is brand new. Will confirm though to be on the safe side.

Idle RPM in Park is usually 850ish (depends on weather, cooler months I keep it just shy of 1000, warmer months 850). Maybe 750 or so in gear, give or take.

I do know the car would probably like a little more initial. Problem is with the motor mount adapters the prior owner used to install the '68 400, the motor sits offset and to the rear of the engine bay more than stock. As such, the vacuum advance can is pointing to (and hitting) the pass. side head heater hose since the vacuum can won't fit pointing at the driver's side (e.g. stock orientation). Right now I can squeeze about 14-16 degrees initial out of it, with the body of the advance can firmly against the heater hose. That's about as high as I want to run the car, given A) the #62 heads / potential compression ratio, and B) the car at hot start will kickback against my Summit gear reduction starter with 16+ initial.

I might try a trick I read here of moving all the wires over 1 post, and then rotating the distributor body accordingly. Thinking is that I can get a little more "wiggle room" for playing with timing, even if it's just to test within a few more degrees one way or the other.

[SA74SD]

Is your timing more advanced when the idle is higher? Does it drop back to "normal" when the idle speed is lower? Is so, check those springs/weights as Cliff suggested. All this assumes you have zero ported vacuum.

[ta man]

Originally Posted by SA74SD

Is your timing more advanced when the idle is higher? Does it drop back to "normal" when the idle speed is lower? Is so, check those springs/weights as Cliff suggested. All this assumes you have zero ported vacuum.

X2 stick a timing gun on it to make sure it has steady initial timing

[Matt Meaney]

just because it's new, doesn't mean it can't have issues. it's a shame how quality isn't what it used to be. seems nowadays, it's necessary to go over new parts with a fine tooth comb, to insure it will work properly before you install it.

curious as to what you find out to be the issue.

[70pontiaction]

Maybe I missed someone else suggesting it, but the first thing I'd try:

Cap the vacuum port on the carb and then drive the car WITHOUT any vacuum advance hooked up.

If the idle does NOT hang up or increase, it IS the vacuum advance causing the problem.

If the idle still hangs up or increases, it IS NOT the vacuum advance. Look somewhere else.

Unless I'm seeing it wrong, this would take some guesswork out of the equation.

Good luck!

[TedRamAirII]

When it idles too fast, pull off the Vacuum Advance hose, if it slows down, Theres Your Sign. ; )

[OrigOwner77]

70pontiaction and TedRaimAirII, that's why all along I've pointed the finger at the vacuum advance can. I can get the problem to occur 100% of the time with vacuum advance hooked up, and 0% of the time with it disconnected/capped.

The others do have a point though, at no / almost no throttle, the ported port shouldn't have vacuum so therefore the vacuum advance shouldn't even come into play. I think this is where I need to begin looking.

Will definitely throw the light on it hopefully this weekend...same with a vacuum gauge.

As soon as I get a chance to dive into this, I'll post updates accordingly.

[Tom Vaught]

I have agreed with you all along that when the issue is like a switch, vacuum line connected ISSUE, vacuum line disconnected, NO ISSUE, that the vacuum diaphragm calibration/internal spring etc was different vs your adjustable unit. Keep after it, you will get it.

Tom V.