A QUICK QUESTION ABOUT MANIFOLD VACUUM

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CFD244

CFD244

"I LOST MY ID IN A FLOOD"
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Hi Folks

The facts..........

Fresh 340 (with about 2hrs run stand time) with 9.3 CR, running a Comp XE262 cam. OEM intake with a 1972 TQ.
Base timing 20* btdc (MP gold shaft distributor.) Engine installed on a run stand, all warmed up..............

Smooth idle at 850 rpm pulling just under 15 inches on the vacuum gauge. Needle dead solid with 20* on the timing light. A/F 12.8

Run it up to 1700 rpm and I see an erratic vacuum needle in the 18-21 inch range. Timing light shows a solid 34* with the extra 14* coming from the ported vacuum signal.......Disconnect the can, back to 20*. A/F 14.2

So my question, does the erratic vacuum signal at 1700 rpm sound normal?

Thanks FABO

EDIT: Vacuum signal measured at the rear port on the carb. base (not the dashpot fitting)
 
Not really. With a some extra overlap its usually least stable at idle rpm. But 3" Hg not sure I'd worry about it unless something else is going on.

Now why the advance isn't coming in before 1700 -rpm that would have me asking questions.
 
Mattax said:
Not really. With a some extra overlap its usually least stable at idle rpm. But 3" Hg not sure I'd worry about it unless something else is going on.

Now why the advance isn't coming in before 1700 -rpm that would have me asking questions.
Click to expand...
I double checked my numbers........That distributor actually comes in at 1500 RPM and puts 14* in by 2000 RPM. There must be a slight variation in tachometers. At any rate, the erratic needle occurs in the fast idle zone (with choke open obviously).
 
My guess is that the VA unit is not deploying fully [ ie. the plunger is not pulling all the way in { see-sawing } ] such that the amount of vacuum is varying slightly. If the VA unit is adjustable, turn the screw fully CW.
 
Depends on the quality of the vacuum gauge. Cheap parts store vacuum gauges made in the last 15 years? Yeah, it's pretty normal. High end diagnostic and high end liquid filled vacuum gauges? Yeah, if those are erratic, you may have an issue. Otherwise, I'd say it's normal.
 
Those numbers on the stand? I'm wondering if the idle will be a little different with the weight of a converter or a flywheel?
 
Demonic said:
Those numbers on the stand? I'm wondering if the idle will be a little different with the weight of a converter or a flywheel?
Click to expand...
Darn right they will be and that's a good point.
 
While the rpm is up , and the vacuum is dancing, watch the timing marks. If they're dancing to the same drummer, put an independent vacuum source on the VA and repeat. If the dancing continues, check for timing chain stretch.
But if the dancing stops, blame it on the VA. To prove it, disconnect the VA, then run it back up to 1700 and simultaneously advance the timing to 34*. If the dancing is back, it wasn't the VA.
So then, Ima thinking your combo may not like 34* @1700 rpm.
So then, still at 1700rpm, and running 34* mechanical timing; just reach in there and retard the timing until it stops dancing, keeping the rpm at 1700. Then read the timing at 1700.
But if nothing seems to work, check your rotor phasing and plug gaps.
I have seen the VA pull the plate over, and the mechanical advance align in weird ways that causes the rotor to be marginally positioned and the sparks go to the wrong tower. And I have seen coils misfire, in weird ways when the plug gaps combine with lean AFRs , and the coil just says, OOps no can do.
And a common mistake is to run the Numbers 5 and 7 wires too close to eachother, causing induction crossfires, and then the engine gets two fires on those two plugs but 90 degrees apart. Not good if one of them is on the intake stroke.
Good luck
 
AJ/FormS said:
While the rpm is up , and the vacuum is dancing, watch the timing marks. If they're dancing to the same drummer, put an independent vacuum source on the VA and repeat. If the dancing continues, check for timing chain stretch.
But if the dancing stops, blame it on the VA. To prove it, disconnect the VA, then run it back up to 1700 and simultaneously advance the timing to 34*. If the dancing is back, it wasn't the VA.
So then, Ima thinking your combo may not like 34* @1700 rpm.
So then, still at 1700rpm, and running 34* mechanical timing; just reach in there and retard the timing until it stops dancing, keeping the rpm at 1700. Then read the timing at 1700.
But if nothing seems to work, check your rotor phasing and plug gaps.
I have seen the VA pull the plate over, and the mechanical advance align in weird ways that causes the rotor to be marginally positioned and the sparks go to the wrong tower. And I have seen coils misfire, in weird ways when the plug gaps combine with lean AFRs , and the coil just says, OOps no can do.
And a common mistake is to run the Numbers 5 and 7 wires too close to eachother, causing induction crossfires, and then the engine gets two fires on those two plugs but 90 degrees apart. Not good if one of them is on the intake stroke.
Good luck
Click to expand...
Timing mark is rock solid no mater where it is, or what I do (in it's respective location relative to the RPM and/or vacuum)
 
Well that rules out chain stretch, flyweight/spring chatter, and VA flutter.

With that 262 cam, the intake vacuum should be rock-solid by 1800>2000rpm. The cam has settled down, and all the intake air should be going in the same direction. The manifold vacuum, should have increased steadily from idle, and peaked in that pocket, and it should NOT be fluttering around 1700.
If the timing is solid, then it has to be; a bad plug, rotor phasing, induction X-fire, the feint beginning of pre-ignition; or maybe, AFR but that, I doubt.
To help in diagnosis, I would check the spark-strobe of every wire, beginning with numbers 5 and 7. You'll be looking for missed strobes, multiple strobes, or out-of-time strobes, at 1700 of course..
 
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CFD244 said:
Hi Folks

The facts..........

Fresh 340 (with about 2hrs run stand time) with 9.3 CR, running a Comp XE262 cam. OEM intake with a 1972 TQ.
Base timing 20* btdc (MP gold shaft distributor.) Engine installed on a run stand, all warmed up..............

Smooth idle at 850 rpm pulling just under 15 inches on the vacuum gauge. Needle dead solid with 20* on the timing light. A/F 12.8

Run it up to 1700 rpm and I see an erratic vacuum needle in the 18-21 inch range. Timing light shows a solid 34* with the extra 14* coming from the ported vacuum signal.......Disconnect the can, back to 20*. A/F 14.2

So my question, does the erratic vacuum signal at 1700 rpm sound normal?

Thanks FABO

EDIT: Vacuum signal measured at the rear port on the carb. base (not the dashpot fitting)
Click to expand...
Upon returning to idle from a rev up in full advance it's lean. Or a miss.
It's 12.8 at idle, then it's 14.2.. right?
Should be 13 something or so at idle and richer at throttle. Unless I'm reading this post wrong.
Is this a dial back timing light?
 
MOPAROFFICIAL said:
Upon returning to idle from a rev up in full advance it's lean. Or a miss.
It's 12.8 at idle, then it's 14.2.. right?
Should be 13 something or so at idle and richer at throttle. Unless I'm reading this post wrong.
Is this a dial back timing light?
Click to expand...
No dial back timing light. I can lean out the mixture with the a/f screws to get into the 13's.

I just found it strange that the mv is perfectly stable at idle, and gets a little erratic in the fast idle zone. I might try the vac gauge on the manifold fitting instead of the car base and see if that makes a difference.

I was just concerned that it may be a vacuum leak........I tried a propane torch around the carb base and intake with no change in rpm.
 
CFD244 said:
No dial back timing light. I can lean out the mixture with the a/f screws to get into the 13's.

I just found it strange that the mv is perfectly stable at idle, and gets a little erratic in the fast idle zone. I might try the vac gauge on the manifold fitting instead of the car base and see if that makes a difference.

I was just concerned that it may be a vacuum leak........I tried a propane torch around the carb base and intake with no change in rpm.
Click to expand...
I'm with the others ... in that it's on the edge of mech advancement during the vac instability
 
CFD244 said:
No dial back timing light.
Click to expand...
Timing tape?

CFD244 said:
I just found it strange that the mv is perfectly stable at idle, and gets a little erratic in the fast idle zone.
Click to expand...
It's spinning at 1700 with no load, a lot of timing and somewhat lean mix. Maybe if you had a load on it it would be slightly trailer hitching or even reving and dying. Run an engine too lean at light to moderate load and you'll know what I'm trying to describe.
MOPAROFFICIAL said:
Should be 13 something or so at idle and richer at throttle.
Click to expand...
I agree. Idealy. But a little richer will seem smoother as long as the plugs aren't fouling.
It seems like more timing than needed for the AFR and rpm given the cam and the vacuum reading.

Here's two data logs off my 340 showing rpm vs. AFR when opening the throttles very slowly.
One is in neutral and the other driving the car.
Mattax said:
... a couple logs showing slowly increasing rpm with small throttle opening.


1712173075191.png
Click to expand...

The AFR gets leaner with increasing rpm, but only when there is load.
 
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I was wondering if the vac source location plays a role in it?
CFD244 said:
No dial back timing light. I can lean out the mixture with the a/f screws to get into the 13's.

I just found it strange that the mv is perfectly stable at idle, and gets a little erratic in the fast idle zone. I might try the vac gauge on the manifold fitting instead of the car base and see if that makes a difference.

I was just concerned that it may be a vacuum leak........I tried a propane torch around the carb base and intake with no change in rpm.
Click to expand...
 
CFD244 said:
No dial back timing light. I can lean out the mixture with the a/f screws to get into the 13's.
Click to expand...
Since its on the stand, there's only so much you can do. But don't worry about the AFR number. Its just a number, even assuming the exhaust is long enough and the idle conditions are good enough the WBO2 is getting interpretted correctly. A CO meter is better at idle, but again its not important.

For a given timing and throttle position, turn the idle mix screws in until the rpm starts to drop off (or vac drops off), then back out to the point it was before it dropped off. Then I'd make it another 1/16 or 1/8 richer. Now go take a reading and note it down just so you have a record to refer to.
 
Thanks for everyone's input. :thumbsup: I will revisit this issue with your suggestions when I fire it up next time.
 
Do the test again as in post #1....with the vac adv disconnected & plugged.
 
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