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Pressure and volume is not the same thing. Pressure on a closed end system is the result of the pump volume at intake being reduced at its release. The volume is the result of how often that happens in strokes per minute.

The carburetor however presents a leak at the end of the above described system so in this reality the pressure is a function of leakage against pump volume. If the leakage rate is greater than the pump volume rate which is chamber volume times strokes or revs per minute, then the pressure falls.

So if you are able to compare at a given engine RPM the dead head pressure against the operating pressure you can see if the pump keeps up. Obviously you will need a test rig to do this as the engine won’t run without a fuel supply.

In reality fuel pressure from an engine driven pump will fall at high RPMs where fuel flow rate feeding the pump runs short of time to fill the pumping chamber on the intake side while the engine is demanding a higher fuel flow rate so the pump outlet pressure falls.

However, I don't see this as an idle situation unless the pump or the cam lobe or the push rod between the pump and cam lobe any one of these being worn to a point where the pump does not produce a full stroke or that the pump’s valves leak such that it is unable to maintain enough volume to hold the pressure up.

Pressure wander at idle is normal as the pump is not stroking fast enough to hide the stroke pulses. These never go away it’s just as the RPMs go up the typical gauge can’t react fast enough such that you can see the pulses.

The valves in a fuel pump are pretty simple they are a spring loaded disc usually plastic that is closed against a metal seat. The intake disc spring faces into the chamber so on the down stroke the chamber volume increases and atmospheric air pressure in the fuel tank wanting to fill the enlarged void in the fuel pump pushes the fuel through the lines and valve into the pump chamber. As the pump moves to the upstroke the spring closes the inlet valve while the building pressure keeps it closed. On the outlet side the building pressure pushes the outlet disc outward compressing its spring allowing fuel to flow out to the line to the carburetor. As that stroke completes the spring closes the valve while the vacuum forming in the pump chamber keeps it seated through the pump’s intake cycle. This is how mechanical diaphragm or electric diaphragm, piston or vibrator pumps operate. Turbo pumps don’t need valves though better ones my have them for drain back control.

So you can see that from cam lobe to push rod in the case on the SBC to the pump’s lever arm to its connection to the diaphragm to the diaphragm return spring to ruptures of the diaphragm to wear or failures the pump valves are all things that can upset if not fail fuel delivery. Add to these fuel tank venting, filter and line restrictions all play a role.

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Discussion Starter · #102 ·
So does the fact that the revs dropped at idle, does that indicate it was flooding then?
and it was 7psi before?
Would just like to know if it was worth while installing the regulator. Thanks
 

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With any carb it’s worth using a regulator.
and it was likely flooding a tiny bit.
did you get a vacuum gauge too?
 

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Discussion Starter · #104 ·
Ok that makes sense. Thanks for all your responses.
And as you guys suggested, I hooked up to the (PCV) vacuum port at the carb, using a tee.
The pictures are attached, but basically, at idle 500rpm the needle was a bit shaky at 13.
At 1000rpm the needle was steady at 19, 1500rpm steady at 20.5, 2000rpm steady at 22.
Thinking it's shaky at 500rpm is just because the idle is reduced with fuel regulator installed.
Not sure, so I would really appreciate all your feedback on what this actually indicates.

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It’s not shaky at 500 because of the lower fuel pressure.
Likely you need to fine tune your mixture screws.
Have you adjusted them at all?
 

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What is the cam the 500 RPM idle is way low even for a stock mild GM cam. The idle if this has a mild stock cam is telling you either or both the ignition or cam timing is late. The slightly shaky needle at idle that smooths out as the RPMs pick up is shot valve guides. None of these have anything to do with a bad needle valve in the carb.

Correcting the ignition advance will pick up the idle speed with no changes to the carb. advancing the ignition timing will to some extent cover any excess wear that is causing the cam to fall retarded. This being the timing set which gets to how many miles are on the timing set?

Bogie
 

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Discussion Starter · #107 ·
Here in the USA we can borrow tools from local parts stores for free (with deposit). Do you have anything like that? Trying to help but you need to do your part.

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Hi,
I did the vacuum test as you suggested, using a tee on the PCV hose and I posted the results.
Just wondering what you think of the results and what can you tell me about it.
Thanks
 

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It would only rattle if the vacuum is not steady. Put a vacuum gauge on it and take a video for us please. You can use that port on the hose near the distributor.

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Need video of the 500 rpm shaky reading (not photo). This was to diagnose the pcv valve rattle. Does it still rattle? When? Thanks. Maybe just misfiring at 500 rpm. Vacuum seems great at 1000 on up. Are all the open ports plugged? Like the line to distributor?

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Discussion Starter · #109 ·
Need video of the 500 rpm shaky reading (not photo). This was to diagnose the pcv valve rattle. Does it still rattle? When? Thanks. Maybe just misfiring at 500 rpm. Vacuum seems great at 1000 on up. Are all the open ports plugged? Like the line to distributor?

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No nothing plugged.
its all still operating, I just tee'd into the PCV hose. Did I mess up? Sorry.
Can do it again, what do you want me to do?
And I put in a brand new AcDelco PCV valve and it still rattles at idle.
 

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No nothing plugged.
its all still operating, I just tee'd into the PCV hose. Did I mess up? Sorry.
Can do it again, what do you want me to do?
And I put in a brand new AcDelco PCV valve and it still rattles at idle.
Actually you previously posted a photo of where the line going to the distributor had an open port on a vacuum tee near the distributor. This is the port that goes to the underneath side of the original air cleaner (which we have never seen in a photo). You need to plug that open port on the tee when the air cleaner is off, otherwise your vacuum advance will never work and you can cause a lean misfire at idle (which may be what you have).

You have not messed up. I just want a video so I can see the action of the vacuum gauge when the PCV is rattling.



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Discussion Starter · #111 ·
Actually you previously posted a photo of where the line going to the distributor had an open port on a vacuum tee near the distributor. This is the port that goes to the underneath side of the original air cleaner (which we have never seen in a photo). You need to plug that open port on the tee when the air cleaner is off, otherwise your vacuum advance will never work and you can cause a lean misfire at idle (which may be what you have).

You have not messed up. I just want a video so I can see the action of the vacuum gauge when the PCV is rattling.



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ok, I said before in a post, that I re-routed the vacuum lines. So that tee is gone.
I have 1 hose going from dist to carb port.
1 hose from PCV to center port.
and the Auto Trans port is capped, as its stick shift.
So I tee'd the vacuum gauge into a tee i fitted on the PCV hose, an left all other hoses in place.
Is that ok?
And you said the vacuum is 'strong' when rev'd, what does that mean relating to engine?
Sorry for amateur question?
 

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ok, I said before in a post, that I re-routed the vacuum lines. So that tee is gone.
I have 1 hose going from dist to carb port.
1 hose from PCV to center port.
and the Auto Trans port is capped, as its stick shift.
So I tee'd the vacuum gauge into a tee i fitted on the PCV hose, an left all other hoses in place.
Is that ok?
And you said the vacuum is 'strong' when rev'd, what does that mean relating to engine?
Sorry for amateur question?
I understand now. Guess I missed that in the 100+ posts in this thread.

Which port to hook distributor to depends on your advance curve and engine components. Which one did you use? Ported or non ported?

Vacuum at 1000+ rpm is fine. This means it seems to be running quite well at those RPMs without pulling a load - nothing more.

This is the last time I'm going to ask for the video of the vacuum gauge at idle when the PCV valve is rattling. Just post it to YouTube and link to it in your response.

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Discussion Starter · #113 ·
I understand now. Guess I missed that in the 100+ posts in this thread.

Which port to hook distributor to depends on your advance curve and engine components. Which one did you use? Ported or non ported?

Vacuum at 1000+ rpm is fine. This means it seems to be running quite well at those RPMs without pulling a load - nothing more.

This is the last time I'm going to ask for the video of the vacuum gauge at idle when the PCV valve is rattling. Just post it to YouTube and link to it in your response.

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Ok will do. i didn't know how to do it. Thanks for info and I will send video today.
Dist hose on ported vacuum. (BUT it was on manifold tee'd into PCV hose as in picture in post 18).
So is this now causing another problem, as I switched advance hose from manifold to ported??
Should I switch back to manifold, doesn't it depend on the type of dist?
I'm confused.
Should I do video as it was originally with advance on manifold, or as now with advance on ported?
 

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Ok will do. i didn't know how to do it. Thanks for info and I will send video today.
Dist hose on ported vacuum. (BUT it was on manifold tee'd into PCV hose as in picture in post 18).
So is this now causing another problem, as I switched advance hose from manifold to ported??
Should I switch back to manifold, doesn't it depend on the type of dist?
I'm confused.
Should I do video as it was originally with advance on manifold, or as now with advance on ported?
Different people have different ideas on using ported vacuum. But I personally would recommend using manifold vacuum if the distributor originally utilized it and advance curve has not been changed. As long as engine has enough vacuum at idle to fully advance the pot (and certainly your engine meets this), that is your best bet. More advance at idle generally gives higher vacuum and higher RPM for a given throttle opening. The 500 vs 650 you are concerned about may be due to this.

Why not try it both ways? Video of gauge both ways.....

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Also would be interested in knowing where the timing is, in the two cases (ported vs manifold). If this is a 383 with a Comp Cams 12-210-2 (High Energy 268) with duration of [email protected] and only 48 degrees of overlap, the vacuum at 800rpm should be above 17.5" (above 18.5" at 1000 and it is). I'm guessing that 700-800 RPM is a better idle speed for this vehicle.

Am trying to go back to the original issue of excessive oil usage. I'm just trying to make sure it runs properly (not rich, not lean, correct timing, not misfiring due to mechanical problem) and then change to new spark plugs to evaluate the oil usage.
 

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Discussion Starter · #117 ·
Ok Mr. Blazer, your bang on!
The original setup was advance connected to manifold like the picture below
and it runs at smooth idle 650rpm, and the vacuum needle stays in green zone.
Switching the advance to the ported option, reduces idle to 500rpm and its outside green zone.
Also attached picture below of altered setup with advance connected to ported.
Very interested in hearing what you can tell me about it after watching the videos. Thanks.
Trans Am - Advance in Ported - YouTube
Trans Am - Advance in Manifold - YouTube

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Well, I would definitely leave it on full manifold vacuum. That is what I would expect for readings and it is steady enough. Assume no PCV valve rattle now?

Next need to evaluate your distributor timing curve and settings but you need the equipment. Do you have a timing light or can you borrow one? Does it have the dial back feature on the timing light, or do you happen to have timing tape on the engine harmonic balancer?

After timing setting/curve is verified and adjusted (if necessary), then we will finally adjust the idle speed and mixture................... should be running tip-top / and will be ready to change all the spark plugs for the oil consumption testing.

Good job - glad to see you making progress.
 

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Let's talk about oil for just a bit, in case that you have not heard. Are you aware that to get a flat tappet hydraulic cam (like you have) to live for any period of time, you need to use either high-ZDDP oil (preferred) or add ZDDP additive to the oil you are using? This is because most modern oils available have been formulated with far less ZDDP in them to avoid catalytic converter poisoning. ZDDP is an antiwear extreme pressure additive that allows a camshaft lifter to live while rubbing directly on a camshaft lobe (flat tappet), and under heavy pressure of the valvesprings. Modern engines have cams with roller lifters in them, so it doesn't matter for those. For yours and anyone else running a flat tappet camshaft, it definitely does matter.

Break the rules and you risk grinding up the bottoms of lifters and camshaft lobes - then these fine particles of metal gets spread throughout the engine and cause further damage and you need full disassembly to fix.

For the street one needs to shoot for 1300-1600ppm ZDDP in a quality multi-grade automotive engine oil 10w30 or 10w40.

generic additive in UK
ZDDP Anti Wear Oil Additive

1300ppm ZDDP 10W40 Oil in UK
Classic Oils Hot Hatch 10W-40
 

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Discussion Starter · #120 ·
Let's talk about oil for just a bit, in case that you have not heard. Are you aware that to get a flat tappet hydraulic cam (like you have) to live for any period of time, you need to use either high-ZDDP oil (preferred) or add ZDDP additive to the oil you are using? This is because most modern oils available have been formulated with far less ZDDP in them to avoid catalytic converter poisoning. ZDDP is an antiwear extreme pressure additive that allows a camshaft lifter to live while rubbing directly on a camshaft lobe (flat tappet), and under heavy pressure of the valvesprings. Modern engines have cams with roller lifters in them, so it doesn't matter for those. For yours and anyone else running a flat tappet camshaft, it definitely does matter.

Break the rules and you risk grinding up the bottoms of lifters and camshaft lobes - then these fine particles of metal gets spread throughout the engine and cause further damage and you need full disassembly to fix.

For the street one needs to shoot for 1300-1600ppm ZDDP in a quality multi-grade automotive engine oil 10w30 or 10w40.

generic additive in UK
ZDDP Anti Wear Oil Additive

1300ppm ZDDP 10W40 Oil in UK
Classic Oils Hot Hatch 10W-40
OK, that's very helpful. I was going to add Lucas stabiliser when servicing. What do you think of it?
Also can I ask you to clarify does the result on the vacuum gauge mean the vacuum is good on the car?
And does it say anything about the engine condition/wear?
I don't have a timing gun, but let me look into it. I watched a few youtube vids, so have an idea of it now.
 
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