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Discussion Starter #1
Everyone,

I have a idle vacuum question concerning a small block 350. It's a very typical old school small block. Basic specs are:

~10.5:1 compression
GM double hump "186" heads, with mild porting / polishing still 2.02 / 1.60 valves
69Z28 intake with a Holley 750 DP carb.
Lunati 40134 solid lifter camshaft, non roller.
Mallory Unilite distributor with the vacuum advance not connected, timing is 8 degrees initial and 38 all in around 3500.
1 5/8" headers with 3" collectors and 2.5 exhaust.

This weekend after reading a link on this site I decided to connect the vacuum advance on the distributor. The issue is that I don't appear to have enough manifold vacuum to activate the advance at idle. When I rev the car up and let it come back down to idle, now the vacuum advance is activated and I have a totally different idle RPM (way too high). If I lower the idle via the carb screw the vacuum advance drops out again and then the idle is too low. So I checked the vacuum with two different gauges and the engine only has 5" of vacuum at idle. would this be considered normal for a cam with these specs.

Adv Dur Dur @0.05” Lift Int/Exh LSA/ ICL Lash Int / Exh
40134 278/288 239/249 .517"/.543" 106/102 .018"/.020"

If so then I'll leave it alone and go without the advance as I have for a while now. If not I'll look for a leak. I did a quick carb spray check around the intake and didn't find any leaks.

Thanks
Ray
 

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Adding IDLE timing would help bring idle vacuum up a bit.

That cam would want me to use 12/14 degrees if INITIAL timing, and set the degrees stop of the vacuum advance to add 10 degrees more timing, on full manifold vacuum, to give 24 degrees of IDLE timing.

This should help to get a couple more in/hg of vacuum, and make the carb adjustment easier.

You might also have another issue on the carb. To find this info, you will need to pull the carb off the manifold and physically look at the idle transfer slots and how far they are open. This gets a little specific, but I will keep it simple.

If you look at the primary throttle plates and where they sit at idle settings, choke off, you will see on the throttle bore, a hole below the plate, and up a bit, a slot, partly open under the plates. This slot is responsible for the just off idle fuel transfer from idle to acceleration, and is the first system in the carb to "hit" as acceleration is done. If the slot is too far open, the fuel that should be held in suspension in the slot will be used for idle, because the throttle plates will be idling too far open.

Now, timing at idle will effect just where these plates sit in the throttle bores at idle, and if too far exposed, with the plates open too far, idle and off idle vacuum is lowered, causing more issues.

Now, pulling the carb on and off the manifold ain't fun. I suggest puling the carb just once, and when off, use the idle speed screw to set the throttle plates so they only expose the idle transfer slots the required .020 inch distance. Then, use a scribe to make a line on the throttle bore to mark the ABOVE the throttle plate position, so it can be seen from the top of the carb. This will help in not forcing the carb removal to check the idle transfer slots as you progress along in getting it dialed in right.

Once the idle timing is dialed in, and the carb in correct idle plate position attitude, then, see what you have for idle vacuum.
 

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Problem is you have too much cam (valve overlap). It is adversely affecting your car's streetability. I doubt you are going to change the cam because you like the noise it makes.

So in that case return to your previous set up and loose bottom end torque (which you compensate for with a higher numbered rear gear). Of course a higher gear affects your mileage, as does the cam, as it is probably running rich down low (if not you are probably running lean up top).

This is what I refer to as streetability. You have a cam that is only good for high (above 4,200 RPM) operation even though you rarely if ever operate your car on the street at that RPM. All to get that race car sound of a rough idling cam.

Big Dave
 

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As Larger Dave suggests, engine cannot make vacuum when valves are open at the same time. A wider LSA is needed with smaller duration open.
It is a trade-off, either/or, not both.
 

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Ben - Delaware - 67 Camaro
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more idle timing. change bushing in distributor to limit total to 38 when you combine your base (idle) timing and mechanical advance. if you can get a bushing that limits mechanical advance to 18 degrees, then set your base at 18 degrees, this should give you a total of 36. then hook up vacuum to manifold source with a limiter plate stopping vacuum advance at 8-10 degrees and you'll get your 24-28 idle timing the motor wants to idle well and make some vacuum and 36-38 total that it needs. after that adjust carb idle screw to get transfer slots covered as stated above. viola
 

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Vacuum seems a little low for that cam. I would think 8"-10" @ about 1000rpm should be attainable, if you get it tuned well.

I had a 355 sbc 10.5comp
Hyd cam.
244 @ .050 .500 valve lift
Lobe Separation 110/ 106 center line
I was in the 8~10" vac range. I had at least 12 deg initial timing on that engine.


My current SBC roller has more duration and lift, but more cubic inches also. It pulls 12" @900 rpm.
8 deg cranking timing, 24 when it fires and idles, 44 deg. or so @ lean cruise, 36deg. @ WOT. Timing is computer controlled.
 

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Discussion Starter #7
Thanks for the input...

Dave R, I hadn't reviewed the transfer slots as the mixture screws were acting normal so I was under the belief that the slots were covered enough. I'll check them out.

bhodgdon400, I have the whole assortment of adjustment for the distributor and can change the mechanical advance to a lower number and add some initial timing to the motor. I'll give that a try.

Larger Dave, I do like the sound of course but didn't pick the cam for this application. I bracket raced for many years and built this motor roughly 20 years ago for a '67 Camaro that I had at that time, it ran high 11's. When I built the next motor (383, with yet a bigger cam, better induction, more compression etc., ran high 10's then) I kept this motor intact basically. So, when I decided to remove the 302 due to a close call with destruction I threw this motor in my car with my 302 heads and intake, until such time that I wanted to put the 302 back in. I didn't bother to change the cam out and for the most part it runs well but I thought maybe I could improve upon my situation. You are right though, I run a lot of gear, which the 302 liked as well, but I have Tremec TKO600 in the car now so highway isn't too bad. The bottom end of this cam is 3000 rpm, under that and its not as happy.
 

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Ben - Delaware - 67 Camaro
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getting some more initial advance will definitely help with the vacuum signal and tuneability. ask me how i know
 

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I just went through this same situation. I have low idle vacuum and a B28 vacuum can. I tried running full manifold vacuum and once I revved the engine it would idle fast on the can and wouldn't come back down to the original idle speed. I switched to ported vacuum off the carb and it fixed it. Because there isn't any vacuum advance at idle, I bumped up my initial and limited my mechanical. I wanted to keep the vacuum advance for part throttle cruising.
 

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Discussion Starter #10
Okay, so in that case the vacuum advance is only on during the part throttle operation. I could switch to that I believe the metering block has that port. I'll see about adding initial timing as well.

Thanks
Ray
 

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Engines with performance options DO NOT LIKE TWO ACCELERATION CURVES.

Now, the mechanical curve is an acceleration curve.

With the WRONG PORTED vacuum, so is the vacuum advance.

At the sacrifice of idle timing quality.

If it runs better with ported vacuum, and it does not have a functional EGR valve, FIX WHAT IS WRONG WITH IT.
 

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Brett - Leander, Texas 1969 SS396
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Dave, you said something the other day that made the light bulb come on with me. I might not be able to remember it precisely, but basically you said that the transfer slots in the secondaries need to stay at "Square" because it reserves the system air/fuel and prepares the tune for when you accelerate. So I then realized that there are "Sub-Systems" within the carb that need to be set independently of the other "Sub-systems" and they are unique and independent and should be treated this way. My mistake was adjusting the wrong sub-system because it seemed to run better when I did at that moment. For me, at least, it is hard to follow the correct sequence of what to do and when to do it, but I am beginning to grasp the complexity.

Ray, I was having similar issues and Dave steered me into a vacuum advance with a stop added into the rod that pulls the diaphragm so it won't "over-advance" the system. I have all of the parts, I just haven't had the time to install it and try it yet. Using manifold vacuum enables the advance to pull better at idle, but then limits the amount when you don't want it. It sounds very logical once I understood it.

Brett......
 

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Discussion Starter #14
So I removed the distributor last night and set the mechanical advance to 24 deg, put the distributor back in and adjusted the initial timing to 14 deg, or 38 total. With that I still have manifold vacuum too low to activate the vacuum advance. I did use a pump to check the vacuum advance and it is set for 10 deg. But it takes like 14 inches of vacuum to pull it in. I'm of the opinion that the cam in this motor will never allow for than much vacuum. I've not pulled the carburetor yet to check the idle slots. I have to travel the next week for work so it likely won't get done until I return.

Ray
 

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So I removed the distributor last night and set the mechanical advance to 24 deg, put the distributor back in and adjusted the initial timing to 14 deg, or 38 total. With that I still have manifold vacuum too low to activate the vacuum advance. I did use a pump to check the vacuum advance and it is set for 10 deg. But it takes like 14 inches of vacuum to pull it in. I'm of the opinion that the cam in this motor will never allow for than much vacuum. I've not pulled the carburetor yet to check the idle slots. I have to travel the next week for work so it likely won't get done until I return.Ray

Sounds like you have the wrong vac advance for your application. I'm not sure which one you are using, but sounds like you need one that kicks in at lower vacuum. I had a similar low idle vacuum problem and changed my vac can to fix the problem (also made a stop for it to limit the amount of advance). Attached below is a listing of various vac advance units with the vacuum they start to advance and the vacuum at which they are all in. Also shown is the total amount of advance provided. If you want to adjust the rate at which the advance is added (not the total amount added), you'll need one like the Crane adjustable unit. Hope this list helps you select an appropriate vac advance unit for your low idle vacuum engine.
 

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You can buy an adjustable vacuum advance can to set it to operate with the vacuum you have. But as Rufuss said the motor won't like it. Further a lack vacuum means your carb is starved for the signal it needs to operate.

It is the difference in atmospheric pressure and manifold vacuum that powers your brakes and provides the energy to pull gas and mix it with air based upon the vacuum level it sees.

With a cam with a lot of overlap you have little vacuum. The carb thinks you are running on the main jets because the passages in the idle circuits are sized for nearly twice the vacuum you have. Your turning the idle screws and idle RPM causes the transition slot to be uncovered which affects off idle and idle fuel mixture. This usually requires you to modify your accelerator circuit to cover for any off idle stumble.

Race cars with a big cam don't have to worry about idle quality as they never idle (unless you consider 2,400 to 3,600 RPM is idling). This is why BMW gives their owners a switch on the dash that plays radical engine noises through the car speakers based upon throttle position instead of putting a big cam in a street driven car. They know the owner wants to hear a race car roar but hate the poor idle issues.

I learned that it is better in every way to have two different cars. One for the street and one for the track. If you want to race go home and get the car with the slicks on it. As long as it stays well muffled no one will be the wiser when you swap cars to get those slicks (and a 582 BBC instead of a SBC).

Big Dave
 

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On flat tappet hydraulic grind cams, there is an option, but not done the way many think. This option is the ONLY way I do it for radical cams now.

As we all know, a hydraulic lifter uses oil pressure, and trapped oil volume to control free play in a valve train. The lifter pumps up and stays at zero lash, wit a bit of preload. Cam timing stays same throughout the rev range.

Alternate option, some years ago, we heard about, and used Rhoads, and Crane "bleed down" flat tappets that bleed oil volume down as oil pressures are lowered by rpm's. This was done to alter cam specs to allow for a smaller cam timing at idle and off the power curve, to help make the engines run better when off the power curve.

Now, lets look at an exhaust valve event. The valve opens, piston pushes spent mix out the valve as exhausted gas. Part of this system uses a header, or well designed manifold to help suction out burned gasses. Suction from the exhaust system also helps to pull the burned gas out of the cylinder and head.

During the suction event of the exhaust, the intake begins to open, and the exhaust suction in the cylinder head and exhaust system helps start inlet mixture fill events.

If the inlet valve opens with a weak suction eve3nt, the mixture flow into the chamber and bore is weak, and takes longer to complete.

Bleed down lifters take duration out of a lobe profile, and in the case of the intake valve, starts later with less duration, but has a stronger signal.

In changing both lifters to bleed down, the inlet suction is enhanced, but the exhaust is dampened down.

Now, consider what I do with bleed down lifters. I use them ONLY on the INTAKE valves in an engine, not both intake and exhaust. This gives me a stronger pull for the inlet mixture, and more lower end, with less off power band power loss.

Vacuum is also significantly increased with this option.

On adjustable vacuum advances, none are the same vacuum adjust ability range wise. Crane works down to the lowest vacuum levels, the others are nowhere near as low operating vacuum range on full slack vacuum adjustment.
 
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