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Discussion Starter #1
My Holley carb idle was a bit too low, 600-700 rpm in park.
So I adjusted the idle speed screw to bring it up. It came up too high to 1500.
So I backed it off again and started making tiny adjustments. I was going for 800-900 rpm in park or neutral.
I'd adjust, rev it, adjust rev it and with each tiny adjustment, it came up a tiny bit but not enough. Then after several tiny adjustments it comes way up to 1500 again.

I can get about 700 rpm or 1500. I can't get it to a middle ground.

What's going on with this? What should I look into? :confused:
 

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WJen you are making the adjustments is the throttle linkage disconnected? I ask because I found that with the rod in the throttle arm, there is only so much turning of the screw before the arm can't go back anymore because the stud is contacting the rear of the arm hole. You try to make small changes, but when it contacts the back of the arm hole you have to turn a little hard and that makes it jump up too high. Try making the adjustments with the throttle rod separate from the carb. Just an idea.
 

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Set the idle screw to about 1 1/2 turns from bottom...then adjust the sec stop scew and mixture screws to your rpm idle.
Fine tune on the idle scew and mixture scews

OR it could be a too light a spring on the cent advance, that starts coming off at the slightly higher rpm.
Or a vac advance that doesnt activate at the low rpms, as u slowly increase rpms, the vac increases and activates ...increasing advance...increasing rpms.
 

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The vacuum advance is my bet. Your vac advance is probably connected to a ported vacuum source.
As you increase the throttle opening, you are starting to get vacuum on that port which adds vac advance and increases your idle speed.
 

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The vacuum advance is my bet. Your vac advance is probably connected to a ported vacuum source.
As you increase the throttle opening, you are starting to get vacuum on that port which adds vac advance and increases your idle speed.
Ya, that's what I was thinking too. Try moving the vac advance to a full-time vacuum source, and try to re-adjust. Moving from ported vacuum to full-time vac should bring your idle speed up to 1500-ish, then you can back off the idle speed screw to get it back down...
 

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If the VA is attached to ported (above butterflys) and it is the VA, then the idle screw is being opened to far, which indicates the sec butterflys are to closed... see my post above.

remove the dizzy cap, attach a vac pump and gauge to the VA, put your finger in the end of the advance lever, suck...note at what vac the lever pulls the dizzy mechanism.

put a Vac gauge in the manifold vac and establish what the idle engine vac with VA disconnected...

If the VA is connected to manifold vac, the VA should be all in at about 1" above the idle....
 

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Discussion Starter #9
Sorry, just got back from a river rafting trip that almost went bad. I'm beat up now.

OK so you guys are on the right track because my VA is not attached to full manifold vacuum. Its on a port on the primary metering block.

I will move that tonight when I get home!

Then I will report back with the results....

Thanks!
 

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Discussion Starter #10
OK,

So just some more information for you that I might of left out.
Moving down the VA line of thought, I probly forgot to mention that I have a big cam and at idle, I've got about 5-6 inches of vacuum.

At 1500 rpm, where it seems to like to go when I mess with it, there is better vacuum. Like about 12-15 inches.

I have a Edelbrock Victor 454-O manifold. It doesn't have a manifold port for vacuum. The closest I can get is the base of the carb very near the manifold.
Will that do for vacuum? I already have a larger 3/8 line coming out of there to a vacuum canister for brakes. I could tee off of it.
Will that do?
 

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either of those will work, and another option is to put a carb spacer that has a pipe port, if you have hood clearance to add a spacer. You could also tee off tranny modulator source. Vac can doesn't consume, so it won't affect the operation of anything.
 

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Discussion Starter #12
either of those will work, and another option is to put a carb spacer that has a pipe port, if you have hood clearance to add a spacer. You could also tee off tranny modulator source. Vac can doesn't consume, so it won't affect the operation of anything.
The only time the vacuum canister consumes is when I hit the brakes. But there is check valves in a vacuum canister right? So it should only effect periodically right after I apply brakes right?

As far as hood clearance, I'm gonna need to see, if there is any clearance, its very little, I do have a spacer in there I think, perhaps I could replace it with one like you mention.
 

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drill and tap it if you already have one and it doesn't have an allen headed plug in it already

as for vac can consuming when you hit brakes, think about what happens right before you hit brakes,,,,,cruzin along and some idiot pulls out in front of you and you get off the gas to go for brakes,,,vacuum rises(actually pressure falls:eek:) to highest it ever will in those conditions. Vac can is pulled like a bastard and has negative pressure on the order of way over 20"HG( about negative 10#)with a mild cam, you hit the brakes and your booster consumes vacuum, which the little vac advance can has actually stored a tad bit of for you. When the booster consumes vacuum, pressure will actually rise somewhat(lower vacuum is higher pressure :eek:)(you are equalizing atmospheric to manifold vacuum) but I doubt you could see it on most gauges because the pistons sucking against the closed butterflies at high speed would replenish it in the blink of an eye. And really, the pressure equalizing doesn't happen until you let OFF the brake anyway, because the booster is just a bigass vac reservoir with check valves that uses that pressure differential to YOUR advantage.
 

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as for vac can consuming when you hit brakes,
Exactly as described above is what happens in the REAL world
The only time it maybe an issue, is idling out of a gas station or driveway, tap brake 1st time, is ok cause of trapped vac in the brakes, but 2nd time...and rem you are travelling about the speed of a mother and pram, the brakes only have parial vac, and are harder...
and u stop nps.
Now apply this to the real world, it is very rare one touched the brakes twice, at idle speed after pulling away from the pumps and slowing down to check the traffic....and IF u do, it just means u have manual brakes at 2 mph.
 

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Discussion Starter #15
Exactly as described above is what happens in the REAL world
The only time it maybe an issue, is idling out of a gas station or driveway, tap brake 1st time, is ok cause of trapped vac in the brakes, but 2nd time...and rem you are travelling about the speed of a mother and pram, the brakes only have parial vac, and are harder...
and u stop nps.
Now apply this to the real world, it is very rare one touched the brakes twice, at idle speed after pulling away from the pumps and slowing down to check the traffic....and IF u do, it just means u have manual brakes at 2 mph.
Manual brakes at 2mph I can live with.
Just not at cruising speed.
 

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I lived fine with 5 to 6 " vac on our daily driver Camaro...
The last custom LPG cam profile I had for several yrs had low idle vac and an idle of 550 rpms...I only noticed manual brakes a couple times a yr at 2 /3 mph, just pushed slightly harder.
 

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Discussion Starter #17
I lived fine with 5 to 6 " vac on our daily driver Camaro...
The last custom LPG cam profile I had for several yrs had low idle vac and an idle of 550 rpms...I only noticed manual brakes a couple times a yr at 2 /3 mph, just pushed slightly harder.

OK so I moved the vacuum advance to manifold vacuum and I still have the problem.
I'm getting 6-8 inches at about 700 rpm. I increase the idle adjustment screw and it goes up to 1500.
So with vacuum advance disconnected and the vacuum port plugged, I adjusted idle to about 800-900 where I want it. Then, I re connected the VA.
No change until I revved it a tiny bit. The idle came back down to 1500 and didn't go below.

So I called MSD tech line to talk to them about VA and stuff.
I gave them the rundown of what I have going here.
BBC, big cam that gets about 6-8 inches at 700 rpm.
MSD everything
On the distributor, I run the blue bushing and the lightest springs. My converter flashes up past 4400 on launch.
Their advice was to unplug VA and go mechanical advance only. Their reasoning was this car is not going to benefit much from the added economy that VA offers.
I unplugged it and must admit, the idle goes where I want it now.

I post this because I know it is contrary to posts in this thread advocating the use of VA. I have an open mind.
If I'm getting minimal economy, which isn't a priority to me, then what sort of benefits will VA provide?
Lets say for instance, I get about 4mpg without VA.
Would adding it get me 5? or better?
Just theorizing and I respect all opinions here and am thankful for them.
 

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They gave you the band-aid fix response. It works, but not good for economy at all.
Most get over this by replacing the vacuum can on the distributor but didn't you already do that?
Having vacuum advance you do notice better economy and I think even more power, especially at cruise.
 

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My Holley carb idle was a bit too low, 600-700 rpm in park.
so NOW you tell us it's a big cam with 6-8" idle vac.:)

define big cam

I'd probably have it at 900-1000 idle if it really is a big cam. And I'd have it either full locked out, or as msd suggested no vac advance. Vac advance and stupid cams just don't get along together and you go berserk fighting it. Either load up and quit or haulass at idle, a happy medium is only a fleeting thing or a dance you maintain by gigging the throttle. I don't even fool with vac advance on my ZZ-4 with a mellow little hot cam(218/228), too few street miles.
 

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Discussion Starter #20
so NOW you tell us it's a big cam with 6-8" idle vac.:)

define big cam

I'd probably have it at 900-1000 idle if it really is a big cam. And I'd have it either full locked out, or as msd suggested no vac advance. Vac advance and stupid cams just don't get along together and you go berserk fighting it. Either load up and quit or haulass at idle, a happy medium is only a fleeting thing or a dance you maintain by gigging the throttle. I don't even fool with vac advance on my ZZ-4 with a mellow little hot cam(218/228), too few street miles.
I don't know the cam. It was in it when I got the car. I took off the timing cover to yank the timing gears that were in it. I didn't like them and I couldn't hear if the engine was pinging or something like that over the sound of the timing gears. I hoped that something on the front of the cam would be stamped so I could identify it. There was nothing.
All I know about it, is that its a solid roller cam and its a bit wild. I also know that it pulls like hell to 6800 rpm as evidenced in 1/4 mile runs and a dyno run once.
I replaced the gears with a Cloyes true roller chain. It actually seemed to drive better, with the gears in there, everytime I got off the throttle at any speed, it felt like I had just geared down.
 
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