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Discussion Starter · #1 ·
Hey all so I just rebuilt my carb and am trying to dial everything in. it is a Holley 4160 on a rebuilt 350 pretty stock with an RV cam. I set my idle mixture by tightening until it started to big then backed off a 1/4 turn. Currently I have the distributor vaccum on the front port and transmission vaccum to the metering block. Maybe there's a trans vaccum leak because it wasn't running well when the lines were swapped...is this a normal way to run things maybe because less vaccum at idle with the cam? now when checking timing should I take the hose of distributor and plug it or take the hose off trans? I didn't see any timing change when I plugged where the trans is currently plugged in. with the engine hot and idling in park I should be at -10 right?
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Both the distributor and trans should be to full manifold vacuum. (There’s always a debate with some people wether the distributor should be to ported vac or full manifold vac. I dont judge anyone that runs ported vac… i just wouldnt let them work on my car… Kinda like when someone wants a well done steak. )

Ok back on track. Yes, when you set the timing you need to pull the distributor vacuum line off and plug the port on the carb. If you’re not going to be doing any distributor tuning (ie. Changing weights and springs to set the timing curve), then you need set the timing for full mechanical advance. Where ever idle timing ends up is where it ends up. Example. You know your motor wants 34degrees of total timing. Have someone sit in the car and hold the engine at lets say 3,000 or 3,500 rpm so you’re at full mechanical advance (or what ever rpm the timing stops rising). Then set the distributor to 34 degrees. Now let the motor rpm back down and see where the timing is at while idling. It may be 10, 12 or 8 or something around there. Lets say its 10. Now when you plug your vacuum advance back it should add somewhere around 10 degrees so now your idle should be 20degrees.

As far as the trans, It needs full manifold vac because at low or no vacuum it thinks you’re at wide open throttle. So with ported vac where there is no vacuum signal at idle, it will think you’re trying to go wide-open-throttle just sitting there idling or when ever you let off the pedal while driving.
 

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Discussion Starter · #3 ·
Both the distributor and trans should be to full manifold vacuum. (There’s always a debate with some people wether the distributor should be to ported vac or full manifold vac. I dont judge anyone that runs ported vac… i just wouldnt let them work on my car… Kinda like when someone wants a well done steak. )

Ok back on track. Yes, when you set the timing you need to pull the distributor vacuum line off and plug the port on the carb. If you’re not going to be doing any distributor tuning (ie. Changing weights and springs to set the timing curve), then you need set the timing for full mechanical advance. Where ever idle timing ends up is where it ends up. Example. You know your motor wants 34degrees of total timing. Have someone sit in the car and hold the engine at lets say 3,000 or 3,500 rpm so you’re at full mechanical advance (or what ever rpm the timing stops rising). Then set the distributor to 34 degrees. Now let the motor rpm back down and see where the timing is at while idling. It may be 10, 12 or 8 or something around there. Lets say its 10. Now when you plug your vacuum advance back it should add somewhere around 10 degrees so now your idle should be 20degrees.

As far as the trans, It needs full manifold vac because at low or no vacuum it thinks you’re at wide open throttle. So with ported vac where there is no vacuum signal it will think you’re trying to go wide-open-throttle just sitting there idling or when ever you let off the pedal while driving.
Thanks for the detailed response. manifold vac is the front lower port that always has vaccum right? the one on the metering block is ported? So I should get a t and plug both hoses into the front port and plug the vac port on the metering block then set the total timing at 3500rpm.....how do I see the total degrees of the timing marker only goes so high? noob question sorry.
I did not see any timing changes when plugging and unplugging the vaccum what could be the possible cause of this?
 

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Thanks for the detailed response. manifold vac is the front lower port that always has vaccum right? the one on the metering block is ported? So I should get a t and plug both hoses into the front port and plug the vac port on the metering block then set the total timing at 3500rpm.....how do I see the total degrees of the timing marker only goes so high? noob question sorry.
I did not see any timing changes when plugging and unplugging the vaccum what could be the possible cause of this?
Think of it like this. Full manifold vacuum needs to come from UNDER the throttle blades. Wether its on the front, back, side of that carb or coming right off the manifold, thats all considered “full manifold vac”. The only spot where you can get ported vac is ABOVE the throttle blades. Hence why the port on the metering block is so high up. Dont use that port and plug it.

I usually try to make all the vacuum accessories hook up behind the carb so everything looks nice and clean. Yes you can tee the tranny vac and distributor vac hoses together from one vacuum source. Or you can run one from the front of the carb and one from the rear of the carb. Or run any of those directly to the manifold if you have ports for it.

For the timing. If your balancer doesnt go that high, you can get a cheapo “dial back” timing light. The dial back timing lights will offset the number its reading with degrees on the balancer. Example. You currently have 34degrees total timing. Your balancer only goes to 10. So when at 3000 rpm the tick mark on the balancer is another inch past where the timing valves stop, off in free space. Now with a dial back light, you start to dial it back. For everyone one degree you dial back, the tick mark on the balancer will also come back one degree. So if you set the “dial back” to 24, your tick mark will line up at the 10degree mark on the pointer. If you dial it back to 34 then the tick mark would be at the 0 mark.

Here’s another way to put it. Point the “dial back” light at the banacer and keep twisting the knob until you see the timing mark on the balancer line up to TDC 0. Now look at the value of the dial back. This is your current timing.
 

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Discussion Starter · #6 ·
I of course don't have the dial back timing light or a tach but that all makes sense. thanks. I'll check out the sticky now
 

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Discussion Starter · #7 · (Edited)
That was a great thread. I suppose I should look into my vac advance and make sure it matches the application. so with a street car and very mild cam I want it to start at 9-10hg and then I want to set my total timing to 36 degrees at 3k rpm? is there any way to do this without a tach or dial back light?
 

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Since you DO NOT HAVE EFI, nor computerized timing, LEAVE THE DIAL BACK OFF, LEAVE THE DIAL BACK OFF, LEAVE THE DIAL BACK OFF. Dial back is to compensate for computer lag in timing, NOT FOR ANALOG MECHANICAL/VACUUM TIMING CURVES.

If you want to set the vacuum advance up correctly, email and ask for the GM Vacuum Advance Degrees Stop packet, it will help, and you will have a better understanding of how timing really works.

The packet is totally free, no ads, no bull: [email protected]

Once the timing is done correctly, the carburetor will adjust a lot easier.
 

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Since you DO NOT HAVE EFI, nor computerized timing, LEAVE THE DIAL BACK OFF, LEAVE THE DIAL BACK OFF, LEAVE THE DIAL BACK OFF. Dial back is to compensate for computer lag in timing, NOT FOR ANALOG MECHANICAL/VACUUM TIMING CURVES.

If you want to set the vacuum advance up correctly, email and ask for the GM Vacuum Advance Degrees Stop packet, it will help, and you will have a better understanding of how timing really works.

The packet is totally free, no ads, no bull: [email protected]

Once the timing is done correctly, the carburetor will adjust a lot easier.
Uhhhhh what? You got that backwards sir. Typically you dont use a dial back with an efi system because it can have erratic readings.

Ive used some sort of dial back on every car ive touched for the last 20 years. There is nothing wrong with them. I use a nicer digital one. Ive seen this timing gun at almost every speed shop ive been ever been to and i use it at my own shop too.


The only other way to do it is with timing tape on the balancer. But why do that when you can do it on the timing light?
 

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Discussion Starter · #10 ·
I mean since I already have a timing light and that's the cheaper option I might grab the tape. Still need to get a tach but I can check that with a vaccum gauge too right?
 

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I mean since I already have a timing light and that's the cheaper option I might grab the tape. Still need to get a tach but I can check that with a vaccum gauge too right?
You can get a cheapo dial back light from any local parts store for $20. Harbor freight even has them. Thats going to be easier and quicker than ordering timing tape. And remember if that timing tape isnt exactly the circumference of the balancer each degree will be off by a certain amount. Plus the timing light you can use on any car. Obviously cant do that with the tape.

And you dont need a tach. Just have someone inside on the gas pedal raising the rpm until the timing mark stops moving. Then you’re at full mechanical advance.
 

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Both the distributor and trans should be to full manifold vacuum. (There’s always a debate with some people wether the distributor should be to ported vac or full manifold vac. I dont judge anyone that runs ported vac… i just wouldnt let them work on my car… Kinda like when someone wants a well done steak. )

Ok back on track. Yes, when you set the timing you need to pull the distributor vacuum line off and plug the port on the carb. If you’re not going to be doing any distributor tuning (ie. Changing weights and springs to set the timing curve), then you need set the timing for full mechanical advance. Where ever idle timing ends up is where it ends up. Example. You know your motor wants 34degrees of total timing. Have someone sit in the car and hold the engine at lets say 3,000 or 3,500 rpm so you’re at full mechanical advance (or what ever rpm the timing stops rising). Then set the distributor to 34 degrees. Now let the motor rpm back down and see where the timing is at while idling. It may be 10, 12 or 8 or something around there. Lets say its 10. Now when you plug your vacuum advance back it should add somewhere around 10 degrees so now your idle should be 20degrees.

As far as the trans, It needs full manifold vac because at low or no vacuum it thinks you’re at wide open throttle. So with ported vac where there is no vacuum signal it will think you’re trying to go wide-open-throttle just sitting there idling or when ever you let off the pedal while driving.
Wow no doubt . Mostly everyone runs vacuum advance to carb instead of manifold.
Im reseting my 350 on my truck tonight or tomorrow. I'll keep that in mind .
And a very well conveyed simple way to time it along with it.
Noted. Thanks and
Good luck OP
 

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Wow no doubt . Mostly everyone runs vacuum advance to carb instead of manifold.
Im reseting my 350 on my truck tonight or tomorrow. I'll keep that in mind .
And a very well conveyed simple way to time it along with it.
Noted. Thanks and
Good luck OP
No problem. Also let me clarify. When i say “manifold vacuum” i mean any vacuum source under the throttle blades.

For a holley carb this could be any vacuum port that sits at the bottom base plate. Usually one in the front and a bigger one in the back (typically for brakes).

You can also get “manifold vacuum” directly from a port on the intake manifold. Edelbrock intake manifolds usually give you 1 or 2 of these that tap right into one of the intake runners.

Sorry if you already knew this. Just didnt want to make anyone confused.
 

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Discussion Starter · #14 ·
Yeah this is all great advice thanks. I hope my vaccum advance cansister is in the range of what I want for this engine and that my timing is just a bit off then it'll idle and rev great. it's definitely close but not spot on. thanks for the good info. Guess I'm going to get the other timing light.
 

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No problem. Also let me clarify. When i say “manifold vacuum” i mean any vacuum source under the throttle blades.

For a holley carb this could be any vacuum port that sits at the bottom base plate. Usually one in the front and a bigger one in the back (typically for brakes).

You can also get “manifold vacuum” directly from a port on the intake manifold. Edelbrock intake manifolds usually give you 1 or 2 of these that tap right into one of the intake runners.

Sorry if you already knew this. Just didnt want to make anyone confused.
No I was actually equating any port on the carb as ported vacuum so I did stand corrected .
Manifold vacuum = any port beneath the throttle plates whearher on the carb or directly on the intake
 

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No I was actually equating any port on the carb as ported vacuum so I did stand corrected .
Manifold vacuum = any port beneath the throttle plates whearher on the carb or directly on the intake
Correct. Now ill add a caveat to it. On Ededbrock carbs they have both the manifold vacuum and ported vacuum sitting side by side on the bottom baseplate. I guess moral of the story is whatever specific carb you’re working with, verify what ports are what with the manual.
 

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You can get a cheapo dial back light from any local parts store for $20. Harbor freight even has them. Thats going to be easier and quicker than ordering timing tape. And remember if that timing tape isnt exactly the circumference of the balancer each degree will be off by a certain amount. Plus the timing light you can use on any car. Obviously cant do that with the tape.

And you dont need a tach. Just have someone inside on the gas pedal raising the rpm until the timing mark stops moving. Then you’re at full mechanical advance.
GTO44. STOP GIVING BAD INFO, YOU ARE TOTALLY WRONG!!
 

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Discussion Starter · #19 ·
Soo anyone else have info to correct or support this procedure lol. didn't know it got that wild round these streets :LOL:
 

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david ray is the timing person most use and listen to his advice. After all he helped develop the HEI dist for GM
 
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