Question about vacuum advance

Hello
When it should to start to advance the distributor vacuum canister?
My engine idles between 10-11 hg and the MP distributor canister starts to move at 11 hg.(checked with a mityvac)
Should i adjust it for starting to advance later?

It depends on the specific setup. Stock or hot rodded / racing.
Some stock setups used manifold vacuum to add advance at idle, many others did not, using ported (aka timed) vacuum instead.

Based on your post, I am going to guess that you have a modified engine?

If so, set vacuum advance for on road driving conditions. Try driving with it as is. If the engine makes a 'pinging' sound under part throttle load, then experiment with changing the vacuum advance is added and also limit the maximum amount. The goal is no pinging at any part throttle.

For idle conditions, there are two options (again, assuming non-stock engine):
1. Connect the vacuum advance to ported vacuum source. Set base timing higher than stock, probably between 14-20 degrees BTDC depending on how radical the engine is. Vacuum advance now will only get vacuum when the throttle moves off-idle.

2. Connect the vacuum advance to manifold vacuum source.
a. Setup the vacuum advance move at less than 10 "Hg. Set the base mechanical timing low (like some stock engines). The total of mechanical base plus the vacuum advance will be the normal idle timing.
b. Setup the vacuum advance to move above 11 "Hg. Set the base mechanical timing between 14-20 degrees as described in 1.

Which option is easier will depend on the engine and vehicle.
Option 2a has one advantage IF your vehicle likes the early addition of vacuum advance. It means the total mechanical advance will not need to be altered much, if it all, to be proper for Wide Open Throttle.
Option 1 allows the idle to be set up independent of vacuum advance needs.

It is critical that total mechanical advance (including base timing) is not too high during full throttle, high load. If you hear it ping, it is detonating. Get off throttle and reduce timing.

if you are on ported (my choice) I would hookup the hose/adj its tip in point after the initial/total/springs have been finalized. If on manifold I would want it to come in sooner (CW with a 3/32 allen wrench, start with 2 turns), so the timing is stabilized at your hot in gear idle speed as opposed to it coming in right where your idle vac is as you posted but I ain't up on manifold so I would Google it as there are some really in depth/comprehensive threads on it. I'm gonna try it one of these days as it might be the way to go. Ported was one of Ma's partial solutions to emission regs on for the most part DD engines & your number sounds hotter than stock.

Thanks both for taking the time for replying.
The engine is modified but not radical.440+0.30 ,the compression ratio is very low,Mopar alu heads and a lunati voodoo 60303 cam.I have a 750 holley vacuum sec. that i want to change for a double pump carb.

Yes ,it is hooked over the carb butterflies (ported).Right now,the initial timming is set to 16 degrees btdc.

Car runs horribly until it almost reaches the working temp.No overheating issues.
Once it warms up,it fails under heavy acceleration.Is a 4 speed ,if the pedal is violently pressed while running in second gear, it dies for a moment,makes a detonation and spins the tires .

OK. I'll try to help.
The warm up issue is due to choke setup, or lack of it.

What Holley List number?
(That's the number stamped on the Choke tower if there is one).

The timing is OK starting point.
What is the rpm at idle (when warmed up)?

It would be good to measure the timing curve (with no vacuum advance).

When you write "makes a detonation", do you mean loud Bang? And was it in the exhaust or intake/carb?

Check the float level & the accel pump tip in, (should be an immediate squirt when the throttle moves).

Mattax,
I removed the old electric choke and installed a manual one from Holley.
However i only use it for starting the engine.Once that it is running,i need to open it,if not the engine stalls.
The carb is a 3310 (click on the picture):

Both jets are 70.Yesterday i mounted a 4.5 hg power valve.
Rpm at idle is around 850 and it falls to 710 if i remove the vacuum advance hose and plug it with my thumb.
"Bang" or "Pop" came from the exhaust.
Plugs are very black with a lot of coal.Also the car has been at idle most time sine i started it after the rebuild.
I did the driving tests on a closed parcel and i only can drive it in second gear as much.
I cannot drive it on the road until it pass the thecnical inspection and i contract a car insurance for it.
Thanks.

You are in the transition slots. I would definitely check float level/float saturation/pump psi as the black plug as you know is is beyond pig rich which is killing mileage & will wash the cyls. I missed if your initial is in the ballpark. EDIT maybe the excessive idling/goosing the throttle has blackened em to some extent but first I would get your initial up to 15 or more (I run 18 on a bone stock 85 318) with the stock cam adv 4 degrees.

OK. It is difficult to setup a choke, even a manual one. Later you can see if anything can be done by adjusting the fast idle relationship to the choke position.
On an older automatic chokes there is a vacuum actuated 'pull off'. When the engine starts running, the choke opens further due to this pull off. Your new manual choke should cover that vacuum hole. I can send or post a picture if you need.


3310-2 Should be a good carb as long as it has new gaskets.


According to Holley's List, it originally came with # 72 Primary Main Jets, and a 6.5 Primary Power Valve.
Neither of these will effect low rpm.


Just to verify, the vacuum advance hose is going to the capped nipple in the photo you posted?
If so, then the only way vacuum is going to that port is that primary throttle blades are open to far at idle.
The pop in the exhaust is probably a combination of ignitable mixture (unfired fuel/air) and late ignition timing making it hot enough to fire at or after the exhaust port opens.

Based on what you've written, the initial (idle) relationship of the throttle blades to the transition port needs to be brought into correct range. The transition port below the throttle supplies fuel, above it supplies air.*

Remove the carburator and visually set the primary throttle blades so between 0.020-.040" of the transition slots show beneath the throttle. WRITE a note to yourself of how much the idle speed/throttle screw can be turned from .020 to .040"
This way, you won't have to take the carburator off again to know where it is at. Use a drill bits or spark plug wire gages to estimate the opening.

Clean the spark plugs, start the engine (opening the throttles as much as needed to keep it running). Increase the base timing from 16 to 18 or even 19 degrees and see if it will idle when the throttle is brought back down to the .020 to .040" range. You'll have to adjust the idle mixture screws several times as the engine warms up and as you bring the throttle position down. The mix screws can be anywhere from 1/4 to 2 turns out, but something around 1 to 1 1/2 is most common.

If this procedure alone does not succeed in allowing the throttle blade to be closed to the proper range, there is another factor we can address. This is the air supply.
If the engine idle speed is too low with the throttles in correct position and enough timing, more air can be let pass without opening the primary throttle blades.

Traditionally, this was done by drilling a small hole in each throttle blade. Holley did it at the factory on a few carbs.
Other places air can be let in is from the PCV valve (using a fixed orifice) or changing/modifying valves. With a vacuum secondary carb like yours, another method is to slightly open the secondary throttle blades. A little bit should not effect the secondary opening performance. Again, WRITE a note of how much you turned the screw. Otherwise you lose the factory baseline.

*Illustration of proper throttle to transition slot relationship is shown in Mike Urich and Bill Fisher, Holley Carburetors & Manifolds [Holley Part No. 36-73] (HP Books, 1987) pp. 121-2
edit: The section is called Special Procedures for Wild Camshafts.
I'll try to link but you may have to google search as links to copyright material are often temporary.

Sorry for the delay.
Haven,t got the time for removing the carb this week.
That Holley book looks interisting...
About the plugged nipple in the picture,it has been always plugged in.
The vacuum advance is hooked to the lower port of the carb.
subir fotocertificity.com

I will post as soon as i remove the carb to check the transition slots.

That is ported vacuum from the main body !

No problem, just happened to look in this morning.

The capped nipple is ported vacuum source.
Your vacuum advance hose is currently attached to a manifold vacuum source - see attached picture.

(You can always verify the passages and connections for yourself with pressurized air or carb cleaner.)

Urich and Fisher's book is very good introduction with useful pictures and diagrams. Any edition will be fine. At least in the US, a used copy can be bought for much less money.
Urich was Vice President of engineering at Holley in the 70s or 80s.

Coke,
Since the vacuum advance has been attached to the manifold vacuum, it probably was adding to the base timing.

If you want to check base timing. Remove the vacuum hose from the carb and cap the port; Or, remove the vacuum hose from the distributor and plug it. I use a golf tee to plug vacuum hose.

I did cap the port for checking the initial timing...However you are right.The port where the advance is attached ,takes vacuum under the primary butterfly.I really thought it went above them:





Today i had time for removing the carb and taking some pics,


The nipple with the rubber cap that appears in my first post picture, is also connected to a hole wich is under the passenger side primary butterfly.


As far for the transition slots,as you can see in the other pictures ,the primary show an important portion of them under the butterflies
The secondaries are completely covered off for the butterflies :


There is a small screw for regulating this on the secondaries:


Also have found this ,wich is very useful:
http://tmpcarbs.blogspot.com.es/p/blog-page.html?m=1

However it talks about the primary transition slots and how much they should to show.Does not mention how much for the secondaries T-Slots.

I ain't sure how far the secondary slot should be showing but I would resolve that/bump the initial to 18/can on ported/reduce primary slot length to a "square" & tune from there.

Ok i will do
The only thing is that i need to remove carb for adjusting the secondaries slots.

Also,what i have noticed is the primaries do not show the slots in the same lenght.
I can adjust the left one showing 0.020" making the "square".On that point the other one would be showing 0.035" making a rectangle more than a square.

Passing this point also would affect the vacuum advance, because the only ported nipple that i would have for it, is connected to a hole that is just covered by the butterfly.

I ain't positive but I'm thinking something (throttle blade?) might be bent as I'd sure think they should be equal. Until you find out for sure (& if it needs to be addressed) you would not want any less than a square on the one.

if the pedal is violently pressed while running in second gear, it dies for a moment,makes a detonation and spins the tires . [/quote]


Has someone put a small bolt and nut to make the secondary open manually? Seen it a bunch of times. Engine falls on its face and then goes like a scalded dog. If someone has put the bolt to manually open secondary, take it out, problem will more than likely be fixed.

I would take the vacuum hose off of the vac. advance and plug the port. [Unless your trying to get better MPG or need to pass emissions you don't need vacuum advance]. Then set the total timing to 38*. drive and listen for ping. If pinging reduce timing 2* and re test drive. If not pinging at 38*, try 40*. With a bigger than stock cam I would recurve the dist. to get around 20* at idle and 38* by 2200rpm. Remove heavy spring and add Mr. gasket springs, both on same side. Leave the weaker spring alone or make it longer by 1/8" so it doesn't have to be stretched to go on pins. Be careful not to over stretch it or you will have to find another one.

Working through the posts in reverse.
Slot lengths
A slight difference in slots is common. 0.015 ? I don't know. Go ahead and see if there is anything to see. Flashing from the machining is possible, slight tweak to the throttle blades?

My gut reaction is if nothing obvious, Leave them the alone. A person with lots of experience and skill in tuning the low speed might teven them out by extending the slot or chamfering the blade. But if they are made longer it will make tuning harder. Then the blade may need to be shut more which will result in more slot above the throttle and everything may need to be redone (idle air bleeds and idle fuel restriction).

If you want to make a little more even, first measure the width and total length of both. If the shorter one is wider, then leave it alone. Area under the blade will be similar.

Initial Position
Anywhere from 0.020 to 0.035" of the slots showing under the blade will be correct on an older Holley like this. There is no exact magic number. There is no exact magic shape.
WRITE a note to yourself of how much the idle speed/throttle screw can be turned from .020 to .040"
This is the adjustment range you can work with while tuning. With this particular carburator, the upper limit with be the port for the timed vacuum. It must not be exposed at idle.

Secondary Slots
They vary depending on the carb and application. On a carb with idle mixture trim screws on all four barrels, some secondary transfer slot may be exposed. On a carb like the 3310, the secondary slots are pretty high up. The secondary idle circuit's job here is to help insure good fueling when the secondaries start to open.
It's best to leave the factory setup. However, if there is need to get a bit more air in at idle, cracking them a hair more open is one technique supply the additional air needed.

Side note about TMP
The pictures are nice but be careful with anything from Troy! He has some good info but some bad info too. Worst - he won't acknowledge what he doesn't know but will argue ad infinitum. It got so bad he was tossed from Speedtalk forum... that takes real effort.

Timing
Let's work with what you have. Yes as you figured out, because of the vacuum port exposure, the initial timing was not quite the same as what was set. If you want to try 18 initial instead of 16 that's fine, even 18. The important factor is that it is stable within the idle rpm range. And of course, that it starts easily both hot and cold. (Too much initial timing can make starting more difficult.)
You should find that increasing the timing at idle will allow the motor to idle with the throttle blades more closed than you had before.

edit: The idle mix trim screws will probably have to be adjusted each time a change is made to the throttle position.
The engine should respond to changes in the mix trim screws.

edit 2: The engine will not ping driving around at 38*. That's a good test for top end but needs to be done under full load 3rd or 4th gear at wide open throttle. Coke wants to register the vehicle, so wide open throttle can be dealt with later.

Thanks for all the replies
I will try with the 18 initial and 38 total timing and also without vacuum and connecting it to the ported nipple.
No idea what is the issue with slots exposure.I try to check it again with a good gauge set,but the difference is visible.

There is a thing that i don,t understand:
Once that you have adjusted the T-slots exposure, Can,t you touch the RPM screw anymore?
what about if you need more Rpms to keep the idle ?you are going to loss the slots adjustment again.
The same for the ported vacuum cicuit,because the hole is going to be exposed under the blade and is going to turn into a vacuum manifold port

If you mean for the fast idle cam, yes you would be opened past the square for that time till you are off of it & yes above idle you would be on manifold.

I had a problem on my 300 when we still had emissions in Missouri on our old junk. One bank would get in limit for hydrocarbon and the other wouldn't. Turned out it was uneven transfer slot exposure. About like yours maybe a little worse. I tweaked the throttle blades to get them even and passed emissions easily. You should set your blades so you see aprox. .020 transfer slot. If you need to open them more than one turn of the idle speed screw to gain satisfactory idle speed, there is a mod to fix it. Drill a .062 hole in each primary throttle blade between the shaft and the transfer slot, then reset the blades to see .020 of the slot again and install carb and start. If you still need more than a turn open holes to .078 and try again. Many cars that were factory equipped Holley carbs had the holes in the throttle blades.

One thing at a time. Do just the idle. Total later. If you have timing tape or a dial back light, you can measure and plot the advance curve. But that will come later.
Plug both vacuum ports for this portion of tuning and testing.

Not doubting that. Just offered some possible causes and a good reason to take action carefully. The area below the blade will be the fuel restriction and the area above will be the air restriction. Length alone is not the only factor. As its an older version, widths are probably the same, but these are things to look for.

Sure you can. You can adjust the screw a little more or less as long as the throttle plate is in the range of .020 to .040" on the slot.



A. This is why I wrote that it is important to measure and write down the amount of slots shown for each 1/4 turn of the rpm screw. (aka idle position screw or throttle stop screw). With the information written down, you will always know where the throttle blade is without removing the carb. This is also handy when using the screw to hold the blades open for testing other things like timing.

B. With increased initial timing, the rpms at idle will increase.
Yes, you may have to start and warm up the engine with the throttles more open. Then increase the timing a couple degrees before reducing the rpms with idle stop screw.
Next readjust the mixture screws for highest vacuum or best rpm depending on what you have. Then set each a little richer (1/8 turn) since under load it will want richer. It is likely they will not be equal as the tranfer slots are unequal and also the distribution on a manifold is rarely equal.

This procedure above may have to be repeated to get a strong idle.

1) There is no magic number or magic shape. 0.025" or square is a good starting point. Over .040 will be a problem because it effects the off-idle driving. You must know that the "idle circuit" provides fuel for most low speed driving and even some cruising up to 2500 rpm or more.

2) If the engine still will not idle strongly without opening the throttle blades more, and you don't want to increase timing any more, then it is appropriate to let more air in. This can be done by any one of several methods; drilling the throttle blades, making an adjustable airbleed in the center of the body casting, cracking the secondaries open a little, or changing the PCV valve restriction.

I personally like drilling the blades. It provides air right next to the fuel coming out of the idle ports so should help mixing. As Dan pointed, this method was even used by Holley on some new carbs.
However, especially because you are in Spain where parts may not be as readily available, my suggestion is to test out with a reversible method. Either the PCV valve or the opening the secondaries a small amount will be good tests.

see above

Robert mentioned the fast idle cam screw. On the choke mechanism there probably is a fast idle cam and adjustment screw. This screw will change the relationship of throttle to the choke. It may be of some help in getting the choke to work better for you. Try setting it so the throttles are open more when the choke is only part open - the situation when the engine has just started running and you have reduced the choke.

Much older vehicles had seperate manual throttle and manual choke cables. Go back far enough and there was also manual timing...

Urich and Fisher's illustrations explain what happens when the throttle is not in the currect range on the transition slot. google urich Fisher wild camshafts if the link above doesn't work.

Now the first time I drilled the throttle plates (on a 3310-3) it was long before I knew about the timing. Heck I hardly understood how the carburetor worked. It did idle better, but had no power to get up the hills on my drive home. None, zero, zilch. We experimented with some different size wires in the idle air bleeds until it worked well again. Every situation is different, but if you do have to add air at idle, then you may need to make the off idle a bit richer the same way I did.