Setting Timing and adjusting vacuum advance canister?

[91r/t]

Ok.......I am digging into this fine tuning bigtime on my 500" stroked 440 I have. I have an aftermarket Accel distributor in it much like the MP electronic distributor. I have the advance plate set at a 15 degree timing curve. My initial at idle is 20 degrees and total is 35 degrees all in around 2800 to 3000 RPM. My question is hooking up the vacuum advance. It runs alot better with it hooked up. Now here is the problem. It isn't doing jack at idle. When I rev it up it starts advancing the timing with the vacuum hooked up which is normal. I have an adjustable vacuum canister on it but that doesn't seem to do what they say. Everyone says that the amount that the vacuum will add to the timing is fixed and that adjuster just determines at what vacuum it starts adding the timing. Well that's a crock on this one because adjusting that canister with the allen adjusts how much total timing with vacuum is added. I have read multiple sources saying around 50-55 degrees total vacuum timing is about where you wanna be. Well my motor pulls about 12 in/mg vacuum at an idle even with my large cam and it doesn't change the timing there whether its plugged into the vacuum advance or not. I am not sure why it won't activate the canister unless it needs alot more to start opening it???? Any thoughts? Maybe the canister isn't designed to start opening until there is more vacuum??? Just weird because they swear that adjusting the allen changes the vacuum in which it opens but I am reading that in some other canisters the allen controls the amount of timing added and the opening point is fixed.......Insight?

 

[72RoadrunnerGTX]

First, what carb port are you using for the vacuum advance? Above or below the throttle plates? Ported or manifold vacuum?

 

[91r/t]

I knew that was coming lol......I cannot use ported because my carb doesn't have any. I have to run manifold which will and should use the vacuum advance on the idle circuit. My reasoning is the car likes intial timing. It won't turn over hot on anything above 22ish degrees. It was set at 25 and loved it but won't start hot so I was hoping to use the advance to my advantage. it is an 850 Quickfuel carb.

 

[72RoadrunnerGTX]

Ok, manifold vacuum. When connecting, there is no advance on your ~12 inches of manifold vacuum at idle? The allen adjustment adjusts the diagram spring tension working against the vacuum applied. The vacuum cans do have a fixed total advance limit as well. The allen adjustment is used to adjust total vacuum advance relative the vacuum level applied, too tight can effectively block any advance on low vacuum levels. Back out the allen adjustment until you get the advance you’re are looking for.

 

[91r/t]

I was all the way in to all the way out and nothing changed......No advance except the total amount of advance when i had it at constant throttle around 2500 RPM which was weird. I have had a couple people tell me some of the canisters have a set vacuum application and the adjuster adjusts amount of timing applied. I thought they were nuts until I have screwed with this one tonight.

 

[72RoadrunnerGTX]

Have you tested the vacuum advance for holding a vacuum, hand held vacuum pump?

 

[91r/t]

That is next I guess.......That will give me the exact amount it starts opening and fully opens. That would also tell me if it is ruptured and needing to be replaced. Ran out of time tonight.

 

[72RoadrunnerGTX]

For comparison only, this is stock vacuum advance adjustment specs. for a ’72 440 HP application.



Note, distributor degrees listed, double for crankshaft degrees.

 

[lewtot184]

my guess is the vacuum unit may have issues if you see nothing using full manifold vacuum. the vacuum canisters do come with different springs in them; some heavy some light. I prefer ported vacuum for vacuum advances and try to limit them to 50 degrees total (this includes initial and centrifugal advance). I hook up a vacuum gauge to the hose and drive the car to see what it's doing and adjust from there.

 

[91r/t]

Yea I would use ported, but like I said my Quick Fuel carb doesn't have any ported vacuum ports at all. The metering plates where they usually are are blank. It is a higher end race style carb so I am not surprised. All three ports are below the throttle plates. I was hoping to use the manifold vacuum anyway to clean up the idle with some additional timing without hard starting issues. I have to total with both around 50 degrees and total mechanical at 35. Most initial I can get is around 19-20 without the vacuum or I begin to have hot start issues.

 

[91r/t]

Maybe the spring is just stiff in the canister and I am actually adjusting the vacuum to open and it looks like I'm increasing the timing more each turn......not sure. If I do find its a bad canister can anyone recommend a good adjustable unit to grab? It would be neat to have one that came on at a fixed value like 8in/mg or something and you could adjust how much timing was added with the vacuum advance.

 

[lewtot184]

Maybe the spring is just stiff in the canister and I am actually adjusting the vacuum to open and it looks like I'm increasing the timing more each turn......not sure. If I do find its a bad canister can anyone recommend a good adjustable unit to grab? It would be neat to have one that came on at a fixed value like 8in/mg or something and you could adjust how much timing was added with the vacuum advance.

turning the adjusting screw clockwise reduces the spring tension, counter clockwise increases tension. it would be nice if you had one of those small vacuum testers to attach to the canister while your watching the plate move. I would think that 12" of vacuum at idle would be more than enough to operate the vacuum advance. maybe there's an adjustment issue. I have noticed that the more degrees that are in the units the stiffer the spring. there should be a number on the arm that attaches to the advance plate stating how many degrees are in the unit. I've also found that around 18 degrees initial timing is about the limit for easy operation.

 

[91r/t]

Yea I will look on the arm. I remember it having a number the other night when I had it out, but can't remember what it was. I agree with the initial timing on a mild to medium motor but this thing has a comp xe295hl in it which has 110 degree lobe separation with .564 lift which is kind of radical I think compared to most. Also Holley's/Quick Fuel usually run fat at idle so I was thinking the initial would help clean that up. I am going to put the vacuum tester on it tonight if I have time and check to see at what value it engages and operates at. That will tell me alot.

 

[72RoadrunnerGTX]

Total vacuum advance is limited/set by a notch in the arm hence the number stamped on the arm. May also find a letter, “L or R” referring to left or right hand distributor rotation.

 

[91r/t]

Awesome....So maybe when I order one if need be I will order one with the number of advance I want to add?? That would be nice. The number is crankshaft timing cut in half correct? So if the number is 6.5 it adds 13 degrees of crankshaft timing to the mechanical number?

 

[lewtot184]

the old 7 and 8 degree units are probably extinct. most stuff today is 10-12 degree units. you might want to take a good look at the idle air bleed and idle jets. I've had more success tinkering with those for a cleaner burn than trying to make the timing clean stuff up.

 

[91r/t]

I did set the idle air bleeds with a vacuum gauge to .5 turn from fully in. It has 4 corner air bleeds and that is as much as it will take. I did find the floats to be sky high last night too so I adjusted them way down. I have been going 90mph with this project and overlooked them. I am going to run it this weekend with the new float settings and see if that helps a bit. I am reading that if they are way too high they will run pig rich at idle and hard accel. Now I probably should go back and check the air bleeds with the vacuum gauge to make sure it wasn't the floats causing it to run rich. Maybe they will need to be turned out a bit.

 

[lewtot184]

I did set the idle air bleeds with a vacuum gauge to .5 turn from fully in. It has 4 corner air bleeds and that is as much as it will take. I did find the floats to be sky high last night too so I adjusted them way down. I have been going 90mph with this project and overlooked them. I am going to run it this weekend with the new float settings and see if that helps a bit. I am reading that if they are way too high they will run pig rich at idle and hard accel. Now I probably should go back and check the air bleeds with the vacuum gauge to make sure it wasn't the floats causing it to run rich. Maybe they will need to be turned out a bit.

the air bleeds are small screw in restricters on top the carb. the idle jets are small 6-32 screw in set screws with orifices mounted in the metering blocks. i'd start at .073" air bleeds and .036" idle jets. big hydraulic cams can get rather ugly to tune, but doable. if your running an auto trans check vacuum in gear. you may have a power valve fluttering.

 

[91r/t]

4.5 power valve with a 4 speed so its constant and definately not opening which is good there. If the float thing doesn't make it any better I will dig into the air bleeds.

 

[91r/t]

Ok indeed my carb does have a ported plug for vacuum. Hence why I am getting no advance at idle. Had it plugged into the ported side.....this carb is retarded. They put both ported and manifold side by side same size on the bottom plate. Ok then I checked the canister for action.....activates at 15 in/mg which explains why I needed to turn it down to 10. Took the car for a ride with the advance hooked up. Now i know why some people disconnect with a big motor. Runs worse and backfires like crazy off the throttle I think due to the advanced timing. I plugged it up and back to normal. I am running pig rich at idle and need to add air and less fuel bigtime. I mean to the point I reak driving the car and it smokes out of pipes at idle. I am going to take this offline for the carb adjustment and PM lewtot184......I have pretty much made up my mind I need to get it running correct before I even think of adding the vacuum advance and even then I will probably run without it. Not concerned with pollution and gas mileage in a 500in big block. As always......Thanks for all the info!!!