Vacuum advance pod

[rednesss]

I have yet again a dead vacuum advance pod. I've gone through at least 3 in the last couple of years. Anyone else experience this? Is it just the ethanol that's eating them up?

 

[AJ/FormS]

I don't see ethanol as the problem. Mine is about 13 or more years old and has always been on an engine that has always run 87E10, and I've always run a manual trans and about 11/1 compression.
Are you running a PCV system?
Why is that important?
IDK, that's just what popped into my head.
I guess the better question is are you having issues with blow-by gasses?
I guess what I'm getting at is; how do you suppose ethanol would get to the diaphragm? Is it not plumbed to the spark-port, which is plumbed to an area in the carb, just above the throttle plate. So for the bulk of the engines run time, the diaphragm would be under dry vacuum.
I guess you could cut it open and inspect it. I do this by grinding off the folded pinch. Then it just falls open. Then you'll be able to tell if it's perforated,ruptured or all soft and wrinkly.

 

[TrailBeast]

I have yet again a dead vacuum advance pod. I've gone through at least 3 in the last couple of years. Anyone else experience this? Is it just the ethanol that's eating them up?

Don't know, never had one die.
In your area it's totally possible that the fuel is doing it, and one of the reasons I use EFI line for fuel everywhere I can, but that doesn't help a vacuum pod.
I assume your motor doesn't pull enough vacuum to break it, so what's left?

 

[rednesss]

What's the correct vacuum pod for a 1970 340 with electronic distributor? I was going to grab a VC-187

 

[AJ/FormS]

I use whatever I can still order, and that runs as much advance as I can get. Then I cut/grind/file the stops to get me at least 20/22 degrees, which makes my engine happy.
I saw one guy cut the stops right off, and drilled and tapped a hole in the housing to place a screw in a strategic spot so he had an adjustable plate-stopper. Then he really had a "fully adjustable" system. I thought that was pretty clever.
If your engine is stock, that would probably be over-kill.

What are the specs on that can?

 

[rednesss]

I ended up getting a VC-167. I know there's a spec chart out there for most of the cans, my question is more related to what would be the "right" one for a 340 with a pretty run of the mill cam, I usually have about 15-16" of vacuum at a steady state cruise rpm of 2,000. I have an OD so my RPM's are a bit lower than normal.

 

[TrailBeast]

What's the correct vacuum pod for a 1970 340 with electronic distributor? I was going to grab a VC-187

I would have to look it up and of course elevation matters as well.
AJ has it down pretty good.
An adjustable pod with limiting abilities and you can't really go wrong.

 

[AJ/FormS]

I ended up getting a VC-167. I know there's a spec chart out there for most of the cans, my question is more related to what would be the "right" one for a 340 with a pretty run of the mill cam, I usually have about 15-16" of vacuum at a steady state cruise rpm of 2,000. I have an OD so my RPM's are a bit lower than normal.

TrailBeast, I think, is on it too. He's got a pretty sharp tune going on in his ride for sure.
Firstly I'm gonna assume your power-timing and initial-timing and T-port sync are well-established, cuz the cruise-timing has to be the last timing to be tuned. I always map these on graph-paper, before messing them up. Then if I get lost, I can go back to what works.
Secondly;Detonation must not be allowed to occur period.
And then;
Well at your manifold vacuum and cruize-rpm, the the cruise timing might like to be 42 to 52 degrees. It might even like more depending on the build. But getting more is difficult with the factory parts.
But how do you know how much to give it?
Simple. Rev it up in neutral slowly until she hits cruising rpm, then block the throttle to maintain that rpm.Then start cranking in the timing in 3* steps, until the rpm no longer rises. Then reduce the rpm back to 2000. Now read the timing, and record it. Next, retard the timing until the rpm no longer rises, and read the timing. If it it only slows down, then advance it instead until the rpm peaks again. Don't try to find the teensiest change. If you crank in 3* and nothing significant changes, back it up to the last time it did. Badaboom, that 's gonna be pretty close. It could be over 50 and tickling 60, so you'll probably need a timing tape. If the ignition starts to crap out, it's probably because the rotor phasing is too wide,so you'll have to deal with that first.
Now, you'll have to figure out how to get there, from where you're at.
Furthermore, this Vacuum advance is gonna have to drop out pretty quick when you start applying throttle ,to keep the engine out of detonation, with part-throttle loading.So, inside the Vcan nipple is an allen-head adjustment screw which is used as a sort of regulator. You can crank that screw a little this way and that, to affect the rate of advance.You almost need a vacuum pump and timing lite to witness how it works. If you screw it too far, it will,additionally, start to reduce the amount of timing. If the screw starts jamming up;STOP!, you have reached the end of the adjustment range.
Most of the time, some sort of sacrifice is going to have to be made.
With the stock parts getting more than about 42* gets difficult.I got this number by taking a centrifugal rate of advance of .8 degree per 100rpm beginning at 1000rpm; plus 12* initial, equals; 8 + 12= 20@2000. Then I added in 22* from the can, equals 42*. I found 12* initial to be sufficient in my combo. Your engine may like more. Probably not less. The T-port sync should have established it.
Of course cranking in idle-timing will upset your power-timing so you always have to keep that in mind, and after testing, you'll have to fix that.
About the very most you can get is say, 16/18 initial with a very fast centrifugal at all in by 2500, which could be say,18*, and so that could be;18 degrees in 1500 rpm or 1.2* per 100, so that totals 30* at 2000. Add in 22 in the Vcan and that totals 52*. That is probably the max. So now we have established a range of 42 to 52, with a 22* can.
But YOUR combo may not like 30*@2000 under power. It may be wanting to self-destruct with detonation.
Detonation must not be allowed to occur period.
So until the cruize testing is complete, no WOT and easy on the pedal. Now it's road test time. This is a steady state test. It is run on flat level ground, in highest gear,and at exactly the test rpm, and on a day with little to no wind with zero gusting.
So install a vacuum gauge plumbed into the V-can line, and hung where you can see it, then get her up to speed.Get your throttle set to where the mph remains exactly steady@2000rpm. Record the vacuum reading and the total timing including the Vcan timing. Turn around and go back to the start point of the test.Take out 3* of idle-timing, and repeat the test including reading and recording the vacuum and total advance.If the second test shows a higher vacuum reading, that means you were using less throttle to maintain 2000rpm.That would show up as less gas burned per mile. If it shows less vacuum, then that was the wrong way to go. Finally retest with another 3* change, but in the right direction.
If the vacuum reading differences are too difficult or too small to measure, replumb the gauge directly to intake vacuum, put the timing back to the original number and start over.
Once the cruize vacuum is sorted out, and if it's the same as the very first rev-it-up-in-neutral test, and you at that time reworked your power-timing, then you're done. But if it's not, or you didn't, then now is the time.
After this you will need to do a bunch of roll-ons to see if the rate of vacuum advance decay is agreeable to your engine, and adjust it with the tuning screw mentioned earlier.
If all this seems overwhelming, then just bolt on whatever Vcan you got and that's that. It will still be way better than no can at all.

But either way, the cruize fueling might want a little tweaking. Again rev it up to 2000 and block the throttle at that setting. Readjust the idle mixture screws for max rpm. Return the throttle to idle. Make a note of where the screws are relative to lightly seated.write it down. Now enrichen the mixture screws 1/8 turn, and roadtest.

To improve accuracy at determining the cruise rpm, I fabricate a throttle stop, and adjust it to limit the cruise rpm to exactly my target. Then by changing the timing, the mph will change. If the mph increases, then she liked the extra timing, and so on. My throttle stop is a home made device and is a two-stage, so I can get up to speed normally then back out of the throttle and engage the stop. I'd explain it, but Rumble's already snoring.
Detonation must not be allowed to occur period.

 

[kowalski440]

I use whatever I can still order, and that runs as much advance as I can get. Then I cut/grind/file the stops to get me at least 20/22 degrees, which makes my engine happy.
I saw one guy cut the stops right off, and drilled and tapped a hole in the housing to place a screw in a strategic spot so he had an adjustable plate-stopper. Then he really had a "fully adjustable" system. I thought that was pretty clever.
If your engine is stock, that would probably be over-kill.

What are the specs on that can?

I can almost picture that in my head, but it'd be nice if you (or someone) could post one. Sounds like something I'd like to mess with at some point. Small side business idea, anyone?

 

[AJ/FormS]

I think all his work only got him 24 degrees. I'm guessing that's all that is possible,lol.
If you put a vacuum pump on the nipple and evacuate the chamber while watching the pull-rod, you will see the rod stop when the lugs on the arm hit the housing. Those are the lugs that need to be shaved. Or some other stop-system engineered.
I'll tell you a secret tho.
I got me a dash mounted adjustable timing control, that made this tuning a ton easier. It has a range of 15degrees. So I set the timing with the device rotated to 50%, that is to say; I can retard or advance the timing up to 7.5 degrees. Then I buzz the car up to cruising speed. I get the throttle up on my home-made stop and let her settle down. When the mph remains the same for a mile or so, then I crank in a quarter turn,or about 3 degrees of advance, and just keep on driving (Manitoba is mostly dead-flat). If the car gains speed, I crank in some more, but if it lost speed then I retard the timing. The goal is to set the timing to an amount that makes the maximum mph, on the throttle stop. Then when you're all done,and if you gained two or three mph, now you reduce your speed back to the starting point, and you are using less throttle to cruise. In this case less is more; less throttle equals more miles per gallon, cuz the engine is making a tiny bit more torque.
Don't drive yourself crazy trying for that last little bit, it's unobtainable using this method. You'll get to a point where 3 more degrees does nothing. Or wait,,,,,,is it picking up speed? aw gee I'm not sure so in goes 3 more and then it slows down. Chit.
So I left that device hooked up cuz my D can only make about 46 degrees at what I have concluded is the optimum idle and power timings, plus the v-can, at my cruising speed of 2150. But now I reset the device so it can retard 3 and add 12; so with the device I can now cruise at up to 46 plus 12 =58* .
The one caveat tho is to always remember to crank it back when coming off the hiway......... cuz 34 plus 12 equals waaay too much power-timing.
To help me remember,I also have an adjustable rev limiter. So when I go on the hiway, I set the limiter to about 2400.This still gets me a top speed of 70 in overdrive. So if I forget to dial the timing back, I can only get that 2400 out of her, and in second or even third that won't hurt the engine, cuz the max power-timing is now just 37 degrees. And when the engine quits revving then the warning light in my little brain comes on.
You can also use this device to help tune the other timing systems.
Then after you think the timing is right on, then you can start playing with the carb.....................And then it's back to timing.......................
The one timing that changes very little seems to be the Power-timing. The iron headed SBMs I have tuned,on road, with adequate compression liked 35 plus/minus 1. And with aluminum heads, mine was happy with 34 to 32

 

[kowalski440]

Very clever! I like your way of thinking! Thanks for the ideas, I'm saving this thread. That wouldn't
be the MSD knock retard control usually used on supercharged engines, would it?

 

[AJ/FormS]

I believe the MSD does the same thing, but mine was an old Jacob's unit