When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
....... I am indeed running about 11 inches of vacuum at 800 RPM with no Vacuum advance connected.
Great information. You have a camshaft with very low idle vacuum. That is greatly aggravating your tuning problem. BTDT. So now you know you need a can that is "all-in" 2" below that, or 8-9", and will "completely release" below that in the 5-6 range.
13.5 inches of vacuum with vacuum advance canister that .I am certain is bouncing around in the middle of its throw.
Yes using a cam that is not "all-in" at idle will cause the idle to be very unsteady.
what I want/need is a can that starts pulling at about 7 or 8 inches of vacuum. And is all in at 12 crank degrees at 11 inches of vacuum.
That might work, but a can that is "all-in" at 8-9" would be safer. You have a very big duration cam.
all the part numbers on Lars papers dont match up with current part numbers on Summit Racing and or other similar online retailers.
Correct you will not find an exact fit, especially on the 10 degrees part. That requires modifying and limiting one that is close in the vacuum department.
Originally Posted by 4-vettes
....I find if I shim the back side of the slot to hold the rod slightly into the can. It raises the start of the advance to, well, where ever I want it. About 8 inches of vacuum as a start point I'm thinking would be good. I'm thinking it's so low now that I may be getting some additional timing in when I floor it. And that would explain my less than great power when I floor it.
soooo, weld in the back side of the slot to hold the rod slightly into the can. File it until I can get 8 inches of vacuum to just start to move it. (How about 5-6"?)
Now keeping in mind, this thing is giving me roughly double the vacuum timing I'm looking for.
weld in the front of the slot and file it till the rod hits at about 11.5 Inches of vacuum. (or even lower 8-9-10") Now I realize that this may not be perfect. But I'm thinking it'll get me in the ball park.
Congratulations! Yes you are on the right track! But flip your reasoning around. The vacuum advance is "all in" at idle at your 11" vac, giving you about 10 degrees extra advance. That's where it "starts". That is the important spot. As you open the throttle, on the road, the vacuum will drop to 9" vac and start to "release" some of this stored up 10 degrees advance. By the time you get to half throttle, at 5" vac, the "extra" advance is all gone. The idle vac set-point is 20 times more important than the lower vac setting, it's just , well, lower. It's just not as adjustable. If the lower point is a few (3-4-5) inches less it's good. The vacuum setting must be 2" or greater below your idle vacuum reading or the idle will hunt as the advance moves. 2" is dead minimum, 3-4 is OK.
The big issue you are going to have is to find a can that will be "all-in" at 9" vac. Modifying your 10 degree Tawain can may achieve that. You'll just have to check.
Any can you can buy will have to many degrees advance and will have to be limited in the same way. So you might as well work with the one you have.
.
I sense your frustration. Tuning the vacuum advance seems to be one of the most confusing and counter-intuitive tuning issues that goes on with mechanical advance ignition systems.
You might be able to achieve what your engine needs with the proform can. Not your original one. And not your dead one.
The only store bought can I can see that comes close to what you need for an HEI is a VC1703. Because it is "all-in" by 7-9 inches vac.
It might even give you the 10 degrees advance advance as-is. (5 degrees distrib)
Can you get NAPA there? It is Echlin Ignition part ECH VC1703 Echlin is a huge manufacturer. Surely it must be available in down-under.
If it's not available over there, perhaps one of your USA Corvette Forum friends could ship it over there for you.
The other option is summit and get this Crane adjustable unit. It should be able to do anything you need. The vacuum set-point is adjustable, it comes complete with the max advance limiter, and comes with lighter springs also.
Last edited by leigh1322; Apr 27, 2020 at 10:06 AM.
O.K. , so far not much in the way of a part number that works in todays world. So I go out to the garage, armed with a vacuum gauge. And I do some playing around with the vacuum can that came with my Proform "Race" Distributor.
yes laughable, anything but "race".
Anyway, it seems the other 2 cans are just too old to think about getting right. The Taiwanese can. Well it's junk but what do I have to loose.
I find if I shim the back side of the slot to hold the rod slightly into the can. It raises the start of the advance to, well, where ever I want it. About 8 inches of vacuum as a start point I'm thinking would be good. I'm thinking it's so low now that I may be getting some additional timing in when I floor it. And that would explain my less than great power when I floor it.
soooo, weld in the back side of the slot to hold the rod slightly into the can. File it until I can get 8 inches of vacuum to just start to move it.
Now keeping in mind, this thing is giving me roughly double the vacuum timing I'm looking for.
weld in the front of the slot and file it till the rod hits at about 11.5 Inches of vacuum. Now I realize that this may not be perfect. But I'm thinking it'll get me in the ball park.
pulling the bottom stop up I should be able to compensate by resetting the distributor. Once in place and base timing set. I can measure exactly how much vacuum advance I am getting and if necessary, file abit from there.
I'm thinking what do I have to loose?
As per the adding a spring idea. I've looked at this and for the life of me can't figure out how to install such a spring.
still , would be great to get a part number that I can order.
Pics to help you out.
This is before I welded up the slot. You could drill a small hole in the plate to get a straighter pull if you wanted to, but this has been working since 2013.
or here’s another idea drill hole tap it and put a shortened 8/32 bolt in, make sure it’s glued or tac welded or it’ll probably just pull out, then loop the end of the spring over it.
many ways to skin that cat.
Last edited by REELAV8R; Apr 27, 2020 at 10:54 AM.
Some really great info, thanks much guys!
no , no Auto Zone over here. Parts for Chevy's not readily available over here at standard Auto Parts stores. A few specialty American auto parts places. Generally very high priced. And not near my home so I end up paying shipping anyway.
I'm thinking I will start by modifying the unit I have. If I muck it up no great loss as it's no good as it is.
If that doesn't work out I'll look into an adjustable unit. Just a little leary of the adjustable units as the one I bought years ago never worked as advertised. Now I have a lot more cam and 1.6-1 rockers. I need to get this right. I'm just not getting the power out of the engine I should. And she runs hot at low speeds and idle.
I'll be playing with this over the next 2 days. I'll keep you all posted.
Some really great info, thanks much guys!
no , no Auto Zone over here. Parts for Chevy's not readily available over here at standard Auto Parts stores. A few specialty American auto parts places. Generally very high priced. And not near my home so I end up paying shipping anyway.
I'm thinking I will start by modifying the unit I have. If I muck it up no great loss as it's no good as it is.
If that doesn't work out I'll look into an adjustable unit. Just a little leary of the adjustable units as the one I bought years ago never worked as advertised. Now I have a lot more cam and 1.6-1 rockers. I need to get this right. I'm just not getting the power out of the engine I should. And she runs hot at low speeds and idle.
I'll be playing with this over the next 2 days. I'll keep you all posted.
exactly! no great loss since what you got now doesn't work.
When you weld up the slot wrap a small piece of wet cloth around the metal between where you're welding and the vacuum diaphragm. Or alternately tack, cool, tack cool. Don't want to cook the rubber inside the vac can. You can lever the rod out of the way while your welding.
Hot @ idle and low speed is indicative of retarded timing, so on the right track here.
O.K. guys. I am going to try to explain this one more time , With Pictures! I realize your trying to do me a favour by over and over again telling me to use this guy "Lars" part numbers. I have read his papers. His tuning method doesn't sound any different from anyone else I've heard over the last 40 years.
put in this part number to a search.
VC1703.
See! There is no VC 1703. So they change it to a tire.
Put in this part number, VC1838
See, no such number so they try to find something close. No good.
Look up standard motor products vacuum advance canisters.
Note the part number, lots and lots of them with part numbers like this and NO SPECS! none of Mr. Lars part numbers match up. Sooo, I started this thread trying to find a part number for a Vacuum can that will work. I have gotten some great suggestions, and I thank you all for that. And I'm working on it. But wouldn't it be great to be able to order a part that might work. With a part number that is in use today.
So today I welded my cheap Taiwanese vac can.
After a little more filing, I put it in the distributor and vacuum tested it. I'm going about it backwards. But with nothing to loose. Tomorrow I'll get it assembled and timed up. How much vacuum advance do I have??? Don't know. Was working on hitting the stop at 10 inches. Time will tell.
Last edited by 4-vettes; Apr 28, 2020 at 07:38 AM.
I think you can make that can work.
Here is a picture of my 23 degree vacuum can with a limiter installed to limit the rod movement to 11 degrees. I am sorry I don't have a measurement for you, but it looks like the gap is only about twice as wide as the width of the rod. Where it used to be over three times. That should get you close.
Last edited by leigh1322; Apr 28, 2020 at 10:04 AM.
You only needed to weld ahead of the shaft to limit its travel forward, behind the rod does nothing.
you still have a problem with that can requiring 30” of vac to pull it fully.
personally I would use a can that takes the normal 17” ish of vac to modify. But whatever you got I guess will do.
if you have a vacuum gauge some vac hose (pull if off the car if needed) and a “T” you can see where it starts to pull and pulls fully and modify it appropriately with a spring to get it to pull fully ( your predetermined distance) at a specific vacuum level. Putting it on and off the car is gonna take A lot of effort and time that is not necessary. For fine tuning you may need to pull it off and file a bit.
I think you misunderstood. It was original 43 year old can that needs 30 inches. This is the new can that came with my proform distributor. It started pulling at 3-4 inches of vacuum. I felt too soon. Was all in with 23 degrees of crank timing at 20 inches.
I welded the back side because I don't want advance timing "floating in" when I floor it. By welding the "other side " I now have it starting to "tip" at 7 inches. I'd like to get about 10 degrees of vacuum advance. Retime the distributor to run about 37 all in, 17- 18 idle , no vacuum. Then drive it and see how it goes.
Summit Racing is not going to carry a NAPA Echlin Pt#
VC 1703 is a NAPA pt#
Jebby
yes, thanks Jebby. I actually do get that. But you see I have no access to a NAPA store on the other side of the planet. So I was wondering if anyone knows part numbers from other suppliers that I can access. If my experimenting with the proform can doesn't work out, I'll try to get someone in the States pick one up and ship it to me.
I think you misunderstood. It was original 43 year old can that needs 30 inches. This is the new can that came with my proform distributor. It started pulling at 3-4 inches of vacuum. I felt too soon. Was all in with 23 degrees of crank timing at 20 inches.
I welded the back side because I don't want advance timing "floating in" when I floor it. By welding the "other side " I now have it starting to "tip" at 7 inches. I'd like to get about 10 degrees of vacuum advance. Retime the distributor to run about 37 all in, 17- 18 idle , no vacuum. Then drive it and see how it goes.
I see where you are going with that. Every 1" of vacuum moves the rod x thousandths.
So you could set it to 10 inches on a vacuum gauge, measure the rod location, and build weld to that spot. Same with the 5" measurement on the other end. If you look closely at he vac limiter I got from Lars, he restricted it on both ends of the original movement.
Last edited by leigh1322; Apr 28, 2020 at 06:49 PM.
That's very interesting Bazza. I've lived in Queensland for over 12 years and have never seen a NAPA store. We have Super Cheap, Auto Barn and Repco. I'll definitely look into that! But who knows if they carry the same line of parts here. Can't hurt to check it out.
O.K. so I spent a half a day messing around with my distributor.
At this point it was giving me 9 degrees of vacuum advance. Took it back out. Filed it a little more. Now I have 10 degrees of vacuum advance. One of you guys mentioned that perhaps my mechanical curve was coming in a bit soon. So, I double checked that. I have all kinds of springs from various tuning kits I've bought through the years. So I started experimenting. At the end of the day I ended up with base timing of 19 degrees at 860 Rpm with a solid 13 inches of vacuum. 38 degrees full advance. With the timing just starting to advance at 1,100. Total timing at 2,700. And 10 degrees of vacuum advance with the vacuum just starting to tip at 5.5 inches of vacuum and all in at 11 inches of vacuum. A fair bit of playing around with different combinations to get to this. Now to wait for the restrictions to lift so I can test drive it.
O.K. so I spent a half a day messing around with my distributor.
At this point it was giving me 9 degrees of vacuum advance. Took it back out. Filed it a little more. Now I have 10 degrees of vacuum advance. One of you guys mentioned that perhaps my mechanical curve was coming in a bit soon. So, I double checked that. I have all kinds of springs from various tuning kits I've bought through the years. So I started experimenting. At the end of the day I ended up with base timing of 19 degrees at 860 Rpm with a solid 13 inches of vacuum. 38 degrees full advance. With the timing just starting to advance at 1,100. Total timing at 2,700. And 10 degrees of vacuum advance with the vacuum just starting to tip at 5.5 inches of vacuum and all in at 11 inches of vacuum. A fair bit of playing around with different combinations to get to this. Now to wait for the restrictions to lift so I can test drive it.
Those sound like pretty good numbers. I bet it runs a whole lot better now. Maybe a tweak here or there if it wants something a bit different.
) I think you have to log in to get anywhere with it.
