What’s the easiest way to limit the amount of vacuum advance on a Electronic vacuum advance small block Mopar distributor?
I’ve heard some people weld a little stop on the vacuum advance arm to limit its travel.
I don’t have a welder but I was wondering if some of you guys had some other idea’s that would work.
I was thinking maybe making a little tab of some kind and using some kind of adhesive to hold it onto the vacuum advance arm, maybe small dab of jb weld or something similar.
I’ve come to realize that my carbed 5.9 magnum does not like anymore then around 10-12 degrees of vacuum advance.
Right now my vacuum advance is adding 15 degrees and it’s just a little too much.
I’d really like to limit that to around 10-12.

JB weld some strips of feeler gauge there. Welding is too much heat/messy. First tho I would slow the curve (try 2 turns CCW with a 3/32 allen wrench) & see if the eng likes it.

Easiest, here you go

http://www.4secondsflat.com/Mopar%20Mechanical%20timing%20limiter%20plate.htm

OP wants to limit vacuum advance, not mechanical.

The can's have a number stamped in the arm listing the advance.

STANDARD MOTOR PRODUCTS VC190 goes to 11

My vacuum advance doesn’t have a number stamped on the arm, but with the spec sheet that came with the distributor it says the vacuum advance adds 15 degrees at 18-20hg of vacuum.
I’ve already turned the adjuster screw ccw a couple of turns and at the moment I have it to where it’s adding 11 degrees at 18-20hg.
But the issue is when I’m crushing down the highway in overdrive the vacuum increases to around 22-23hg and that of course adds more timing.
I have mechanical timing set at 20 degrees, initial is at 14 and total is at 34, the advance rate with the springs that are in the distributor starts to come in around 1500 rpm and is all inn around 2700-2800.
I did have the mechanical advance set at 16 degrees with initial at 18 and total at 34 and that created a rough spot around 2400 rpm.
I’ve come to the conclusion that it likes a slower advance rate because when I increased mechanical advance from 16 to 20 which slowed the advance rate down from all in at 2400 to all in at 2800 the rough spot cleared up and got much better.
So for the moment I’m leaving mechanical at 20 degrees with initial at 14 and total at 34 all in by 2800 untill I have some time to play around with different springs to see if I can find an advance rate that is all inn by 3k rpm with 16 degrees of mechanical timing.
The problem is all the advance rate graphs I’ve seen there is nothing in between all inn by 2400 to 3500 with 16 degrees of mechanical advance.
And I’ve also come to the conclusion that the vacuum advance needs to be limited by a few degrees down to around 10-12 instead of 15.

I would note the total advance (mech+vac can) at the offending RPM where it "runs rough" then cap the can & bump up the timing to get the same amount of timing at that RPM & see if anything changes, otherwise there are cans that offer less adv but not sure where/how to search, Google or Rockauto maybe.

Just wrap some wire around the vacuum canister arm. Use different gauge wire to change the amount of limit. I have been running this in my distributor for years, and it works fine.

I’ve looked at different vacuum cans online and the lowest one I’ve been able to find adds 17 degrees which is more then what my current can adds.
I’m going to do some more trials with the timing settings, I just never really have the time to specially right now cause it’s the busy season at my job and I’m working 10 hour days 5 days a week and ok the weekends I hardly have enough time to do the things I need to do around the house and for my family.
I’ll get more time to mess with it this winter but untill then time is pretty much non existent lol

I understand, we ain't going nowhere. when the time comes I would adj the initial/total (initial+slots)/springs/vac can: in order

Well from expierence and trial and error I’ve determined where my engine likes the initial timing and the total timing and mechanical advance settings.
It likes 14-18 initial 32-34 total, and now that I have a distributor where the rotor phasing is within range, I’m basically left with figuring out the correct advance rate and the correct amount vacuum advance.
I know my engine prefers a slower advance rate, because when I had the mechanical advance set at 16 degrees that made the advance rate all in at 2400.
There was a rough spot around 2400-2500, it was suggested to me by a couple different knowledgeable guys that the rough spot was the advance rate coming all in a little too early.
Instead of tearing the distributor apart and swapping springs, I decided to ncrease the mechanical advance to 20 degrees because that slowed the advance rate down to be all in at 2800 rpm.
In turn I had to drop my intial timing down 4 degrees, from 18 to 14.
The rough spot cleared up quite a bit, which tells me it likes a slower advance rate.
I’ve been doing some research onthe magnum engines and have found that they don’t require as much total timing with vacuum advance because of the better chamber and that they don’t like 48-52 total degrees timing while cruising down the highway, I’ve learned they like a small amount of vacuum advance like 10-12 degrees for a total of 44-46 degrees while cruising with vacuum advance because of the more efficient chamber m.
So I think once I limit the vacuum advance it will clear up and be happier.
But it’s going to require a little more trial and error for sure.

So Don from FBO Systems was spot on.
I had messaged him and explained the rough spot I was feeling around 2500 rpm in every gear with the 18 degrees of initial timing and 34 degrees total timing with the 2 blue springs in the firecore distributor that had the timing all in around 2400-2500 rpm.
He asked me all the details, what engine, what vehicle, what tire size and what gearing.
I answered all of his questions and he said I think your initial is good and you could back off the total timing a couple degrees, the magnum heads are more efficient and don’t need as much total timing, he said try 18 initial and 32 total.
Then he said your biggest problem that I can see isn’t your advance rate is too fast, all in by 2400-2500 is to quick in that heavier truck with bigger tires.
He said slow that down to all in by 3000-3500.
So yesterday I pulled the distributor back out and took it apart and swapped the 2 blue springs for one silver spring and one black spring. Set the mechanical advance to 14 degrees.
This has the advance rate start at about 1500 rpm and has it all in around 3400-3500 rpm.
Set initial timing at 18 which lands the total at 32 degrees.
Hooked the vacuum advance up and turned it up to add 12 degrees at idle for a total of 30 at idle with vacuum advance.
Took it out for a drive and the rough spot at 2500 rpm is completely gone.
Smooth as butter all through the throttle now.
What confuses me is this engine is advertised at 9:0.1 compression, I’m running 4.56 gears with 33/12.50/15 tires.
So my gearing is more then adequate for the tire size and the truck.
Why was having the timing all in by 2500 an issue?
I’m running premium fuel as well.
I was just shocked that all in at 2500 was too fast, specially with the lower gear ratio.
Is it because the engine had more of a load on it being in a truck and the smaller camshaft is building a lot of cylinder pressure lower in the rpm range?
Those are the only conclusions I can come too.
The throttle response is lacking a little bit compared to when it had the two blue springs in it, but it feels so much smoother going down the road now so I’ll deal with that trade.
I’m sure it could tolerate having the timing all in by 2800-3000 but there isn’t a spring combo available that has the timing all in by 2800-3000 with 14 degrees of mechanical advance.
The closest I could get is the one silver and one black spring which starts the curve around 1500 and has it all in by 3400-3500

can you weaken one or both of the springs to get it to come in earlier (2800-3000) that you wanted. How is the rotor phaseing now compared to where it was with the faster curve (if the vac adv amt was changed) in addition to the springs.

The rotor phasing is still good, a lot of the rough spot I was feeling was the advance rate being too quick. I put one black spring in and one silver spring in which starts the advance rate at 1600 and is all in by 3400-3500.
I guess the thing to do would be to try the lightest spring in the summit brand spring kit that I have along with the Black firecore spring and see where the advance rate ends up with that combo.
It’s alot smoother it just lost a little response down low, I think it would tolerate all in by 2800-3000 with 14 degrees of mechanical advance.
It’s just gonna take playing with the different springs to see if I can find that combo.

I have the extreme timing kit that is not being made anymore. Summit had them for the Mallory's, but they are not being made. I have some on the shelf, if you really want to get into the exacts. it really is easy changing the springs with tiny/long needle nose, without tearing the whole thing apart.

If you or your team members know of a spring combo that starts around 1500 rpm and is all in around 2800-3000 rpm with 14 degrees of mechanical advance that would be awesome.
I would buy the springs from you if you guys can find that combo.
Idk if you guys have a distributor machine but if you do or you have access to one it would be fairly easy to play around with some different spring combo’s to find a combo that is all in around 2800-3000 with 14 degrees of mechanical advance.
Cause my engine likes a lot of initial timing but doesn’t like a whole lot of total timing and it doesn’t like the advance rate to be too quick.
So it’s kind of a picky little small block with timing but a lot of it is because it’s in a heavy, lifted 4x4 truck with big tires.
I think the heavier bigger truck along with the smaller truck camshaft that’s the reason it prefers a slower advance rate, it idles best with 18 initial and 12 from vacuum advance for a total of 30 at idle and it likes the total to be at 32, Magnum heads don’t require as much total timing as the older stuff.
I think if I could find an advance rate that was all in by 3000 it would be perfect.
But right now with the one black and one silver spring it made a big difference and smoothed things out.
I’m going to put a wideband o2 on it soon so I can make sure my carburetor is dialed in good.
Cause right now I’ve got the carburetor tuned by feel and by reading plugs and the vacuum gauge, so I think it’s close but it’s probly not perfect.

Mopar distributor curves are fine tune-able just call me and I'll explain how simple it is. You don't need to buy anything all you need is a little coaching.

Just call when you need help like this.

You could buy an E curve distributor and then you can just program the curve rather than using the witchcraft method of springs and slots and weights and vacuum cans.

OP, the Firecore distributor you say you have is the product of Board member Rick, or Sparky, or SunroofGTX. So when he speaks, you should listen.

Seriously, did you tell us it was a Firecore from the beginning? If so, I missed it. Different distributors have different ways of setting the advance. If it's not a Firecore, why did you mention it?

R.

Andy what is the best one to get?

In the future, if you are working on a Firecore distributor and want advice, it would help if you mentioned right off the bat that it was a Firecore.

Rick, aka Sparky, AKA Sunroof GTX, IS Firecore, or at least a good part of Firecore. So if he has a comment it would behoove you pay attention.

I agree that the use of springs and vacuum pots is much akin to witchcraft. It is true, also, that electronically setting advance is another level of witchcraft. Electronic setting has one advantage over the spring/slot witchcraft and that is this: You don't have to understand how the black box works, you just have to understand that it does work.

R.

Is vacuum advance really needed? If so Why?

It provides additional advance at part throttle conditions when the engine can take the advance (the additional advance would cause pinging at wide open throttle).

The additional advance increases the engine efficiency at part throttle, does not affect WOT.

This is another reason to think about moving to EFI. Even the cheap EFI systems available these days can control the ignition and they have vacuum advance built in since the EFI system needs to know manifold vacuum. I helped install a low dollar MSD Atomic system the other day and it had vacuum advance built in. The nice thing about EFI is that you can program the vacuum advance for both rate and total amount.

I was curious on vacuum advance as the new FBO distributors have vacuum advance. Their mod years ago was adjusting the advance profile and eliminate vacuum advance on the distributor.

To limit the vacuum advance arm travel, I set my initial where I want it, use a timing light and a mighty vac hand pump to pull in an additional 10* and note the amount of vac required for the 10*, pop the cap off, pump the vac to noted amount, mark the vac advance arm with a sharpie right where it passes through the housing, pick a appropriately sized short roll pin and drill the correct sized hole through the arm where you marked it. Drive the pin in and you now have a mechanical stop on the vac advance.

^^^ Ding ding ding I do believe we have a winna.

Put a 5/32" ball in the vacuum hose. Vacuum advance worthless.

i like vacuum advance on anything that sees any street time. if the hose is just removed or plugged, the tune is not complete. take the time to add/subtract any advance you can use [with what ever method you choose], and you will be happy you took the time to do so.

I recently worked over the older MP electronic distributor in my friends GTX. With a Lunati Voodoo hydraulic FT,mildly ported 906 heads and a Holley SD with a 750 AFB, it was a bit sluggish on the bottom end. I put the FBO limiting plate and springs in it, and adjusted the vac advance for an additional 10* at idle. The difference was amazing, super sharp on the throttle response and my friend had to relearn how to take off from a light without spinning the tires. If you follow FBO's instructions, you can get your dist squared away and the advantages of having a vacuum advance that works properly will be obvious. My buddy said he picked up a little mpg to boot!

Easy way to do it on an old Mopar Distributor is to wedge a piece of ~.1-.125 header gasket material between the pin and the slot, takes out about 10-12 degrees of centrifugal and if done properly it can't go anywhere because the pins sling against it as the distributor speeds up. You could weld up the slot but back in the day this was easier. This allowed to advance the distributor and keep the total timing ~34-36 degrees.

It's real good and easy to try to see if it makes the difference you need, you could always weld the slot up later if you want.