Engine:
440 bored .040"
Flat top pistons/Zero Deck
.039" Thick head gaskets
915 heads with over sized valves and harden seats/milled slightly
11:1 compression ratio (4400' altitude)
1.6 Roller Rockers
Comp Cam XE285HL-10 Duration @ .050 241/247
Lob lift .364/.364 LSA 110
Stock steel crankshaft
440 performer manifold set up as true dual plane/1" phenolic spacer
Holley 870 street avenger (went two jet sizes lower per Holley for altitude - 76 primary/80 secondary)
Hedman Headers 1 3/4" tubes

Ignition:
MSD6AL
Mopar Performance Distributor

Questions:
From what I have read Total Timing should be about 36 degrees.
I have the initial timing at 18 degrees and it runs and starts fine but when the vacuum and mechanical kicks in (2500 rpm) my total timing is 48-53 degrees.

I tried turning the vacuum advance adjustment with a 3/32" Allen wrench clockwise (first source I read) and went one turn at a time until it bottomed out. NO difference. So I backed it out to where it was and then went another full turn counterclockwise but still no change.

My thinking is that the vacuum advance is not the problem but rather the mechanical.

Is welding the slots the only way to remedy this situation?

Sorry for the long post.

If you have 36 total mechanical advance with the vacum advance unplugged and around 50 or so with the vacum advance that is normal and fine. You only have that much at part throttle when vacum is high and the eng can use that much but when you floor it you will only have the 36 total since the vacum drops to near 0 at full throttle. Ron

Start with one thing at a time.. unplug the vac and
plug the carb port and see when and how much mech timing
you have

My vacuum is nearly zero at idle.
It goes to 17 hg when I rev the motor.

Mr_P_Body, I will give that a go tomorrow after work.

I downloaded and followed an article called Mopar Timing.
It said to remove the vacuum advance hose from the distributor and hook a vacuum gauge to it. I know this is the distributor port on the Holley Carb and questioned in my mind why they wanted that one.
The same article recommended adjusting the vacuum advance to reduce timing.

Is there a step by step article available that is trustworthy?

I dont know of any but I havent looked... I play with
the dist in a few different segments.. first when on
the engine I figure out what the engine wants for
base timing then what the engine wants for total timing..
then I put the dist on a dist machine(I have access
to a old one).. I test it for when the timing starts
to come in at(thats adjusted with the springs).. then
I test for the total mech advance.. thats adjusted
by the slots... the last dist I got has adjustable
advance with just a simple screw(I believe its a MP
part).. but if you have slots then its weld up the
slots to limit the total mech timing.. the vac pot
can have too much travel and pull more timing than
needed and you can adjust that travel... if you want
to slow the vac to the can you can get small in line
vac restrictors to slow it down but it just slows
it.. the vac advance is more of try it and see thing
since your not on a dyno.... but if you think about
using a stock dist on a modified engine your increasing
the base timing and all of the numbers move up by
that amount(difference of the stock base timing to
the new modified base timing) so that dist need to
be modified

Also remember the vacum advance is mostly for part throttle driving when intake vacum is still high. It like ported vacum will drop to about 0 at wide open throttle as the only vacum that wont is venturi vacum which is not a real strong signal but strong enough to work vacum secondary carb diaprhams. Your total mechanical at 36 is not a bad starting point and may be fine for your combo depending at what rpm its all in. My self my dist does not even have a vacum advance as its basically a race dist thats all mechanical advance and I have full mechanical avdance by about 1800 rpm. But the vacum advance is good for a street car as it helps gas milage and helps part throttle power and most cars have 50 to 55 total with all mechanical and vacum advance in. But when you floor it you will only have the mechanical advance because as we said at full throttle you wont have any vacum to work the vacum advance since the throttle plates are wide open there will be no vacum in the intake. It may help you if you know that what they call ported vacum that alot of vacum advances are hooked to are actually intake vacum but the port they hook to is above the throttle plate so it will have no vacum at idle with the throttle plates closed because the port is above the throttle plate. As soon as you step on the gas a bit it uncovers the port and the vacum advance starts working. But it is intake vacum that will drop to 0 when floored. Intake vacum ports are usually refered to that when the vacum port is below the throttle plate or anywhere on the intake as that will be full intake vacum at idle with the throttle plates closed becuase the intake vacum ports are below the throttle plates. Ron

There is a lot of misconception about how all this works, because of you either view it from a performance perspective or from an emissions perspective.

For performance, the vacuum advance MUST be hooked to a port below the throttle blades for it to work correctly. If the port you are hooking it to is at or near "0" until you rev the motor; you have it hooked above the throttle blades and its on the wrong port.

You want it hooked to a port that has full vacuum at idle and subsequently cruising speed. When the throttle is opened, the manifold vacuum will instantly drop towards or all the way to zero depending on how far you stomp on it.

At that split second, all the added timing from your vacuum advance that was maxed out at idle and/or at cruising speed drops to no added timing via the vacuum advance pod. It saves the motor from detonation as you roll-it-on harder than a mild acceleration, floor it, go up a hill at heavy throttle under load in too low a gear, or just punch it.

At that point, only your mechanical advance, along with your initial timing is dictating your total advance.

It's easy when you think of it this way.

Also, what ever your cruise rpm is on the highway, ideally (if you don't have too much compression) all your mechanical advance should be all in (READ: use the correct advance springs to allow full advance by your normal cruise RPM) When you are at light cruising speed of the freeway or just idling in the driveway all your vacuum advance will be in for maximum fuel mileage and dramatically increased off-throttle responsiveness. Typically, your timing will be up around 50 to 52 degrees total with the vacuum advance hooked up and around 32 to 36 total when it is not. There are a lot of variables at play that dictate those final optimal timing choices like fuel quality, compression (both static and dynamic, altitude, combustion chamber and piston design etc, etc.

.

You want it hooked to a port that has full vacuum at idle and subsequently cruising speed. When the throttle is opened, the manifold vacuum will instantly drop towards or all the way to zero depending on how far you stomp on it.


This can be misleading to some. If you just step on the throttle 1/4 to almost half way down you will still have a strong intake vacum signal when cruising. If you go past half throttle to maybe 3/4 then yes the intake vacum is dropping off and of course will drop to about 0 when at wide open throttle. I have seen it alot over the years when testing eng vacum for clogged exh systems and so on. And I dont mean it to be smart its just I see alot of people think as soon as you step on the gas a bit your vacum is dropping to zero which when you go right to almost 1/2 throttle the vacum drops a tad and then will stabilize at about the same vacum as intake vacum is at idle if you stay at that throttle posistion cruising along. Dont matter if its ported or intake as they are both intake vacum anyway just one is above the throttle blade and one is below. When cruising you usually wont loose the vacum advance until you go past half throttle or more when the vacum drops. Ron

Okay, so I'll plug the upper vacuum port on the carb and run the vacuum advance on the lower one.

Hooking the vacuum gauge to this port will definitely help me in fine tuning the idle mixture screws as I couldn't get any change at all on the upper port - but I understand now why that is so.

I'll also find the rpm where the mechanical is all in and the amount of mechanical advance.

If I am understanding y'all correctly, it is the initial and mechanical timing combined that should be about 36 degrees?

Might take me a couple of days to get this done since my schedule is packed with necessary work related things.

Thanks for taking the time to educate me.

Find the initial timing point where the engine kicks back when hot on the starter or where idle vacuum is greatest at a constant rpm, whichever is lower. Kick back minus 2* or highest vacuum/back of timing so 1" drop.

Set the rest of the timing events using the above as your foundation. It's an exercise in finding the efficiency point of the engine at idle.

Setting mechanical advance is easy if the MP distributor has a mallory advance system in it. Use drill bits to set air gaps. No need for the keys. Old style requires welding or jb weld in the slots.

Simple test at idle, twist your distributor a little clockwise, if it picks up rpm, it wants the timing.

I copied the boat guys who run blowers, and use a switch to kill the 12v circuit to the ignition while I begin to crank the engine. Once its spinning good after a second or so, I flip the 12v ignition switch and she lights every time. Most of the time, I only have to do this when its really hot out, but it's a lot cheaper than a timing start retard. I run 22 initial 36 total, all in by 2000 rpm. No vacuum advance. I only have 7 to 8 inches of vacuum at idle as it is. Plus no one makes and adjustable one that's decent.

Although, I did here that the Firecore vacuum advance distributors are being blueprinted and the range of initial timing adjustment available are being improved by the man at Four Seconds Flat. If I could get 21 initial and 36 total with an added vacuum advance that brings me up in the 45 to 50 total advance at cruise 2200 rpm, I'd be interested.

Eventually, I may just drop the nearly $400 on the newer PC programmable MSD unit and lock out my distributor, but that still leaves me with no vacuum advance for increased fuel efficiency in 5th gear.

I'm in the middle of changing, welding up the advance slots on several stock Mopar distributors(non Mallory)to reduce the amount of mechanical advance. I have found most BB Mopars will start really well with 18 degrees or less BTDC and run very well with between 34 and 36 total mechanical advavance. I shoot for 14 to 18 initial timing and 34 to 36 degrees total Most of the stock Mopar distributors I have worked on have the amount of advance stamped into the bottom of the advance plate, most of mine say 13, look at yours and go from there

FBO is not a dealer, but you can call and get some specific spring advance rates. Also, Ron Wilson or Bob George will be happy to discuss exacts to your needs. Thanks.