rebuilding a distributor

[dropped_71chevy]

I need a blow up of an hei distributor and then detailed instructions on building the distributor

I know I can buy a new one for $120 but I want the experience of rebuilding my own so that way I gain some knowledge for future use

Thanks

Tim

[/<eystroke]

This is long.

The web page http://www.kendrick-auto.com/ignition.htm has images and
names to a lot of the parts you will see when you tear into your
distributor.

Here is a exploded diagram.


Here is what is under the rotor.


I'm typing from memory so please excuse any mistakes:
First check the up and down movement of the shaft. Any excessive play
will affect the timing when the engine is accelerating or
decelerating. You will want to put in washers or shims on the bottom
of the shaft between the gear and the lock washer to fix this.

To get the shaft out, you have to press out a pin in the gear on the
bottom of the shaft. Before you do notice which way the indention on
the side of the gear aligns with the rotor. You want to make sure you
put it back the same. Once you get the gear off the shaft, the shaft
will slide through the assembly and the advancement plate (which holds
the inner pins) will then slide off the shaft. Where the advancement
plate rides on the shaft is where you need to clean and polish the
shaft. If the pins have worn loose and are leaning over go find another
distributor. This will cause irregular idle and you will never get the
timing to stay where you want it.

The rest of the mechanical stuff (weights, springs, pins) is very easy
to understand and work with.

The vacuum advance assembly is what a lot of people don't understand.
The canister should be connected to a ported vacuum. Which means there
is no vaccum at idle but as rpms climb the vacuum raises, but then
drops under heavy engine load. So at idle, there is no or little vacuum
so there is no advancement. As the RPMs climb the ported vacuum climbs
and pulls in the diaphram which advances the timing. I'm inserting a
text I've found that explains how to the three timing (initial,
mechanical, and vacuum) adjustments work together.

HEI SETUP FOR PERFORMANCE written by Damon Nickles This article deals primarily
with older-style non-computer controlled HEI distributors. This is becuase
there is so much performance left in them vs. stock AND since many people have
ditched the computer in their 3rd gen cars and gone with a totally non-computer
controlled engine. Please note: All carbureted V8 Fbodies since 1981 came from
the factory stock with a computer controlled, internal coil HEI distributor.
Most Fuel Injected 3rd gen V8s use a modified version of the HEI known as a
"divorced coil HEI" (not dealt with here).

A word first about those internal coil computer controlled HEI distributors:
Computer controlled versions work basically the same as the old non-computer
versions except that they don't have any mechanical advance mechanisms inside
them- the ECM determines the advance curve electronically. There is NO WAY to
change the advance curve of a computer-controlled HEI distributor- timing is
controlled ENTIRELY by the ECM's programming! The only change you can make in
the advance curve, other than buying an aftermarket performance chip for the
ECM, is to manually advance the base timing (usually worth a little HP by
itself). Check your GM service manual for the correct way to set the base
timing for your particular engine/year. If you have a computer controlled
distributor in your car right now you can't do much to increase performance
other than to make sure it is correctly communicating with your car's ECM and
upgrade the coil to a better unit. This is not a bad thing- it leaves funds
available for parts that WILL make the car faster!

DON'T USE A COMPUTER CONTROLLED DISTRIBUTOR IF YOU HAVE DITCHED THE ECM OR ARE
RUNNING WITHOUT ONE!!!! You will get NO ignition timing advance AT ALL from the
computer-controlled distributor plus lousy performance and lousy mileage. If
you have disconnected the 4-wire ECM connector going in the side of the
distributor or the wiring between the ECM and the distributor is damaged you
will, again, get no advance (and a check engine light) Yes, you can remove a
non-computer controlled HEI and drop an old-style mechanical advance
distributor in its place- it will physically bolt right in. But it will cause
the ECM to throw a code because the ECM looks to make sure the
computer-controlled portion of the distributor is out there every time you
start the engine. Once you begin to take things out of the ECM's control you
pretty much have to go all the way with it- replace both the
computer-controlled distributor, the computer controlled carb (Qjet) and
deactivate the check engine light.

That being said, the NON-computer controlled HEI can be made into a great
distributor for a street or street/strip car. Its simple, easy to tune, and
plenty powerful to light off any naturally aspirated engine up to 7000 RPMs if
properly equipped.

Let's start with the coil. Its located on top of the distributor between the
plug wire towers under a plastic cover. It's the "engine" that makes the
sparks. Stock its capable of about 35,000 volts and so-so total spark energy.
Its fine for a naturally aspirated street motor that rarely sees the high side
of 5000 RPMs. It will provide the energy to jump a plug gap of .040-.045 with
no problem in these applications.

You can upgrade the coil with an Accell or MSD replacement coil that will jump
the voltage up to about 42,000 volts and total spark energy will also jump
about 10-15%. There are even hotter coils than this from Accell, MSD and others
that will give you the same 42,000 volts but a LOT more total spark energy
(like 50% more). Either one of these "super high output" coils is a worthwhile
upgrade for a hot street motor- you'll get better response and HP especially at
higher RPMs where the stock one hits a kind of "wall". A new coil is also a lot
cheaper than a complete MSD ignition amplifier setup and at least 95% as
effective at making HP in most naturally aspirated applications (the MSD will
still give you slightly better mileage & emissions due to it's multi-strike
spark capability below 3000 RPMs). With either hotter coil you can open the gap
up to .045-.050 for just a smidge more HP. There are 2 different versions of
the HEI coil and you need to make sure you get the right one. The only external
difference is that one has red and white power leads, the other has red and
yellow power leads. You will need to know which one of these you have stock to
order up the appropriate aftermarket upgraded coil.

For blown or nitroused applications I always recommend an MSD (or equivalent)
ignition amplifier box setup. You can do it with a stock HEI + hi-po coil but
you'll likely have to close the gap way down (.030-.035) to make it work well.

THE ADVANCE SETUP: Centrifugal and vacuum advance (non-computer controlled
distributors ONLY!):

Centrifugal advance assy. on the HEI is pretty darned good. The stock weights
and advance plate are perfectly acceptable for all but the highest HP/RPM
engines. Centrifugal advance is used to advance engine ignition timing relative
to an engine's RPM. With more RPMs, more advance is needed- UP TO A POINT (more
on that later). ALL stock HEIs that were installed in V8s are designed to have
a total centrifugal advance of 20 degrees, +-1 degree due to production line
tolerances. This is as measured at the crankshaft (10 at the cam).

The HEI's centrifugal advance is susceptible to old age, though. Typically the
centrifugal advance weights wear their pivot holes into an "oval" or eat a
trench into their pivot pins OR BOTH. This is bad and no attempt to change the
advance curve should be made on a distributor that suffers from these problems-
fix it first or get another HEI to start improvements on (I can buy them for
$20 a piece in decent shape at local junkyards- just make sure you are getting
the right one for your engine- they were used on ALL makes of GM V8s and all
look very similar). Also, the centrifugal advance plate (that pivots around the
main distributor shaft as the centrifual advance moves it), near the top of the
distributor shaft sometimes gets gummed up and sorta "sticky," slowing the
advance curve and generally preventing the centrifugal advance assembly from
working correctly. If your centrifugal advance doesn't "snap" back when you
twist the rotor with your hand and let it go then you have this problem. You
need to pull the distributor shaft apart and clean everything out, especially
up top, before you proceed with upgrades.

The stock HEI also uses a vacuum advance canister to further advance engine
timing. Vacuum advance's purpose is to compensate for and engine's LOAD.
Manifold vacuum is an excellent indicator of an engine's load. A lightly loaded
engine can tolerate more spark advance than a heavily loaded one for better
fuel economy, emissions, and to keep the tip of the plug hot enough to keep it
from fouling with combustion contaminants. The centrifugal and vacuum advance
work together, but independent of each other, each adding its appropriate
amount of timing advance, to supply the correct TOTAL spark advance to the
engine under all RPM/engine load conditions.

Tuning for performance (changing the advance curve):

1. Centrifugal: Stock the advance mechanism is pretty good but the stock
springs are usually way too strong, causing the advance curve to come in too
slowly, if it ever gets fully advanced at all. All you need are the right
springs and the right initial advance setting. Most Small Block Chevys like
about 32-38 degrees total advance at WOT. Since we know already that the HEI
has 20 built into the stock mechanism the first thing we need to do is set the
initial advance correctly- that means you need an intial advance between 12 and
18 degrees (you might want to retard it 2-4 degrees for daily street use just
to build in a little safety margin).

Now all we need to do is make sure the centrifugal advance comes in at the
right RATE- relative to engine RPM. You want it "all in" by about 2800-3200
RPMs for a typical street performance motor. (Additional advance above this RPM
point is neither needed or wanted- increased turbulence in the combustion
chamber offsets the need for further ignition advance beyond this RPM level)
You do this by changing the centrifugal advance springs to lighter ones. IF you
use the Crane advance kit like I do you are looking to install one Blue (heavy)
spring and one Silver one (medium). These springs are located directly under
the rotor and are easy to remove/replace by hand or with needle-nose pliers.
These springs will give you an advance curve that starts at about 800 RPM and
ends at 2800. If you don't have the Crane kit then install whatever springs you
have and check the advance curve with a dial-back timing light (or use a
"timing tape" wrapped around your harmonic balancer) and a tachometer, swapping
springs until you get it close to these specs. It doesn't matter if the springs
are not "matched" side to side- you can install one heavy one and one light one
and it will work fine. Please note that getting the advance in sooner does NOT
build peak HP, but it does build quite a bit of bottom end torque. This mod
will have you grinning ear-to-ear with the nice seat-of-your-pants improvement!

2. Vacuum advance: Stock cans typically provide 22-24 degrees of advance. This
is WAY WAY WAY too much if you have recurved the centrifugal and initial
advance as described above. You will get "3 rocks in a coffee can" kinda
detonation.

Can you just leave it unplugged? Yes. Your highway mileage will be off by about
5MPG and your plugs will load up with crud within just a few thousand miles.
For a race car or a weekend street/strip can this is probably no big deal. For
a daily-driver street car, forget it.

Vacuum advance for the street: You want about 12 (crankshaft) degrees total
vacuum advance if you run WITHOUT a functional EGR system, 16 degrees if you
run WITH a functional EGR system. Regardless, you want it to come in between
about 5 and 15 inches of manifold vacuum. I have found the most expedient
solution to be the Crane advance kit once more. Install their can with about 9
turns (tighter than the loosest setting) on the adjustable advance can spring.
IMPORTANT!- Also, use the little "lockout" cam that comes with the Crane vacuum
advance to lock out AS MUCH ADVANCE AS POSSIBLE. This will still leave you with
about 12 degrees of available vacuum advance. If you set it with 2 notches LESS
lockout than the maximum you will end up with about 16 degrees available vacuum
advance- perfect for those of you running a functional EGR system valve.

So your "typical" advance curve will look something like this:

14 degrees initial advance + 20 degrees centrifugal + 16 degrees vacuum adv. =
50 degrees total

If you are in the 45-55 degrees range for total advance you're in the ballpark.
Each engine is a little different and what works for your engine might be a
little different than what works for someone else's.

That's about it for distributor setup. You're ready to rock and roll. Now all
you have to do is install it correctly and plug everything in correctly. Here's
a few tips:

Number 1 plug wire should be at the front of the distributor just to the
driver's side of centerline. The vacuum advance can should be pointing roughly
at the passenger side front tire. If that's not your #1 plug wire, or your
vacuum can is pointed in a weird direction then you have the distributor
installed off by a few teeth. It won't hurt performance if timing is still set
correctly but plug wire routing gets a bit messy.

The vacuum advance can should be plugged into a "ported" vacuum source on the
carb. This is a vacuum port on the carb that provides NO vacuum at curb idle.
When the throttle is opened past idle the vacuum kicks in and starts your
advance curve for cruising/part throttle.

WATCH YOUR IDLE RPMS WHILE YOU SET INITIAL ADVANCE TIMING!!! You note that the
centrifugal advance curve that I recommended above starts at about 800 RPMs. If
you try to set your initial timing with the engine idling ABOVE this RPM point
you will NEVER get a true initial advance reading since the centrifugal with be
partially advanced at that point! Set it with the idle temporarily slowed WAY
down if you have to but DO IT RIGHT!

A final word about that HEI "module:" Lotsa mystery around this little
"thingamajig." This is just the little electronic brain located on the floor of
the distributor housing with 4 wires going into it (2 per side). All it does is
read the magnetic pickup signal from the magnetic pickup assy. around the
distributor shaft and then using this reference signal, tell the coil when to
fire and with how much "dwell". I always recommend a good stock GM module, not
a parts-store cheapie. No reason to go crazy and get one of those $70
"super-high output modules here"- I have tried them and they do nothing for
performance. But get a genuine GM one. Stock GM modules incorporate what they
call a "variable dwell" circuit that reduces dwell at lower RPMs to keep the
coil from over-saturating. This is good for sharp performance and long coil
life. Some parts-store modules don't have this circuitry in them.

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From http://www.jeepster.org/swchapter/hei-2.html

Getting it to Run Right - Recurving for Mileage and Performance Just installing
an HEI is not all that needs to be done. You have to "dial it in" to make it
work like it really can.

The stock HEI's vacuum advance diaphragm is far too sensitive to work with the
older carburetor's ported spark vacuum system. It will advance to fast, too
much, and too soon, hurting performance, mileage, and possibly burning valves
and pistons. Leave the vacuum line off till you make the modification.

You'll need to get an adjustable vacuum advance diaphragm from your parts
store. It will run you about $20 or so, but it's well worth it. Parts stores
will probably have to order it, speed shops will probably have it in stock.
Make sure it's for a GM HEI.

Before installing it, check the instructions to make sure, but all I've seen
needs to have the Allen adjustment screw turned all the way counter clockwise
to DECREASE sensitivity. You do not want it to work fast, you want it at the
least sensitive position.

Install it carefully as per instructions. If you have trouble reaching the
mounting screws apply a little vacuum to the diaphragm, it turns the plate out
of the way.

Install the limiter cam the new adjustable diaphragm came with. Set it so it
only travels about 1/2 the way it did without it. You can make marks on the
distributor body before and after the cam is installed to see where to set it.
1/2 way should give you about 14 degrees vacuum advance. Without that limiter
it will be worse than it was with the original advance diaphragm.

Clean the damper and the pointer. It will help if it's painted black.

Roll the engine to TDC. Since you don't have timing marks beyond 10 degrees,
you need to make some. We are going to make a $100 degree wheel for free.

Use dabs of white, yellow, or silver paint to make it easy to see against the
black background. The tip of a small nail or wire makes a good brush..

Put a tiny dab of paint along the notch in the damper. This is your normal
timing mark. Put another larger dab right on the 0 mark on the stationary
pointer and a tiny one on the 10 degree point.

When the notch and the 0 are lined up exactly, the timing is 0.

Rotate the engine till the scribed line lines up with the 10 on the stationary
pointer. Put another dab on the damper exactly where it lines up with "0" on
the pointer. Put a "1" next to it. This is now another 10 degree mark.

Move the crankshaft another 10 degrees till your new "1" mark is opposite the
"10" on the pointer. Now put another mark on the damper next to the "0" on the
pointer. Mark it with a "2" This is now your 20 degree mark.

Again, move the crankshaft 10 degrees more till the "2" mark is opposite the
"10" on the pointer. Put another mark on the damper next to the "0" on the
pointer. Marking it "3". Right, that's your 30 degree mark.

Again, put the "3" opposite the "10" on pointer, mark the spot next to the
pointer's "0" with "4" It should end up like this: 1 - 2 - 3 - 4 -


Be as accurate as you can.

What we did is made the degree marks on the damper rather than the pointer. The
reason you heavily marked the 0 on the pointer is that now is your new
reference. When the "2" is opposite the 0, it's timed at 20 degrees. We can
read all the way up to 40 degrees ("4") now, the original pointer only went to
10. Disconnect the vacuum advance hose. Start it, take a reading - this is
your "Initial Timing." It should be about 10 degrees.

Run it up to cruise speed, about 3000 RPM. Notice the marks moved as you revved
it up. Now your "2" should be near the big "0" on the pointer. You can
interpolate what it is, like 23 etc.

The difference between the reading at speed and the Initial is how much your
mechanical advance moved it. It should have moved it 10 to 15 degrees more than
the initial. So if you started at 10 degrees, it should have gone to 20 to 25
degrees. That advance is simply the weights inside the distributor, under the
rotor. Kits are available to modify this curve, but stock is better for
off-road performance, since the odd-fire loves making low RPM torque.

Now reconnect the vacuum line to the advance. Take another reading at idle to
make sure the vacuum isn't affecting it at idle, it should still be at the
Initial reading. -- If it moves the timing you have the wrong type of vacuum on
the line. It needs ported vacuum, not manifold vacuum. Ported vacuum is near 0
at idle and gets stronger as you accelerate till about 1/2 throttle, then it
decreases again as the throttle open more. Manifold vacuum is strongest at
idle. It's important you use the right vacuum source, otherwise the system will
work backwards!