Chevy HEI Distributor problems
 

Hi Guys, I know that I'm in danger of being lynched for having a Chevy 383 in my Cobra but it's quite common here in the UK.

Anyway I have a HEI dizzy in the motor which was supplied last year by the Engine Factory. I have a problem in that the ignition timing appears to have a mind of it's own.
With the base ignition set to 10 deg BTDC at idle vac disconnected and plugged, the engine runs OK until I rev it above 3000 rpm then It begins to misfire and backfire as though the timing has gone mad.
Also when I connect the vac advance up the engine cuts out almost immediately unless I give it throttle at which point again it misfires and backfires as if the ignition wires were crossed or the firing order was wrong. (it isn't)
The Carb is clean with no blocked jets etc. No air leaks etc.

Can anyone shed any light on the problem?
Or is the dizzy just toast?

Have you looked inside the dist?? Sounds like vacuum advance isn't working right...check the springs, make sure you don't have a broken one. make sure the weights are working correctly...total timing should be about 38 degrees....

Yes I already did all of that stuff, all looks OK inside springs all on, weights all free etc.

how old is the harmonic balancer?? outer ring might have slipped and timing is off..need to check TDC with plug out of #1 and see where timing mark is

I can only assume that the damper is a year or so old like the rest of the engine.
I must admit I'had similar thoughts I'll try to check it out tonight.
Assuming that the woodruff key is in the crank I cant see how the hub could move relative to the crank.
But the outer hub relative to the inner is a different matter.

Kev sounds like maybe the cap & rotor is bad also belive the the advance is working (because it gets worse when hooked up) check and see if you have any play in the dist- shaft any is unassessable.
Ken

Here are a few items to check as they are the know faults or componets that can fail. Check rotor for black line on bottom of rotor in the center for carbon tracking or blow through, replace if needed also check cap for same as crossfire from jumping to wrong cyl can happen. The green wire from the module that goes to the moving plate can have wire insulation failure from moving with vac advance all the time and become a short. Have module checked at chain parts store like Checker for failure with RPM and coat with generous amont of white heat paste between module and dist. body or unit will fail in short time. Even a bad plug wire will misfire with load or RPM so if a little crazy run your hands along length to see if you get shocked ( can look at wires and move around in dark for sparks if not up to 40KV shock!!! Pull any plug wire and put a plug in wire and place in area you can watch end od plug. Start engine and look at plug to see if spark is neon blue, a yellow orange color means bad coil as a rule MUST BE NEON BLUE SPARK. Do not put plug test on polished valve cover as it will leave a mark !!!. If damper is wrong just time by ear and then check to see if in expected range. If needed can explain how to check with a dial indicator on # 6 cyl. but long thread needed to explain.
If I had to guess 1 or 2 even bet=it is a bad plug wire from header burn 2. bad cap/rotor 3. green wire to advance plate 4. module is on or off as norm but electronics can be wierd to trace. If happening for a while you could have fouled a plug and now have 2 problems to mask eack other. All this ASSUMES NO computer in car that can be a whole new deal.

When you "give it gas" the vaccum signal drops real low and the vaccum advance does not "advance" without vaccum. It would be as if the you unplugged the advance unit.

Since it seems to run better with the vaccum unit "advancing" this is a clue and here is why.

The base plate that rotates when the vaccum advance unit "pulls on it" repositioning the small wiring leads that provide voltage to the ignition sensor mounted on the base plate. IF those wires are broken inside the insulation (where you can't see it) they may be making better contact when they are moved. When the base plate returns to it's normal position (you step on the gas, the vacccum drops, etc.) the wires are NOT making good contact and the sensor voltage drops.

Inpsect VERY closely the small wires feeding the sensor for signs or insulation cracking, etc. Inspect ANY wires that are affected by the base plate "moving" under vaccum conditions.

Ernie

Vac advance is just for emmisions and economy anyhow. I remove from all speed toys any way. All engines love higher advance from stock setings. Each engine has its own personality because of cams,headers etc.. but advance initial timming until it bumps against starter and a few more degrees until it starts to ping when loaded like when low RPMS in a higher gear. STOP RIGHT THERE and remove 4 degrees from that point. That is your max initial timing (say about 20 to 24 degrees) then subtract that number from 39 or 40 degrees total timing and use that number for desired maximum mechanical advance said differently 40 degrees max minus 24 initial is 16 degrees mechanical and lock down the advance plate so it will not move and you have your optimum performance in MOST CASES. If your bumping agianst starter when hot just mash the throttle 1 time before hitting the key. This will flood engine for a nano sec to allow starter to gain RPM clearing its throat before starting. This does not work with fuel injection and computers so do not create yourself problems. But carbed hot rods love it.

All very good advise posted!

I do have a question for you though.

Have you removed the carburator off the engine? If so did you install a new carb gasket? If the answer is yes & no then my guess is that the vacuum advace is getting manifold vacuum not ported vacuum from a poor seal on the carb to manifold.

A GM stock Hei units usually only produces 15-18 degrees of advance. Your case 18 +10 equals 28. I am guessing that with weights & springs only, your distributor may be maxed out at the 3k you stated. The stock vacuum advance unit usually will add 24 more degrees.

A SBC under light throttle cruise performs best at 52 degrees of total timing.

If you need to replace that distributor, may I suggest you try a MSD Pro Alumn Billet HEI. Really nice unit!
Good luck ,
Bud

Bud, I don't know of any SBC that perorms best with 52 degrees total advance.....most SB's are in the 32-40 total range...they should be pinging like crazy with that much advance.

Hot fingers,

The key words are under light cruise. I was shocked the first time I heard it also. I am somewhat old school also, meaning 38-40 max also as much, and as fast as possible. Not any more!
Check it out and then repost.

My MSD HEI will advance the total timing to 45 with the vacuum advance connected at very light cruising. When I run it with the advance disconnected I have 34 @ 3400 total timing, 13 +21. It is the best setting for me, as I am running Fast Burn Heads. I get a slight amount of detonation @ 5300+ if I do not use 94 Sunoco.

Bud

MSD is a great product from my experiences but why not go for the Std or non HEI so you gain room with smaller diameter, remote the coil also a benifit from room and heat soak and more brands of wires available plus you gain a rev limiter in the deal with a 6AL to boot. The MSD has a very user freindly adjustable curve feature also.
What is your manifold vac now and what is historic for your cam/engine? We have slid off problem a bit but still having fun and great bench racing secrets being brought up. What the heck if you throw money at all the stuff we are selling your problem has to be fixed. If it is not the wires as I expect problem to be as a betting man still giving odds of 2/1 . Please post the fix so we know what it was.

Long, but very informative, I would think it may also apply to Ford Engines.

from
JohnZ
Tech Team


posted 03-23-2003 03:41 PM
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As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

After reading through all of the posts here I didn't see anywhere that describes when the problem started.
Was it after attempting a tune-up, suddenly running crappy, progressively worse.
This may be a stupid question but if you were doing a tune up on the car did you maybe get a crossfire cap by mistake? ( they have all the rightside wires coming of the right side of the cap etc.)

There is something missing from the original info that is needed to get down the right path, I'm just not sure what it is.

George thanks for the detailed info. How useful is a vac. adv. unit in a motor that is not pure stock or has many aftermarket modifications. My question is intended to learn your point and not argumentative. I understand its place in a stock motor and agree it is useful in that area. As most Cobras have aftermarket camshafts with the typical larger cfm potential carb with a fat primary and fatter secondary systems with enlarged cc acccelerator pumps not to mention the aggressive driving habits of myself and many of the brotherhood are in the stereotypical majority.
Locking the plate down stabalizes timming in high RPM runs and removes another variable for faults in a pure performance driven mindset. I do not have any vac. adv. system on my toy. I must also come clean in the fact I removed all the items mentioned above to install the Holley 4DI Pro-jection system and am using the variable timing programs (but electronic) with all the goodies (MAP, TPS, closed loop 02 sensor, IAC and more) I have written all my own maps and compensation charts because none of the stock stuff worked with my Big cam and 542 cubic inches at 13 + comp ratioon gas. Not even mention the modifications to the valve body and special lock up converter in the Automatic O/D tranny

I like the vacuum advance feature for "cruising". At LOW rpm the centrifigal advance doesn't provide enough advance to "optimize" the burn process. Generally speaking the Vac advance will increase effeciency and thus fuel mileage in a cruise mode (light throttle, low rpm).

At wide open throttle or high rpm (race mode) there is very little if any vacuum and advance is handled by the weights. If your not interested in "cruise" mode, focus on the "race" mode and keep the rpms up.

This logic would apply to a heavily modified engine or a stock engine. Using "weights only" will provide ABSOLUTE control of the timing at any given rpm, desirable for a "race" application to eliminate a major "variable" and allow precise "tuning" of the engine. In a race application you could care less about "gas mileage"!

Ernie

Ernie,
From your photos listed it looks like you are running a 650 Vac sec Holley on an open plenum long runner manifold and I'll bet you have massaged the curve and jetting. Have you played with the runners and combustion chambers to max performance and milage as well? What mileage have you charmed out of your toy?
I have been building the full tilt boogie motors for so long I have forgotten how nice passing a fuel trailer can be. I have to trailer to any run over 40 or so miles 1 way from the shop. I need the Sunoco GT plus at 112 octane.
This insanity is by choice and would not have it any other way but I am building a stocker:3DSMILE: :JEKYLHYDE for customers and wife to drive as we speak. I had grand ideas of just throttle stopping and loading a very conservative fuel map for customer rides.... What a joke, This baby will swap ends at the drop of a hat and throwing the keys to a first time driver is not an option. Isn't that the greatest problem ever to have...

vettestr,,,,,,the car is still awaiting it's final destination at Ed's shop in Ohio and I'm still in Hawaii. It has about 1400 miles on it so it's not been driven much, let alone "tweaked" at this point. My 14 year old son wants it for his first car,,,,,,,(aint gonna happen, LOL).

Ernie

The post grew a life of its own but back to original issue. Have you fixed or found problem and if so what was it ???