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So, still working on getting the GTO dialed in with the fresh engine and this is, as you may recall, my first experience with a Holley.

A little over a week ago I had gotten to the point where I was ready to start trying to tune the carb. My new AEM A/F meter was showing it was pretty rich at cruise so I undertook to lean it up some. At that point my replacement module for the DUI HEI had come in so I installed that too, since it's "hotter" than the standard module I used to first get the car running. This point will turn out to be significant later.

I put in 3 "numbers" leaner jets in both ends of the carb, and took it out for a test drive. Popping! Banging! So bad that I though surely it was going to blow a hole right through the hood. This is not good. I brought it back and went back to the original jets ---- and it did nothing to fix the problem. I tore the carb down again, looking for anything - pinched gasket, leaking power valve... didn't find any smoking gun so I tried it again --- same result. It was even doing it sitting still in park/neutral. As soon as I'd start to feed it throttle, it would shoot flame 2 feet high out of the carb. I fought this problem for two solid days. I put in the other module it had been running on -- no change. I relashed all the valves, even checked all the pushrods for damage, and pulled the spark plugs in preparation for bore-scoping the cylinders and running a leakdown test. There had been some 'hard spot' pulling the engine through by hand to lash the valves that had be concerned, but doing the same thing with the plugs out found it to be smooth and even through two full revolutions. I decided to reconnect the carb and spin it over with the starter while looking at it, and that's when I found a problem. Float level. The front bowl was spewing fuel out of the vent as the engine turned over. This turned out to be fuel pressure which I fixed by dialing down the regulator, but it was a mystery how it changed "on its own" - it still is.

However, even though that made the backfire problem somewhat better, it didn't fix it.

More carb changes, going richer - going leaner with no results. I even talked with the techs at AED, the carb vendor, and they didn't have any ideas that helped. After fighting this problem for a solid week with no results I was seriously starting to doubt everything I'd done and getting severely discouraged and frustrated.

I pulled the distributor with the intent of checking oil pump drive shaft end play and everything associated with it. That's when I found something else. The pivot points in the advance weights were worn significantly, so much that they allowed the weights to sit in the orientation that you see in the photo. With them sitting like this, when the rotor is installed it clamps down on them and keeps them from moving - at all - creating a situation where the only advance the engine will ever see is what it has for initial timing.

I moved all the electronics (module, cap, coil) over to a 'backup' HEI I had, put it in, timed it, and this morning I drove it again. Perfect - no drips, no runs, no errors. I've gone back to the original leaner jets that I wanted to try a week ago when all the misery started, and they're fine. In fact I can probably go a little leaner still.

Whew

Bear
 

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Always nice when you find the problem.
I swapped distributors, messed with timing and carb jetting with no luck.
Car ran good just no get up and go.
Finally while staring into the engine compartment out of ideas I started to rev it up and noticed the air cleaner top pulling down.
I flipped the top over and took it out for a run.
What a difference!
I ordered an open element air cleaner from Summit, the stock unit is great for car shows just doesn't let enough air in.
 

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Typical problem with the large cap HEI's, because nobody ever takes the rotor off and lubes up the works. Another issue is the reluctor to main shaft, they also carry grease to lube them, but, nobody ever takes the shaft assembly apart to lube them. Pins are still available from various sources, EBay has sellers for them.

As far as the "hotter" module, a sticker alone does not make a better HEI module than stock.
 

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Discussion Starter #5
Typical problem with the large cap HEI's, because nobody ever takes the rotor off and lubes up the works. Another issue is the reluctor to main shaft, they also carry grease to lube them, but, nobody ever takes the shaft assembly apart to lube them. Pins are still available from various sources, EBay has sellers for them.

As far as the "hotter" module, a sticker alone does not make a better HEI module than stock.
It's a DUI - Davis Unified Ignition - they have a very good reputation "in the hobby". Whether deserved or not, I can't say. DUI claims their "Dyna Module" has more dwell time than stock, leading to longer coil saturation resulting in a hotter spark.

Bear
 

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I am very well aware of DUI, nothing to write home about, especially over clocking the input to 16 volts. Obama-Care boasts we all have free, fantastic health care, some people actually blindly believe it. If lipstick is put on a pig, it is still the same pig.

Have you ever actually measured the dwell times for different HEI modules? I have. Adding a decal to a GM 990 series performance module isn't anything to boast about.

"longer coil saturation resulting in a hotter spark", and, more coil failures, common on in cap large HEI's, no matter the coil used. Then, as the coil is failing, it takes module after module out, "for no apparent reason", until the coil finally fails, long after the last person insists "It can't possibly be the coil". Also, insisting on using giant spark plug gaps does the same thing.

I see, and fix that stuff EVERY DAY.
 

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That's great, Dave.

No, I haven't measured the dwell myself because I don't know how to do that on an HEI module. I'd have that job covered just fine on a points system. Perhaps if you were to proactively share your experience and knowledge with the folks on the forum here it would be helpful.

Bear
 

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... Adding a decal to a GM 990 series performance module isn't anything to boast about...

"longer coil saturation resulting in a hotter spark", and, more coil failures, common on in cap large HEI's, no matter the coil used. Then, as the coil is failing, it takes module after module out, "for no apparent reason", until the coil finally fails, long after the last person insists "It can't possibly be the coil". Also, insisting on using giant spark plug gaps does the same thing.

I see, and fix that stuff EVERY DAY.
Whoa - so are you saying that the Davis DynaMod is really just a GM 990 with a sticker on it? I think the GM part number is GM 10482820 ? If that's the case and those are still available, that could save some money. Are they still available?

What do you like for plug gap on HEI's in Pontiacs? Davis says 0.050 to 0.055 but that seems huge to me and I'm not running that much either.

What's your overall opinion of Davis?

Bear
 

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Sorry for my ignorance, its been many years since I really fooled with a distributor, but how does the centrifugal advance weights being locked in a non-advance position cause the symptoms you experienced? Did that also lock the vacuum advance?

I would expect lacking performance with reduced throttle response, but flames out of the carb because there was no timing advance?
 

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Well, this build has a fairly rowdy solid roller cam that has 251/257 degrees duration at 0.050 tappet lift, so there's lots of opportunity there for a mis-timed spark to ignite the "wrong" charge, or some flow that's still in the intake manifold itself. In the case of an extremely "late" ignition, which would have been the situation here, the charge in that cylinder could have easily still been burning "vigorously" when the exhaust valve on that cylinder opened, and since cylinders 5 and 7 are adjacent to each other both in proximity and in firing order, a late fire on #5 could have been igniting a partial charge in #7 while its intake valve was still open.


Anyway, that's my story and I'm sticking to it.

Bear
 

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Lets take a closer look at this. How many of you have done extensive research on modules offered by the various sources, and coils, spark plug gaps? I fear not many.

I have. In fact, I use a testing device that has shown me that using the SAME coil in cap HEI distributor, power source, with the ONLY change being the various HEI modules offered, that no significant output level changes occurred, except for ONE module, the MSD. And, I have also tested these different setups in another tester I built, that I use for EVERY conversion I do. This one uses a chamber that has 8 spark plugs in ot, and can stand 1,800 PSI, while being able to see each spark plug and the spark arc it makes, and with the electrical tester in place with it. I do this to everything I build.

Ever see an ACCEL performance HEI module? It has an ACCEL sticker on the face of its body, with a very small bump in the decal, from a raised area under the decal. Ever seen a NAPA TP45 module? It has the SAME raised area, but NO decal. One has to realize that both ACCEL and NAPA electronics are owned by Echliin, along with a few other brand names. One set of tests was done with less than one minute changeover time between tests, same HEI, same scope, same power source, first test, ACCEL module with decal, next, Echlin/NAPA TP45....NO DIFFERENCE WHAT SO EVER, except the price, ACCEL, $78.00, NAPA, $24.00. I still use the NAPA modules, they work well, and match other good quality modules of all sorts. I use the NAPA modules because they are good price, they work excellent, they do not fail, and NAPOA stores are everywhere, and online. I also find O'Reilly Master Pro replacement modules to be of the same quality as the Echlin, with a comparable price.

As far as a lot of big time name brands of HEI modules, there was literally NO significant difference in all of them vs a good GM or aftermarket replacement module, no matter the decal on it.

If you want to see a great variety of the same HEI modules, with all sorts of different maker/brand decals on them, go look for 4 pin HEI modules on eBay. It will enlighten you.

Spark plug gaps and various designs have also been tested, and significant changes have been recorded. First, the worst spark plugs are those with multiple negative electrodes, then, Iridium, which is for emissions engines, and then, different versions of the negative electrode design.

The whole idea is to expose more of the center electrode to atmosphere, to light off more fuel molecules, so, there have been different ways to do it. Drag races used to "J Gap" plugs, they would cut the end of the negative electrode and move it back to the other side of the center electrode, opening the area exposed, they worked well, but did not last on the street. Next, the various multiple negative electrodes that still shroud the center electrode, wrong way to do it. Two ways I do it, either round off the end of the negative electrode to match as much of the diameter of the center electrode as I can, opening up some area around the electrodes to hit more mixture, and, taking the end of the negative electrode and forming it into an arrow point, exposing the sides of the center electrode. The best for a two stroke CD ignition outboard engine is a "surface gap" design, which I have seen advertised recently on the various seller's, Summit, Jeg's, and others, no negative electrode, only a flat surface with a round gap between center and body, they do not work in 4 stroke engines. The older V series fine wire center electrode pluts were a step forward, a short step, they still shrouded the center electrode, and so did the V gap ND's. My favorites, Auto-Lite and NGK.

Spark plug gaps show NO performance increase when opened up past .045, which is what we found when we first designed the large coil in cap HEI. To prove that, GM had some division engineers try to run spark plugs with .060 and even .080 plug gaps, with extensive coil/module failure results in low compression, emissions engines on Olds, Buick and Pontiac vehicles, all under full warranty. THE fix was to replace the failed electronics with the same as what was in the distributor, and run the spark plug gaps back down to the original design .045. FAILURES STOPPED. The reasoning those engineers had, that didn't work, was that if they could get more area between the electrodes, the less fuel molecules could be used between them, to light the extremely lean emissions mixtures off. The real world fixes were to reduce the spark plug gaps back down to .045 maximum, so the ignition systems lived, and to fuel inject the engines, for better mixture volume and mix control.

Now, why do we have to be concerned about spark plug gaps, especially giant ones? Well, the reason is compression pressure resistance, which has nothing to do with resistance in the electrical system. It has EVERYTHING to do with the pressure resistance the spark plug encounters to get the compressed mixture fired off efficiently, and in some cases, at all. THIS is why I built my firing pressure chamber, so I could see the actual plug firing under pressures encountered inside a running engine, not a spark from a plug laid across a valve cover.

When cylinder pressures INCREASE, spark plug gaps should be closed up. Is the fuel volume lowered when a higher compression is encountered/ NO, is there more area between fuel molecules in higher compression engines? NO< just the opposite, more molecule4s of fuel are more tightly packed together, along with the pressures they are held in. So, with a more dense fuel volume, we do not need a wider gap to "reach out and grasp the farther apart fuel molecules", now, do we! NOPE. in fact, engines that have higher compression ratios and moe opened spark plug gaps usually have significantly higher fuel light off issues only cured by closing the spark plug gaps down.

What spark plug gaps are used in ultra high compression engines? Well, lets take a look. Supercharged, turbo'd, nitrous, no matter the ignition system used? .028 to .032. Also, engines used in Top Fuel, Funny Car, and blown Alcohol drag racing use a 66 ampere MSD electronic magneto. Those mags are so dangerous, the timing is set with a bar that goes around the mag, with insulated hand holds, by people with gloves on that insulate the guy setting the mag. EVERY team has one member that is certified in CPR, just in case someone gets shocked by the mag, and stops breathing. Compression pressures usually run on those engines atmospheres, between 5 to 5 atmospheres, which is extremely high. Those engines have the highest compression resistance of any automotive based engines, and they run, with that big a magneto, between .018 and .022 plug gaps. so much for setting the plug gaps at .080 for your 11.00:1 street engine, just because someone that hasn't even tried it, literally only THINKS SO, or, heaven forbid, only ran an engine on "A DYNO".

When this came up, I had a person from Team Chevelles email me on what he needed to move the coil out of the cap to a remote oil filled coil for his large cap HEI. In his email, unsolicited, he stated he had read the same sort of stuff I am posting here, and closed his .060 spark plug gaps back down to .045, against every bit of advice to keeping them open, from very well intention'd members there. His results, the engine ran significantly better. He had arbitrarily set those gaps when he added the spark plugs, under recommendation from other persons on those tech boards. He said it was to prove to himself after hearing from everyone else that I was leading everybody wrong, that I was "full of **it". Didn't work, all it did was make his engine run better. What a let down. It'll run a lot better with the remote mounted oil filled coil when he gets that done, too.

Please realize, I get this all the time, and my only intention is to say it exactly like it is, from what I have found in my own testing, been involved directly with, and good info from what savvy others have found, and not from internet innuendo, over-advertising hype, and/or other's egos. I am here to help, not to advertise, not to confuse. I haven't posted here in a couple of months, from one very arrogant and aggressive member that has an agenda of some sort against me, when he has never used any of my products, only defames (It ISN'T you, Bear). It is easier to just go fix stuff for people that have done it the way the other guys had him do it, that didn't work. Arrogant? NOPE, just the way it all shakes out. Long winded, yes, now, we are all on the same level, and can go back to the FRIENDLY discussion about all this stuff.

I am also a car collector, with 127 cars of all makes, including 9 Pontiac's, one with large holes drilled into the frame at the factory to make it a race car.
 

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Noted, and thank you for the information.

With that being said, do you see any connection with an HEI distributor, showing correct static timing, but having the advance weights stuck causing the symptoms the OP has described?
 

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Discussion Starter #14
... Spark plug gaps show NO performance increase when opened up past .045, which is what we found when we first designed the large coil in cap HEI. To prove that, GM had some division engineers try to run spark plugs with .060 and even .080 plug gaps, with extensive coil/module failure results in low compression, emissions engines on Olds, Buick and Pontiac vehicles, all under full warranty...
Thanks Ray, that's useful information. I've got mine gapped at .048 right now so I'll tighten them up a smidge.

...I am here to help, not to advertise, not to confuse...
Awesome, I can sure use it. Did you say whether or not the DUI module is in fact just a re-stickered version copy of someone else's? If so I missed it. If it's electronically the same as a different one that's less expensive and just as reliable, then I'd sure like to know.

Bear
 

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The board's UGLY HEAD raised itself over night, the arrogant adverse person returned, so, I am going to step away for a while, go fix stuff he and others have advised to be done, so they will run right, instead of being the mess they were made into.

I can see where the weights locked up could cause exactly what ear experienced. I see things like that, and worse, all day long, every day.

What we have to remember is, although the basic design of both GM points and HEI distributor mechanical advances is primarily the same, they vastly differ.

In the points distributor, the start and stop points of the curve are set with positive stops through the points plate, and with a bushing, degrees can be restricted down. this leaves the weights, and center curvature to give the actual curve degrees. The4re are only 4 different weights for points distributors, one for the early cast iron distributors, and the rest for different weighted applications, stock has 1, Corvette uses the stock, and the two others.

An HEI DOES NOT USE THE SLOTS AND PIN AS A START STOP DEVICE. Take a look at Bear's picture, look at the slots, you see BOTH ends of both slots, the pin does not touch them. So, HOW does the HEI curve start and stop limit? ITS IN THE CURVATURE of the combination of weights and center.

Now, the ONLY performance large HEi GM has ever produced is for carburetted ZZ series crate engines, with 22 crankshaft degrees, weights 41, center 375. These parts were derived from other large HEi's, all emissions parts that just work right together for a performance engine. MOST aftermarket companies now producing large HEI's now use a copy of the ZZ weights and center. Now, yes, there are probably combos of HEI weights and center that will work for other performance applications, but, NONE of them are paired together, and there are NO lists and charts to tell us what is what. And, the aftermarket "curve kit" is NOT for street use, it makes 16 degrees of mechanical advance only, and are designed to remove the vacuum advance, FOR DRAG RACE USE ONLY.

If anyone can't stand that there are no charts for the weights/center combos, feel dree to go out and get some parts, and start testing and listing, there are only 437 different weight/center combos used on the large GM HEI's, and ALL of them are for emissions engines.

Also, if anyone gives advice to "weld up, braze up, pout a screw in the slots" to change the curve in a large HEI, say "thank you", and make tracks for the hills as fast as is humanly possible, they do not know what they are doing.

Bear, as far as what module is actually behind what decal, I tested 7 of the ones you are asking about, against various others, with, and without other decals, red, black, blue, white, gray, and even stock GM regular and 990 modules, and did not find significant differences in any of them. With thwt, who cares whom built them, I use the one I feel is reasonably priced, good quality, and easy to obtain everywhere.

I am going to the shop now, I have a Pontiac points distributor to convert to EFI-HEI for a Holley system that was messed up by someone else, and do not care to get involved with the UGLY HEAD, whom I am sure will be right along when he sees my name on a post here. I simply refuse to have a battle of wits with a totally and completely unarmed person. .
 

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Discussion Starter #16
What we have to remember is, although the basic design of both GM points and HEI distributor mechanical advances is primarily the same, they vastly differ.

In the points distributor, the start and stop points of the curve are set with positive stops through the points plate, and with a bushing, degrees can be restricted down. this leaves the weights, and center curvature to give the actual curve degrees.
Yep, similar, but actually different.

Bear, as far as what module is actually behind what decal, I tested 7 of the ones you are asking about, against various others, with, and without other decals, red, black, blue, white, gray, and even stock GM regular and 990 modules, and did not find significant differences in any of them. With thwt, who cares whom built them, I use the one I feel is reasonably priced, good quality, and easy to obtain everywhere.
I searched for the GM 990 and found apparently that it has been discontinued from GM, so sources are limited to folks that happen to have a N.O.S. supply of them. Dave, which ones generally available at retail are "the same" or close?

Bear
 

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Still sneaking up on getting everything dialed in, most recently getting the Brand H carb to be happy. My AEM A/F meter was telling me it was still too fat nearly everywhere during the drive over to Dallas this past weekend, so yesterday I set out to address that and also to get the accelerator pump dialed in. After all, the supposed advantage of these carbs is their flexibility in being tuned, right?

I've changed the accelerator pump cam to the 'orange' one from the 'pink' one the carb came with in order to get a bigger shot earlier. Throttle response isn't quite "instant" yet but it's tons better than it was so I must be getting into the ballpark. The "blue" cam which is the largest and most aggressive seemed to be too much because I could see the A/F swinging super rich momentarily during the pump stroke. I've also leaned 'both ends' to 75/85 (from the original 81/91) and have gone to a 3.5 In Hg power valve from the original 4.5 (the rating is how much vacuum is required to hold it closed, so the lighter 3.5 should be opening a little later and delaying the power enrichment circuit activation).

This thing is serious. I took it out yesterday and hammered it a couple times from a rolling 2-5 mph start. Even with the Nitto drag radials on, it lit them up and spun all the way through 1st gear, upshifted, and kept spinning part of the way through second before finally hooking up. The TH400 was auto-upshifting at about 4800 RPM. I think on my "to do" list is going to be tweaking the governor in the transmission to raise the shift point some.

I really like the new converter I bought through Cliff Ruggles. It drives almost like a stocker until I hang my big toe over into the carburetor. Very little slip at cruise - on level ground a steady 70 mph is right at 3000, maybe 3100 RPM with 27.7" tall rear tires and 3.50 gears. My previous converter would have been running about 3300-3400 at 70 under the same conditions. That doesn't sound like a big difference, but it is.

Bear
 

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Great news, Bear! As you know, I've been advocating the benefits of a measly 300-400 rpm drop at cruise for years and years. Most folks don't think it makes much difference, but as you said, it sure does---especially on a road trip. Simply changing the 3.55 gears for 3.36's in my '65 made a huge difference in cruise comfort: lower rpms, less fuel used, lower engine temps. I'm a huge Cliff Ruggles fan myself---everything he does is based on real-world results from personal experience. The parts and combos he uses and recommends simply WORK.
 

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Now that the ignition gremlin is sorted out, I'm a little puzzled by what I'm seeing on my new shiny AEM digital A/F meter. At very light cruise, say 2000-2500 RPM when I'm reasonably sure the carb is probably still running just on the transfer slots, the meter shows really rich - 10:1 or even in the 9's - despite me having dropped it way down on jetting from where it was originally. If I roll into the throttle a little, not WOT but just enough to produce what would be 'normal' acceleration in any other car, it jumps up to the mid to upper 13's which is sort of where I'd expect. What has me baffled right now though is why does it show to be so rich at steady state cruise and then go leaner when I apply part throttle? Makes no sense... Well, I'm sure it will once I unravel the mystery, but right now it doesn't.

Bear
 

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Great story Bear. I'm glad you got it sorted. that would've drove me nuts. BTW on my HEI with a hotter coil i have run plug gap from .035 to the present .045. i see no difference in performance or plug condition. and yes, that is a rowdy cam....
 
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