427 TI Distributor Centrifugal Advance Malfunction - What would happen?

[Richard M.]

I think I've discovered a unusual problem. I'd like to get your opinions as to what a scenario would be if this condition occurred.

A problem on a TI ignition 427/435 that only shows up after engine normalizes and driven for several miles. It could be 2 miles, or 20 miles. Engine runs fine, bring it up to 3500-4000 rpm while driving. All is fine. Reduce throttle/RPM. All of a sudden the engine begins to miss, stumble, and feels like ignition is erratic. Engine making strange "internal" noises, and sometimes backfires. Stop, let things cool down, restart and all is fine.

All TI serviceable items have been replaced with known good/new parts. Module, Pickup coil, Harness, Ign Coil.

Tough problem? It has been very difficult to get it to repeat.

This is what I'm thinking could be happening. I discovered that the distributor football on the shaft was welded by a prior owner. I also discovered that the Rotor tip is not aligned with the gear dimple. It's about 45* off. Not a big deal, but just a observation.


I finally found that the Rotor center/bottom was actually hanging up on these repair welds and the rotor would not return to it's quiescent state. I turn the rotor by hand and it sticks advanced. I discovered this by chance.


This white rotor(below) was in this distributor a long time before. Never showed this problem. You can see some marks on it from where it may have been rubbing on the spot welds, but never exhibited the conditions with this rotor. It moves freely by hand.


It seems that when the centrifugal advance activates(AMA Spec is 30* @ 3800 RPM) and advances the timing all is fine, but when the engine RPM is reduced, the "new" (black) rotor is retracting intermittently. No fault of the rotor. It's the modified shaft on the distributor.

How badly would the engine run if this happened?

In my mind, this....If #1 was intended to fire, it was now potentially near #8 (30* closer) and firing it. #8 may be on it's INTAKE stroke with the Inlet valve open, or maybe on the start of it's COMPRESSION stroke. The spark now occurs on #8 instead of #1. Some fuel burns, and a possible backfire. Next in line is #4. #8 fires but #4 is in a similar state as the #8 example. Etc, etc. The engine runs like it's completely out of time. Strange noises and even seems like it's internal. There must be some unusual activity inside the engine as the firing is way off on each cylinder sequentially. We also notice a pungent odor of unburned fuel.

EDIT....( I shaved the backside of the rotor to help clear the raised spotswelds. All seemed fine. Not so. I think that on acceleration, that the distributor shaft may be raising slightly. If so, it may be exerting pressure on the underside of the rotor and causing it to continue to hang up when deployed and not returning.

Also, to help understand.... Why then does the engine run fine after the condition occurs? I think that when the engine shuts down abruptly, especially if there is a backfire, that the centrifugal motion and torque of the distributor shaft then returns the rotor platform back to it's at rest position. Engine starts up fine and runs fine. The car ran fine for about 10 miles, with the operator babying it back home and never reaching engine RPM exceeding 3000 RPM.)

Tomorrow we will change the rotor back to the original and ensure that it doesn't hang up when advanced. Until then, what is your opinion regarding how a engine would run in this scenario? Could it exhibit the conditions I've described? I'm merely trying to understand if this is the smoking gun or not.

Thanks,
Rich
ps We will ultimately get the distributor rebuilt with a new shaft to do a correct repair.

[John M.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Hi Rich It would seem that having the distributor stuck in an advanced condition could cause detonation, those noises, and possible damage to something. I had problems with a TI in my 69 and fixed it by putting in a points distributor, temporarily, which at least confirmed it was the distributor. I'd be a little worried about those noises.
John

[Philip A.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Could it be a timing gear/chain issue and the distributor mods you see are/were an unnecessary band aid fix?

[Richard M.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

John, Yes the noises are not good but we shut down when the condition occurs. The engine starts right up and is fine until it gets to high RPM.

Phillip, The engine was completely rebuilt with a new high quality timing gear/chain set. Yes we discussed it seems like a chipped tooth but it is not consistent after the condition occurs. If it was a bad chain/tooth it would happen always.

I have a SB TI distributor here and I'm going to cut up a old rotor to check clearance out of curiosity.

BTW, the car is 1500 miles from my reach. I have a very knowledgeable engine guy on it up there.

I need to get the distributor properly corrected, but for now trying to understand the problem.

Rich

[Duke W.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

The pickup coil wires are constantly being moved by action of the VAC. This can result in strands breaking, which can cause the symptoms you report. It will usually start out as an intermittent condition, but eventually completely fail. Disabling the VAC is one way to test for this condition. If the problems cease or are even mitigated failing pickup coil wires could be the culprit.

Also you can exercise the VAC with a Mity Vac while measuring continuity with an ohmmeter. Resistance should remain constant. Pumping the VAC to the fully advanced position then quickly releasing the valve simulates what happens when you open the throttle about halfway or more from a cruise condition.

This statement caught my eye:

"This is what I'm thinking could be happening. I discovered that the distributor football on the shaft was welded by a prior owner. I also discovered that the Rotor tip is not aligned with the gear dimple. It's about 45* off. Not a big deal, but just a observation."

Actually, this may be a big deal. If the football was welded on in a different angular position than OE, the rotor may not be lined up with the correct cylinder with the OE wire indexing causing intermittent cross fire, which can cause backfires. There are only two positions the dimple should assume - either lined up with the rotor tip or 180 degrees out.

Dressing down the underside of the rotor to provide sufficient clearance is a good idea, but also reducing shaft end play to the 2-7 thou spec for single point distributors is a good idea to provide adequate running cleanance and reduce spark scatter.

If you can borrow a TI distributor to test on the car and the symptoms go away, that isolates the problem to your distributor, and as a minimum you should get a new shaft that's properly manufactured to a GM print.

Duke

[Richard M.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Thanks Duke,

I should have mentioned, that this distributor has been in the engine many years and running fine, so the football/dimple oddity apparently had no effect on prior operation. The engine ran fine and never had these issues. The only thing I did to the distributor was install the new rotor, and that in itself IMO then exacerbated the clearance problem to those spot welds. For the record, we WILL get this distributor properly corrected in the future.

Buried in my text I mentioned we just installed a new pickup coil yesterday. We thought that could be the problem too, but not the case. We tested the newly installed pickup coil on the bench and got correct readings, and when installed in the engine the car started right up and appeared to have a better sound, and as my friend put it, a much more "snappy" response.

This morning I located a NOS TI distributor from a friend. Unfortunately for a small block, 1970 LT1. Part# 1111491
It is missing the special channel blocked area in the lower shaft oil groove. I don't understand why the BB distributor has that blocked groove for engines after 1967. I am reluctant to use this in a 427.

I did a quick experiment using a old rotor. I cut half away and installed it on the above distributor. As you can see, there is no clearance for the bottom of the rotor against the football.



I'm still waiting to hear from my friend if he had a chance to work on the engine this morning.

===

[Patrick B.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Rich -- the LT-1 distributor will work fine. The blocked groove is not really needed for the big block. I am using a 70 LT-1 distributor in a 69 ZL-1 427 and it works fine.

[Richard M.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Rich -- the LT-1 distributor will work fine. The blocked groove is not really needed for the big block. I am using a 70 LT-1 distributor in a 69 ZL-1 427 and it works fine.

Patrick,

Did you specifically verify that the oiling system is properly oiling the top end valve pushrods and rockers?

I have never understood the purpose of the blocked groove and it's usage on the post-1966 427 engines.

This morning I've been researching this issue and I'm at a loss.

Rich

[Richard M.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

I found more info, and I'm now more confused than before.....

https://www.forums.ncrs.org/showthre...-BB-cam-oiling

https://www.forums.ncrs.org/showthre...r-in-BB-engine

https://www.forums.ncrs.org/showthre...k-Distributors

[Patrick B.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Rich -- The blocked groove distributors came with the early Big Blocks with grooved rear cam bearings and grooved rear cam journal. 1967 and later big blocks have the oiling groove in the block and use a regular rear cam bearing and cam, but I don't know when they changed the distributors. My ZL-1 has been running a full grooved distributor for years. I verified the upper oiling with an electric drill and and an old distributor (with the weight plate cut off) that I use as an oiling tool before I first started the engine. The oiling tool was a small block distributor. I have also used it successfully on 65 and 66 big blocks without actually thinking about the groove difference. You probably have an old small block distributor that you use for an oiling tool also. If so, give it a shot and verify for yourself that the upper oiling is unaffected.

[Richard M.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Rich -- The blocked groove distributors came with the early Big Blocks with grooved rear cam bearings and grooved rear cam journal. 1967 and later big blocks have the oiling groove in the block and use a regular rear cam bearing and cam, but I don't know when they changed the distributors. My ZL-1 has been running a full grooved distributor for years. I verified the upper oiling with an electric drill and and an old distributor (with the weight plate cut off) that I use as an oiling tool before I first started the engine. The oiling tool was a small block distributor. I have also used it successfully on 65 and 66 big blocks without actually thinking about the groove difference. You probably have an old small block distributor that you use for an oiling tool also. If so, give it a shot and verify for yourself that the upper oiling is unaffected.

Patrick, Thanks, in fact you reminded me of a forgotten task....

When I had this engine rebuilt, I did in fact use a cutoff SB distributor priming tool to prime the engine. I completed the build of the "short block" assembly done by my machine shop. I installed the heads and associated top end hardware. I then primed the oiling system and yes, the top end oiling reached the rockers.

My confusion stemmed from the above referenced threads, where there appeared to be doubt by some of the participants that the blocked groove/"partial annulus" distributor went out of service in 1968 versus 1967. Those threads never specifically closed with total agreements by all on that issue. I was left with a level of uncertainty.

I still have a engine that has a intermittent ignition related issue and am working on it with my remote troubleshooter. I can purchase the 1111491 SB distributor above from a friend and send it up there, but I must be 100% sure it will be compatible with that engine. I will ultimately send the original 1111258 distributor out for repair of that shaft and proper rebuild.

I'm sure you understand my concerns.

Thanks,
Rich

[Patrick B.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Rich -- I just read some of the old posts and I see what you mean about the lack of consensus. However, I did use the same small block distributor oiling tool to test the upper oiling of my own 67 427/435 (January 351 block) prior to first starting it with the original 258 distributor.

[Richard M.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Thanks Patrick, Now you can see where I was uncertain, however I think I may have something to add more credibility to the issue. For reference, here are photos showing the lower distributor shaft area oiling ports on a 1967 3904351 427 block. It's owned by a friend and I just took photos.


Forward hole


Rearward hole


It would now seem logical to me that a distributor either with or without the blocked groove/partial annulus lower shaft area would supply oil to the destination oil galley in this engine.

Rich

[Richard M.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

"This is what I'm thinking could be happening. I discovered that the distributor football on the shaft was welded by a prior owner. I also discovered that the Rotor tip is not aligned with the gear dimple. It's about 45* off. Not a big deal, but just a observation."

Actually, this may be a big deal. If the football was welded on in a different angular position than OE, the rotor may not be lined up with the correct cylinder with the OE wire indexing causing intermittent cross fire, which can cause backfires. There are only two positions the dimple should assume - either lined up with the rotor tip or 180 degrees out.

Duke

Duke, Thinking more carefully about your statement above....

If the football was welded on without regard to gear dimple to rotor tip at 0* or 180*, wouldn't it just be in a new gear position? When the distributor is installed at #1 TDC, with the balancer timing mark set at 10* BTC, and the Rotor Tip pointing to the OE #1 position of the Cap, wouldn't that now be a "pseudo" correct installation?

This distributor had the spot welds for many years and never exhibited the present condition. Not until I installed the new heavier duty rotor, and then misdiagnosing other faults(see #1 to #4 below), did I see where it was hanging up on the spot welds and then did a temporary fix by grinding the underside of the rotor.

This is a very long and detailed summary of the history of this issue for reference.....

There has been a intermittent issue that over several months had tricked me. Several "fixes" that I thought corrected problems, but not so as time progressed. This is a recap of my previous diagnoses, which has now led me to believe I was tricked by making misdiagnosed repairs to items shadowed by a ghostly condition, which I ultimately feel is a intermittent centrifugal advance return.

1- The first time I had a problem was right after engine break-in. I installed the new rotor before break-in, not knowing or noticing the Dist shaft spot welds. BTW, the lesson learned..... is that I will ALWAYS now check a dist shaft advance football for spotwelds!!

I would have a problem intermittently trying to start. Occasional backfires. I had Holley stumble issues which were caused by debris inside the carbs(all 3), then after correcting that and reducing float level was better. Time later I finished the restoration and re-installed the body and completed the car.

2- After several test rides after restoration completion, one time I accelerated at high RPM and the engine missed, ran rough, and made strange noises. I thought there was a valve train issue. I even pulled valve covers suspecting broken valve springs. All okay. I pulled all spark plugs. #2 was black. I tested plug wires, and #2 wire was intermittent open. After re-crimping the wire, no start. I then decided it must be the ORIGINAL AC Delco TI module, as they're know to fail when Spark plug wire removal occurs.

I replaced the TI module with new, engine started right up and ran fine. A few short test rides, IIRC, not exceeding 3000 RPM, and I thought all was well. I then replaced ALL Plug wires and plugs with new, after testing each one for continuity, another test ride for miles, all was fine.

3- A week or so before the car was going back to my friend, I ran the engine to normal temp, tested timing, carbs, temps, etc. At 950 RPM apx 25* advanced with B26 VAC connected to Manifold vacuum. All was fine. I had it idling for a while and then took it for a aggressive test drive. As soon as I reached high RPM, engine began to stumble and run rough with loss of power. Very similar to the first time in #1 above. I feathered throttle and turned around and barely made it back to the shop. Disgusted and frustrated, I closed the shop door and took a break. After engine cool down and after a few hours I tried a restart. No start. I put a timing light on the coil wire. I saw spark.

I started pulling plugs again. All perfect. I then started testing the new Plug wires, all good. I pulled the Coil wire off of the old D207 coil and something strange flew in the air.......OIL. The Ign coil was leaking oil. I removed the coil, and saw the screw holding the coil wire ferrule was loose. Oil was dripping. I was very careful trying not to get any on my skin as I know it's not healthy.

I acquired a new #263 TI coil a few days later, installed it, ran the engine to temp, and test drive was fine. I'm certain I got it to higher RPM, but don't recall exactly. All seemed fine.

4- Now the ghost has returned. Intermittent no-start. Last week we changed the Dist Pickup coil. Car ran fine until a high RPM run. Back to the original problem, rough running, etc. Shut down a while, starts up fine and ran fine at lower RPM for miles. Yesterday the car went to a local car show, 20 mile roundtrip, and ran fine. No high RPM takeoffs were performed. It actually took first place.

Summary of diagnosis and faults:
1- Bad Plug wire and foulded #2 plug
2- Bad AC Delco TI Module
3- Bad D207 original Ign coil
4- Changed Dist Pickup Coil

Today we will continue diagnosis. We will revert back to the original rotor and test. Then if deemed necessary, we will change the TI Module with another new(spare to keep in the car), then TI Harness using a tested new M&H part.

I will update when the results are in.

Rich

[Duke W.]

Re: 427 TI Distributor Centrifugal Advance Malfunction - What would happen?

Each gear tooth is 27.7 degrees and the cap terminals are every 45. So depending on the orientation of the football and initial timing there there is likely some misalignment between the rotor tip and cap terminals. They are wide enough to tolerate some misalignment, but I don't know how much.

Too much misalignment could cause misfire or cross fire. Being as how Bubba appears to have worked on you distributor, I would recommend obtaining correct unmolested parts. That may not solve the problem, but at least you can eliminate Bubba's work as a cause.

Duke