427 TI Distributor Centrifugal Advance Malfunction - What would happen?

[Dave S.]

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

Rich,
You installed a reproduction #263 coil......right.? Maybe that coil has gone bad.......

[Richard M.]

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

Duke, I follow now. Although I thought that just by turning the distributor as normally done to set timing, any error in gear to tip misalignment would be corrected by default. The effect of what you describe though, is evidenced by a slightly abnormal placement of the VAC with respect to the intake/Dist shield. In order to get 10* static timing, the VAC is in fact very close to the Ign shield. Babba strikes again and I feel angry at myself for not seeing this oddity long ago.

As usual, what we don't see or notice in a restoration, usually bites hard later. I'm certainly guilty of that here.

Rich,
You installed a reproduction #263 coil......right.? Maybe that coil has gone bad.......

Dave, Yes the 263 is the reproduction, replaced when I found the oil leaky old D207 which I suspected failed hot. I saw a nice NOS 263 for about a grand, but opted out of that selection. I'm not really liking the idea of a 50 year old used D207 either though.

I also have a spare repro 263 there if needed.

I don't know if the new existing repro is bad. Difficult to test until engine gets hot. Probably a good test would be when the failure occurs, is to use a fan or freeze spray to cool it down.

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General update....
Today I tackled more remote diagnosis with the troubleshooter. We checked the centrifugal advance operation. To my surprise, it was fine and with a few twists by hand it returned to it's stop properly. So I think I'll have to rule out the spot welded Bubba'ized distributor shaft for now.

Unfortunately, further diagnostic test results were inconclusive, and a bit frustrating. We changed the TI Harness, using a tested spare new unit, and a TI Module which was a spare. It then had NO Spark. We replaced the TI Module back to the one that was working/suspect @ high RPM, and spark was then good. TI Harness therefore good.

The TI Modules require a solid ground with toothed lockwashers under the mount screws. We ensured that was the case. We tried it 2 times, and I had the troubleshooter verify module case ground to the internal circuit board ground. All was good. I also had him verify the harness ground and that the radiator support was well grounded. The radiator support gets ground via the frame horn mount bolts AND the Engine harness ground where a lug is attached to the horn relay mount. All tested good.

We stopped the exercise, as I now need to get a replacement TI Module from my supplier tomorrow.

I now may have a intermittent bad module which only fails at high RPM, another that's DOA, or a bad coil. Or a combination of all.

We likely won't get back to tests until later in the week. I'm not ready to swap in a points distributor....yet.

Rich

[Richard M.]

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

Update..... It was not the distributor.

The high RPM fault was diagnosed to be a defective TI module(M&H). At high RPM the module was breaking down, causing erratic ignition operation, mistiming, detonation, and poor performance at lower RPM. The failure my be attributed to "infant mortality", a condition which exhibits itself after only a short time of usage. They have a one year warranty so I will contact them for a replacement ASAP. I bought it earlier this year.

The K&B modules that were failure suspects, were NOT failures. I had them returned to me and passed my bench tests....Details HERE. Final results at post#34. They were returned and one was reinstalled and works fine, at ANY RPM. The other will be kept there as a spare. We suspect a interconnect or bad case ground was the reason for initial TI ignition failure.

The old Delco TI module which helped us confirm proper high RPM operation during this remote troubleshooting exercise, is now safely tucked away in a cabinet and will someday be encapsulated in clear resin and framed to become garage wall art.

Rich

[Richard M.]

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

Duke, et al..... As another follow up and for future reference, here are some photos of what you described showing how a misaligned shaft football affects the rotor tip to cap position alignment. We've changed the "non-OE/welded football" bubba'd distributor shaft on the initially described distributor in post#1 with a new replacement. This will eliminate the chance of spark scatter due to the non-OE configuration of the modified shaft football. Why this distributor never exhibited a spark scatter problem for many years of use is unknown, but I have some thoughts about this later.

These photos(a OE TI distributor I have here) show the placement of pole piece tips to stator tips, which when aligned, produce the signal to trigger the amplifier for spark generation. This has been confirmed using my bench test(more info HERE). When the stator tips are aligned, the rotor tip is precisely aligned with a distributor cap spark plug wire position. This is the design.

I used my cutaway rotor so I could see the alignment. When the tips align, the rotor tip precisely aligns with a plug position. I marked the base of the distributor with cap installed to place a witness mark of the plug wire position center. This is when spark occurs from the TI module to the ignition coil.












The modified distributor, as simulated below using my borrowed TI unit, is approximately where it's located on the wrong/modified shaft when the rotor tip is aligned with a cap wire position. The gear dimple is about 90* from the rotor tip. When the stator tips align, producing spark trigger, the rotor tip is now NOT aligned with a cap wire position. It's nearby, but likely as mentioned the width of the rotor tip may help offset spark distance, but there is likely a limit before it scatters.


Here are my thoughts. The engine had this modified shaft for many operational years. I never took a baseline timing measurement on the engine before disassembly so I have no idea of its prior advance map. I also never attached a vacuum gauge to check idling running results. The VAC control was a MS201 15 connected to ported vacuum. When I rebuilt the engine and setup the plan for full time manifold vacuum I used the B26(VC1765) VAC anticipating a stock build running at about 14" to 15"-Hg at idle apx 950rpm. The B26 is all in at apx 12"-Hg so I would be within the 2" rule.

I still have the original MS201 15 VAC (spec max 15* cam advance) and just did some vacuum gauge test measurements using my Mityvac. It begins to move at 12"-Hg. At 15"-Hg it's approximately through 1/3 of its travel, which is apx 5*(cam)advance. At 18"-Hg it maxed at apx 2/3rd travel, which would equate to 10* advance.

With the engine originally setup with ported vacuum, at idle, there would have been zero advance. At increased rpm off idle, the VAC would then become active and produce no more than 10* advance(20* at the crank). I do not have prior rebuild baseline data, but assuming the OE L71 configuration with a maximum of about 15"-Hg at cruise, the prior build would have allowed a limited amount of VAC advance. This may be a reason why the engine never exhibited conditions of spark scatter due to the modified shaft football.

Now, after rebuild, I did my conversion to full time manifold VAC to generate a scientific advance map for better normalized idle/traffic/ambient temperature and fuel economy, the modified distributor now became apparently intermittently dysfunctional. I believe that this exacerbated a inherent spark scatter within the distributor under certain conditions.It may be that the higher ambient and higher engine temperatures added to this condition.

Additionally, the prior engine configuration used the original Delco TI module. When I replaced that with the later technology silicon based TI modules, which by design are more sensitive to distributor pickup coil trigger pulses, it may have contributed to some intermittent ignition operation under specific conditions. I am now not convinced that the M&H module was the reason for the high RPM failure. It may have in fact been the distributor. More road tests using the corrected distributor will either prove or disprove my theory.

Rich
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[Duke W.]

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

Now that you have the distributor back to is "as designed" geometry I expect it will perform as new. Let us know.

The VAC is likely original. I know the 201 15 was OE on L-71, and it was not available from service parts after the late sixties. VACs are known to freeze up or the diaphragm can leak. It's also possible that the diaphragm that I believe is a reinforced elastomeric material can harden with time - like a tire and still function, but go way out of spec. That's why it's important to check function on an old VAC, especially if you think it's original, as part of a normal tune-up.

I believe the main power transistor closes to cut off primary current when the rotating pole piece is just past the stationary pole piece and the induced voltage drops off rapidly.

The TI distributor can be static timed on installation similar to a point distributor. With the balancer notch set at your target initial timing on the timing tab, once the dist. is properly installed rotate the housing such that the rotating pole piece is lined up with the stationary pole piece or just past and snug the bolt, but just enough to allow the distributor to rotate with some resistance. This should get you within a couple of degrees of the target timing value and the engine should start easily.

Then do a final check and adjustment, as required, with a timing light and tighten the hold down bolt. With the distributor properly secured the initial timing should never drift off your setting as long as no mechanical or electrical problems develop in the system.

Duke

[Richard M.]

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

Thanks Duke, I'll certainly let you know how the car tests go.

Yes I thick this MS201 15 is very old and not to be trusted. However, I also have a reproduction MS201 15 and it is way off spec too. I'll have to test that one again. I think I've heard you say it may just be a B1 in diguise with the restamped MS# on it. Did we ever figure out what "MS" stood for?

I'll may try to capture a few accurate timing waveforms at my bench test setup with respect to the rotating pole piece tips/stator tips to see if I can show their physical relationship to the coil pulses. Not sure if I can see with the eyes though. Ideally, a camera to shoot a video of the tips and the waveform at the same time and run in slow motion, may visualize the result.

Rich

[Richard M.]

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

A follow up regarding the distributor shaft. We replaced it some time ago with a new SHP shaft and the engine has had no problems at high RPM since.

The modified shaft compared to a reproduction I had here. The repro is a base shaft, not SHP, but can clearly see the football rotational positions with respect to the shaft gear mount pin hole and slot are very different.