B-28 follow up

[Dennis D.]

Re: B-28 follow up

Using rubber band method to restrict centrifugal advance.
Can the amount of centrifugal advance coming in at idle be determined given the data in the pics? Timing 12 degrees btdc at the crank.


Vacuum attached no rubber band
20230910_134513.jpg

Vacuum attached rubber restricting weights
20230910_133726.jpg

Gauge tee's. No change either way
20230910_134624.jpg

[Dennis D.]

Re: B-28 follow up

Btw... chart from TI specialies

Think the info in the CSM is wrong

20230910_164112.jpg
Screenshot_20230910_164100_Photo Editor.jpg

[Duke W.]

Re: B-28 follow up

With 12 degrees initial and the full 16 from the VAC total idle advance with the centrifugal tied up and not contributing advance should be 28. but your reading is 22 and it rises to 34, 12 degrees more, at only another 56 RPM with the centrifugal free, so the it appears the centrifugal is adding 12 degrees at idle This makes no sense at all.

I previously asked you to test and document the centrifugal curve, but you have not posted that data.

The AMA specs data for the ...491 distributor says start at 1000, maximum 26 @ 5000. This is in engine degrees and speed as is the CSM data. As I stated before the TI Specialty data is in DISTRIBUTOR degrees and RPM, which is half the engine values, so the two sources are close. I assume Dave has a distributor machine, so he works with distributor degrees and speed. When testing on the engine we work with engine degrees and speed.

I'm having a tough time making out the numbers in the photo of the CSM page even if I expand it, but it looks like it has errors, and it doesn't jibe with the AMA Specs. When the service manuals were printed that was it. There were no revisions., so the errors are there forever. The AMA specs were a "living" engineering documents and revisions were made through out the model year to reflect spec changes or correct errors. I'll take the AMA specs as gospel.

I'm not familiar with your timing light. Mine is an older model with a dial... rev it to whatever turn the dial to bring the balancer notch back to zero on the timing tab, and the dial reading is advance. So the centrifugal is suspect, but be sure you're using the timing light properly to get accurate data.

Remove the cap and rotor, and post a photo of the centrifugal mechanism. Then test - at least get the start point RPM and a few measurement every 500 to 1000 revs up to about 3500. It may have some extremely light springs that are causing it to start to deploy way too soon.

Duke

[Dennis D.]

Re: B-28 follow up

I'm reluctant to test the centrifugal advance at high rpm. As I mentioned originally, getting the car back from the shop something seems off. Story for another time.

Thought I read where you mentioned testing a new B28 and the tester only getting 10 degrees not 16. Is that a possibility here?
I didn't think to hook up the vacuum pump to the running distributor and maxing the advance can, then check the timing.

This is an old pic, since there is a B28 can in there now. Have no idea what the 540 means. I don't think they relate to any degree

Screenshot_20230910_184918_Gallery.jpg

[Duke W.]

Re: B-28 follow up

Your reported data indicates the B28 is only providing 10 degrees max advance.

It's always been my understanding that the last two digits of the three digit number stamped on the cam assembly (or rotating pole piece with TI) is the max centrifugal advance, which is based on the length of the slot. I have never seen one greater than 32, and since the spec for max centrifugal of your LT-1 is 26 degrees I find this part suspicious.

Post a photo with the weights and springs installed.

As far a centrifugal is concerned you must test (VAC disconnected and plugged) it to find the RPM start point then test in 500 RPM increments to at least 2500. What you will read is initial plus centrifugal, and if are are absolutely positive than you know initial, subtract that from all data points for the centrifugal data

Until you provide the requested centrifugal test data I can offer not further advice.

Also, what is the "other story" about the recent work done on the car. Since the problems started after this work was done it's highly likely that there is a connection

Duke