B-28 follow up

[Dennis D.]

2 B-28's from rock auto showed up today. Along with a new mityvac gauge. Both units tested out starting around 4 and all in at 8.
Tested the unit in the LT-1 figuring it was leaking, but got same results. And it's holding vacuum.

My issue is when I pull the vacuum hose there's vacuum at the hose. (Have it attached to manifold vacuum)When plugged into the B-28 there is no difference in idle.
Pulling the cap I watched as vacuum is applied, the magnetic pickup rotates counterclockwise. The centrifugal advance doesn't move, unless i move it by hand clockwise.That correct?

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[Hank D.]

[Duke W.]

Re: B-28 follow up

The vacuum pump allows you to check the start and stop point in inches Hg. But it does NOT verify how much advance is added. That can only be checked on the engine, and you MUST be CERTAIN that the speed at which you cheek added vacuum advance is BELOW the centrifugal start point, so you need to TEST for the centrifugal start point RPM.

Using real stiff springs is one way to make it easier, or tie up the centrifugal mechanism with a stiff rubber band.

In the nearby '69 vacuum advance thread the installed supposed B28 did not met the 4-8 stop-start vacuum specs. The replacement did, but appears to add only 11 degrees at about 8" and above, so it apparently doesn't meet the max advance spec of 16 degrees.

Duke

[Dennis D.]

Re: B-28 follow up

When idling at 850rpm and I pull the vacuum hose from the distributor, I was expecting a change to the idle and there was no change.
There is vacuum to the hose that I will get a reading. Last I checked it was between 11-12.

[Duke W.]

Re: B-28 follow up

Assuming full time vacuum advance when you pull the hose off the VAC revs should increase due to the extra air, but the idle will likely be rough because it's now lean.

Then, if you plug the hose, engine speed should reduce to below what normal idle speed is with full time vacuum advance, and may even stall due to the reduction is advance.

I hope understand that to measure idle vacuum you must "tee" into the the VAC signal line assuming full time vacuum advance. I know of cases where the owner simply removed the hose from the VAC and plugged it into the vacuum gage. So now the engine configuration has changed due to the loss of added advance from the VAC, and the vacuum readings are meaningless.

Duke

[Dennis D.]

Re: B-28 follow up

I get that. I do tee a gauge to check vacuum advance. I expected some sort of change pulling the hose, since it releases the pull on the magnetic pickup.

Think you gave me info to get started tomorrow. Car came back from shop and seems off.

[Duke W.]

Re: B-28 follow up

When you pull the hose off the VAC check how much vacuum you are seeing. No change in engine speed when you pull the hose is what usually happens with ported vacuum advance, so something doesn't add up.

Duke

[Dennis D.]

Re: B-28 follow up

Warmed engine. Idle 875-900rpm.
Attached vac gauge tee. Vacuum11-12 at idle (vac attached.)
Pulling hose idle raises a bit. Vac same
Plug hose, engine want to stall.
Dial back timing light shows 12 degrees btdc at crank.
Reconnect hose timing jumps to 40 btdc.
Looks like a light and heavy weight spring. Am I getting some mechanical advance?

[Duke W.]

Re: B-28 follow up

The CSM will tell you when the centrifugal starts, but you need to test it to be sure. This is a common mistake... measuring "initial" timing at a speed above where the centrifugal starts and it's a particular problem with SHP engines that often need to idle above the centrifugal start speed. For example, the 365/375 HP 327s centrifugal starts 700, but these engines will rarely idle stably long enough below 700 to actually check and adjust the initial timing. If in doubt tie up the centrifugal advance with some stiff rubber bands.

Another solution is to use the "Total WOT method"... with the VAC disconnected and plugged, rev the engine a few hundred revs above the speed of max centrifugal (that you previously tested) and with a dial back timing light set the advance as high in the 36-40 range as the engine will tolerate without detonation. This is easy on the 365/375 HP engines because the OE centrifugal is all in by 2350 engine revs, but some engines full centrifugal may not be in until as high as 5100. Most of these "lazy" centrifugal curves should be quickened with lighter springs up to the detonation limit.

On the 365/375 HP 327s a 900 idle speed adds two to three degrees centrifugal, so with 12-16 initial, 24 centrifugal and 16 vacuum total idle advance could be as high as 35 degrees, and total cruise advance at light load at 2350 and above could be as high as 54 degrees. The high overlap mechanical lift cams need a lot of low sped-light load advance because of the high exhaust gas dilution from all the overlap that slows combustion propagation.

The Duntov and LT-1 cams have less overlap, but they like about the spark advance map as the 30-30 cam engines.

Duke

[Dennis D.]

Re: B-28 follow up

Going by gm spec for the 1111491 distributor, centrifugal at 850rpm=5 degrees dist.(10 crank)
With 12 dbtdc +10 centrifugal + 18 vacuum =40 at the crank?
My reading with b28 if using gm centrifugal spec.
I need to figure my actual centrifugal.

[Duke W.]

Re: B-28 follow up

I don't understand your post. What is the centrifugal start point and what is the max centrifugal, crank degs. @ engine RPM, as listed in the CSM and AMA specs?

The test for what is actually is because there's a good change it's been changed at some point in the car's life.

If centrifugal in distributor degs. and RPM is 5 @ 850, then that's 10 @ 1700 at the crank. Since were not working with a Sun distributor testing machine, lets deal with crank advance and crank RPM, and not intermix the two.

Duke

[Dennis D.]

Re: B-28 follow up

Factory spec sheet. My distributor Circled in red.

I used those figures and applied to what crank advance I was getting in real time, vac connected.( 40 dbtdc).

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[Duke W.]

Re: B-28 follow up

Those specs are clearly in distributor degrees and RPM. Read the footnotes. So you multiply by two to convert to crankshaft degrees and RPM. Thus the centrifugal curve is 1 @ 1150, so it probably starts at about 1000. Max is 26 @ 5000, which is very lazy.

The best spark advance map for the LT-1 is the same as the 365/375 327s... first two entries on the chart, 062 and 064, and set total WOT advance as high in the 36-40 range as the engine will tolerate without detonation.

Maybe even more because of the fact that the HEI retards advance above the max centrifugal point, Read note 3.

What document did this chart come from?

Duke

[Dennis D.]

Re: B-28 follow up

Yes, I see that. Thanks. Just trying to figure whats effecting the timing im getting at the crank.
Going to restrict the weights with rubber bands and recheck the timing at the crank. Comparing to previous readings.
What should expect to see from vacuum as well as centrifugal advance

[Duke W.]

Re: B-28 follow up

This thread is getting tough to follow. I don't want to speculate on what you expect your measurements to be. You need to test and report the following data.

1. Start and stop RPM points of the centrifugal advance (the VAC must be be disconnected for this test)

2. Initial advance measured below the start point of centrifugal that you measured. (VAC disconnected) If full centrifugal is still the OE 26 then initial needs to be 10-14 for max WOT advance to be in the optimum 36-40 range.

3. Max WOT advance (VAC disconnected) degrees @ RPM. With the OE centrifugal you will have to rev over 5000. If you're not comfortable with that get a Mr. Gasket 928 spring kit and install the black springs).

4. Total idle advance, VAC connected, state degrees @ RPM

Once the above data is in hand we go from there.

Where did the TI distributor specs chart come from???

Duke