B26 bushing

[Carmelo P.]

1967 L79
Replacing a 236-16 vac with a B26 .
The 236 bushing was in found pieces.
Does the B26 also need a bushing on the pin riding in the slot ? Thanks...

[Harry S.]

Carmelo, my understanding is that the bushing was actually a spacer. The spacer(s) came in different thicknesses depending on the vacuum can. The spacer controlled how much vacuum was actually generated by a particular can. So, I believe the answer to your question is yes. I'm sure others will chime in.

[Duke W.]

The specs for the B26 are start at 6", 16 deg. @ 12", and I don't think they have a bushing. Bushings on OE VACs were there to limit total advance, and I don't recall that OE 236 16 VACs from service parts have a bushing, but I can't be sure.

The only way to tell how much advance is provided by any VAC is to test on the car. This is done by starting with a high idle speed where the centrifugal has clearly started with the VAC hose disconnected and pluged. Then, while watching the timing notch retard while reducing idle speed, determine the engine speed that centrifugal is fully retracted. If necessary, install stiff centrifugal springs for this test or temporarily tie up the centrifugal advance mechanism with rubber bands if you can't verify that centrifugal is fully retracted before the engine stalls due to to low engine speed, and set initial timing in the range for the specific engine configuration.

Once you can get the engine to idle at a speed that you know for sure is below the start speed of centrifugal advance and you know initial timing, connect the signal hose to the VAC. This will increase idle speed, so lower it down to below the speed that centrifugal starts, measure total idle advance and subtract initial. The result is the amount added by the vacuum advance, assuming it meets The Two-Inch Rule.

Duke

[Owen L.]

Would it also work to hook up a mity-vac to the vac-can while the engine is idling and pull vacuum to whatever in.hg you're testing for? For this method, I don't think dropping to zero mechanical advance would be necessary to get vac-advance readings. (yes?)

[Duke W.]

The trouble is that as you pump down the VAC it will add advance, which will increase engine speed, likely to the point where centrifugal advance is added.

IIRC the '67 L-79 centrifugal is start @ 900, 30 @ 5100, but check the AMA specs and CSM to be sure. The other issue is that it may not be OE as it was a common modification to quicken even back in the sixties, so you need to verify.

If it is OE then set initial timing with the VAC hose disconnected and plugged at 750, and the engine should idle stably if it has the OE cam. Then connect the signal hose to the VAC and reduce engine speed back to 750. You are now reading total idle advance. Subtract initial and you have the amount added by the VAC. You could also use the vacuum pump to determine not only added advance, but also the amount of vacuum required to bring it all in.

I always recommend buying VACs locally and checking that the part pulls the pin to the limit at spec plus or minus 1" Hg. If not, don't accept the part.

I'm assuming you have an OE cam, and if so it should idle at 750 @ 14-15" inches, so a 12" VAC meets the Two-Inch Rule. I also recommend you install lighter centrifugal springs to bring all 30 degrees in earlier - say 3000 or so.

I never understood why the L-79 spark advance map was set up with with laziest centrifugal curve from the base engine and the most aggressive VAC from the 30-30 cam engines. Given idle behavior a 12" VAC passes The Two-Inch Rule, and if you have 93 PON fuel available. even with the original CR, it should tolerate the very aggressive centrifugal of the 30-30 cam engines, which starts at 700 and should bring in all 30 degrees by about 3000, and 6-10 initial gets total WOT advance into the optimum max power ranger of 36-40.

Dukef

[Owen L.]

The trouble is that as you pump down the VAC it will add advance, which will increase engine speed, likely to the point where centrifugal advance is added.
Duke

<forehead slap>Ah, yes, the idle will rise with any timing advance so it becomes a problem in keeping the two separated in the readings. </forehead slap>

[Carmelo P.]

I'll try the B26 without the bushing first and see how that works out.
Distributor and 236-16 vac advance appear original...
Thanks for the responses...