Re: Just Bought Some 100LL Av Gas From Our Local Airport
It's likely that the installed VAC is original. Nevertheless, it's more aggressive than necessary, which can lead to part throttle detonation like you describe, so I suggest you obtain a B26 and install it. Because in recent years there are reports of VACs being significantly out of spec, I suggest you buy it locally and check with a vacuum pump that the pin starts to pull at 6" and is to the limit at 12" doesn't leak before you accept it. Save the 236 16. Do not throw it away. Functioning OE VACs manufactured by GM, identified "MS XXX YY" stamped on the mounting bracket are getting rare because they have not been manufactured by GM for over 30 years.
To install, pump down the installed VAC to pull the pin to the max advance limit, remove the two screws and wiggle the pin out of the stationary pole piece base plate. Don't force it. Take your time. If it resists disengaging, try using and awl or similar tool to push down the pin from the top. Pump down the new one and install.
Go through the idle speed/mixture adjusting procedure in the CSM with a target idle speed of 750. If the final idle screw position is about 1.0 turn out from the seat that's was I've observed on other L-79s, and vacuum should be 14-15" with a slight lope. The vacuum gage should be teed into the vacuum advance hose. See the test equipment photo in the tuning seminar. The golf tee is for plugging the VAC signal line when setting initial timing because to do this the VAC must be disabled by disconnecting the hose and the vacuum leak plugged.
Let the fuel tank run down to near empty and install about 2 gallons of 93 PON and test for detonation. Repeat this at least three times to fully dilute the effect of the residual avgas.
I expect your centrifugal springs are original. The centrifugal specs are listed in '67 CSM and AMA specs. Memorize them! Then use your timing light to test and report the acutal start and stop points. If the springs are original the stop point is about 5000, and you may not want to rev it that high, but rev to at least 4000 to 4500.
Once the B26 is installed and the avgas is purged set original timing (at engine speed below the centrifugal start point.) at 4 degrees BTDC. Test for detonation and if none advance to 6 degrees and retest. Continue testing for detonation with 2 degree increased increments in initial timing until you detect detonation or you reach 10 degrees, which ever occurs first. Depending on the results of this test you can look at quickening the centrifugal curve.
Duke
It's likely that the installed VAC is original. Nevertheless, it's more aggressive than necessary, which can lead to part throttle detonation like you describe, so I suggest you obtain a B26 and install it. Because in recent years there are reports of VACs being significantly out of spec, I suggest you buy it locally and check with a vacuum pump that the pin starts to pull at 6" and is to the limit at 12" doesn't leak before you accept it. Save the 236 16. Do not throw it away. Functioning OE VACs manufactured by GM, identified "MS XXX YY" stamped on the mounting bracket are getting rare because they have not been manufactured by GM for over 30 years.
To install, pump down the installed VAC to pull the pin to the max advance limit, remove the two screws and wiggle the pin out of the stationary pole piece base plate. Don't force it. Take your time. If it resists disengaging, try using and awl or similar tool to push down the pin from the top. Pump down the new one and install.
Go through the idle speed/mixture adjusting procedure in the CSM with a target idle speed of 750. If the final idle screw position is about 1.0 turn out from the seat that's was I've observed on other L-79s, and vacuum should be 14-15" with a slight lope. The vacuum gage should be teed into the vacuum advance hose. See the test equipment photo in the tuning seminar. The golf tee is for plugging the VAC signal line when setting initial timing because to do this the VAC must be disabled by disconnecting the hose and the vacuum leak plugged.
Let the fuel tank run down to near empty and install about 2 gallons of 93 PON and test for detonation. Repeat this at least three times to fully dilute the effect of the residual avgas.
I expect your centrifugal springs are original. The centrifugal specs are listed in '67 CSM and AMA specs. Memorize them! Then use your timing light to test and report the acutal start and stop points. If the springs are original the stop point is about 5000, and you may not want to rev it that high, but rev to at least 4000 to 4500.
Once the B26 is installed and the avgas is purged set original timing (at engine speed below the centrifugal start point.) at 4 degrees BTDC. Test for detonation and if none advance to 6 degrees and retest. Continue testing for detonation with 2 degree increased increments in initial timing until you detect detonation or you reach 10 degrees, which ever occurs first. Depending on the results of this test you can look at quickening the centrifugal curve.
Duke
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