73 L48 tune up

[Duke W.]

Re: 73 L48 tune up

A 15" B22 would be best, but try the 12" B26. You don't have to remove the dist. to swap out the VAC. The secret is to pump it down to pull the pin all the way. Then remove the screws and carefully wiggle the pin out of the hole. Pump down the new one and reverse to install.

If no detonation is okay for the long run.

Duke

[Douglas C.]

Re: 73 L48 tune up

A 15" B22 would be best, but try the 12" B26. You don't have to remove the dist. to swap out the VAC. The secret is to pump it down to pull the pin all the way. Then remove the screws and carefully wiggle the pin out of the hole. Pump down the new one and reverse to install.

If no detonation is okay for the long run.

Duke

[Duke W.]

Re: 73 L48 tune up

My recommendation is to always use the least aggressive VAC that meets the Two-Inch Rule. More aggressive than minimum can cause part throttle and transient detonation, but not always.

Duke

[Douglas C.]

Re: 73 L48 tune up

My recommendation is to always use the least aggressive VAC that meets the Two-Inch Rule. More aggressive than minimum can cause part throttle and transient detonation, but not always.

Duke

Thanks Duke, good information. Also, maybe a small point but I like to use the plastic bushing for the canister rod. Here is the only place I know to get them. Doug 20220409_234908.jpg

[Anthony C.]

Re: 73 L48 tune up

i have a set of orignal NOS AC R44T if that helps you

[Duke W.]

Re: 73 L48 tune up

Thanks Duke, good information. Also, maybe a small point but I like to use the plastic bushing for the canister rod. Here is the only place I know to get them. Doug

I'm not familiar with that part. Most VACs offer 16 degrees total advance, and in most cases I don't see a need to reduce this, but it comes down to the specific engine one is dealing with.

Duke

[Douglas C.]

Re: 73 L48 tune up

I'm not familiar with that part. Most VACs offer 16 degrees total advance, and in most cases I don't see a need to reduce this, but it comes down to the specific engine one is dealing with.

Duke

The reason I mention this is I always thought this thin bushing was intended to take up any clearance between the cannister rod end and slot, sort of guide it straight. Thought it was also part of the old original canisters and effectively created the correct degrees. Somehow I remember seeing this many years ago and thought it was part of the original canisters. The part I listed is so thin it seemed more to guide the rod than really effect degrees. Here is an old pic of an original canister.vacuum_2.jpg

Doug

[Joe L.]

Re: 73 L48 tune up

The reason I mention this is I always thought this thin bushing was intended to take up any clearance between the cannister rod end and slot, sort of guide it straight. Thought it was also part of the old original canisters and effectively created the correct degrees. Somehow I remember seeing this many years ago and thought it was part of the original canisters. The part I listed is so thin it seemed more to guide the rod than really effect degrees. Here is an old pic of an original canister.[ATTACH=CONFIG]111190[/ATTACH]

Doug

Doug------


The bushing you picture is not the same as the bushing originally used on the vacuum control rod. The original bushing had no flange and was thicker. It was 1/4" OD. It was once available from GM as a separate part under GM #1961234 but was discontinued without supersession in May, 1991.

[Douglas C.]

Re: 73 L48 tune up

Doug------


The bushing you picture is not the same as the bushing originally used on the vacuum control rod. The original bushing had no flange and was thicker. It was 1/4" OD. It was once available from GM as a separate part under GM #1961234 but was discontinued without supersession in May, 1991.

Thanks Joe. Maybe that is what I recalled some many years ago, I just find these things interesting.
Doug

[Duke W.]

Re: 73 L48 tune up

I've never seen a bushing like that on a single point distributor VAC. Some have a rubber sleeve that limits vacuum advance. That bushing looks like it might my an HEI part that the weights rest on.

Duke

[Douglas C.]

Re: 73 L48 tune up

A 15" B22 would be best, but try the 12" B26. You don't have to remove the dist. to swap out the VAC. The secret is to pump it down to pull the pin all the way. Then remove the screws and carefully wiggle the pin out of the hole. Pump down the new one and reverse to install.

If no detonation is okay for the long run.

Duke

I now have the NAPA VC1802, marked B22. But, checking on the bench with Mityvac see pics. Begins to move around 7" and full extension at just about 17.5". My engine has a rock steady 19" at idle, so I'm not quite within the 2" rule. I have a B26 which starts at 7" and all in at 12".
As to movement of the canister rod to full extension the B22 moves .240 inches, B26 moves .20 inches. So not sure if both will get 16 degrees vacuum advance. Duke, you probably have an answer on the measurement vs. degrees effect? Which canister would you suggest using.
Doug 20220413_143159.jpg20220413_143244.jpg

[Douglas C.]

Re: 73 L48 tune up

I now have the NAPA VC1802, marked B22. But, checking on the bench with Mityvac see pics. Begins to move around 7" and full extension at just about 17.5". My engine has a rock steady 19" at idle, so I'm not quite within the 2" rule. I have a B26 which starts at 7" and all in at 12".
As to movement of the canister rod to full extension the B22 moves .240 inches, B26 moves .20 inches. So not sure if both will get 16 degrees vacuum advance. Duke, you probably have an answer on the measurement vs. degrees effect? Which canister would you suggest using.
Doug [ATTACH=CONFIG]111237[/ATTACH][ATTACH=CONFIG]111238[/ATTACH]

AMA specs show canister starts at 6, and max 15 @ 13.5.

[Duke W.]

Re: 73 L48 tune up

A B22 specs should nominally be start at 8", 16 @ 15". If yours isn't all in until 17.5" you can use the modified Two-Inch Rule... call it the 1.5" Rule, however if you still have ported vacuum advance The Rule doesn't apply.

Your B26 is essentially nominal spec.

I can't correlate pin movement to advance, but both the B22 and B26 should offer nominally 16 deg. at the crank.

If you can get the idle speed lower you'll lose a little vacuum, so you might need to install the B26 if you have full time vacuum advance.

Duke

[Douglas C.]

Re: 73 L48 tune up

A B22 specs should nominally be start at 8", 16 @ 15". If yours isn't all in until 17.5" you can use the modified Two-Inch Rule... call it the 1.5" Rule, however if you still have ported vacuum advance The Rule doesn't apply.

Your B26 is essentially nominal spec.

I can't correlate pin movement to advance, but both the B22 and B26 should offer nominally 16 deg. at the crank.

If you can get the idle speed lower you'll lose a little vacuum, so you might need to install the B26 if you have full time vacuum advance.

Duke

I'm running stock setup. No vacuum advance at idle or first or second gear, but it's hooked to full vacuum, TCS just only allows function after second gear. Factory calls for 12 initial but I have it at 16. I just barely then get 36 all in at 3,000 rpm with lighter springs. Stock 73 carb has idle passably holes under throttle plates so it's tricky to get idle speed down. I'm able now to idle at 800. Doug

[Anthony C.]

Re: 73 L48 tune up

hi just was curious, what is your fast idle rpm's on cold start ,, my 75 L48 is at 1600 rpm's however some folks say it should be around 1200 to 1300 any help would be great thanks