73 L48 tune up

[Douglas C.]

For full tune up and best running, stock engine, suggestions for best parts. Not show judging. Tks. Doug

[Duke W.]

Re: 73 L48 tune up

Most electrical parts for post War vintage cars are manufactured by Standard Motor Products, a Tier 1 OEM supplier and are available in a number of different brands including NAPA/Echin, Niehoff, BWD, and yes, even Delco. GM has not manufactured these parts of at least 30 years. They just put SMP manufactured parts into Delco boxes.

SMP also has their own brands at retail, Standard, and Blue Streak.

So the bottom line is purchase on price and convenience.

Duke

[Douglas C.]

Re: 73 L48 tune up

Most electrical parts for post War vintage cars are manufactured by Standard Motor Products, a Tier 1 OEM supplier and are available in a number of different brands including NAPA/Echin, Niehoff, BWD, and yes, even Delco. GM has not manufactured these parts of at least 30 years. They just put SMP manufactured parts into Delco boxes.

SMP also has their own brands at retail, Standard, and Blue Streak.

So the bottom line is purchase on price and convenience.

Duke

Ok thanks. I'll go to my local NAPA and get Echlin points, condenser, cap, rotor. Probably cross reference AC R44T plugs to NGK plugs.

Doug

[Duke W.]

Re: 73 L48 tune up

Suggest you go with R45T plugs. Slightly hotter plugs have proved to be less likely to foul going back decades. There are several points sets that wiil work on your engines. I wrote an article about it that was published in the So Cal chapter newsletter and The Corvette Restorer. An e-copy might be on this site, but I know it's on the CF C2 discussion in a thread started by SWCDuke... easy search.

Duke

[Douglas C.]

Re: 73 L48 tune up

Suggest you go with R45T plugs. Slightly hotter plugs have proved to be less likely to foul going back decades. There are several points sets that wiil work on your engines. I wrote an article about it that was published in the So Cal chapter newsletter and The Corvette Restorer. An e-copy might be on this site, but I know it's on the CF C2 discussion in a thread started by SWCDuke... easy search.

Duke

Thanks Duke. I'll look for your article, always very informative. Agree on plugs, thinking of extended tip R45TS or NGK UR4. Really like the NGK's. Always also liked over the years NAPA Echlin CS-786 points and RR175 condenser. Thanks again, I'll read your article first. Doug

[Douglas C.]

Re: 73 L48 tune up

I did the basic tune up on my 73 L48 and the car runs very well. It is completely stock, NCRS restoration. Have driven and it runs around 230 degrees, just as owner manual states. So, I don't want to remove any emissions items but would like to cool it down. I know the stock distributor needs recurring but am I right the TCS is going to limit timing advance anyway. Can I simply plug off the TCS and then recurve distributor to cool things a little? Want to be able if judged to simply plug items back in.
Thanks, Doug

[Duke W.]

Re: 73 L48 tune up

You can probably bypass the TCS by running a hose directly from a full time manifold vacuum source to the VAC, like teeing into the choke vacuum break, and verify that the existing VAC passes the Two-Inch Rule. A 15" B26 VAC would be the best if a manual trans and 12" B26 if a TH400.

The best centrifugal curve would be max 30 all in by 3000-3500, with 6-10 initial for 36-40 total WOT advance, as high in that range as the engine will tolerate without detonation, and total idle advance will be 22-26, which will considerably reduce EGT and heat transfer to the cooling system as exhaust gas traverses the head exhaust port.

Full time vacuum advance will definitely cool down the engine and surrounds and reduce the propensity to detonate. You could replace the 195 degree thermostat with a 180, but it might not do that much because the OE fan clutch is setup to tighten with typical radiator outlet air temp when the coolant is about 195-200. If you do decide to go with a 180 stat get a fan clutch that is set up to tighten at a lower temp as was the case with pre emission controlled engines.

Duke

[Mark E.]

Re: 73 L48 tune up

I perfer SMP's premium brand, Blue Streak. For points, I recommend their DR-3575C or DR-3575CX "LubriPoints and Condensor Assembly". It's a points/condenser "uniset" for ease of installation, and features a threaded terminal for the wire (vs. a clip-on connection that some use), a felt lubricant wick, and copper plating. Beware of "performance" points with heavier spring tension for reducing float at high RPM- something your car doesn't need and which can increase distributor cam wear.

Regarding ignition timing, a search will find quite a few threads on this topic, including many featuring posts by Duke. One of the easiest ways to wake up a '70s engine and reduce temperatures is to change ignition timing. A good starting point is 36 to 38 degrees total centrifugal, including 8-12 degrees initial, with TCS bypassed. Duke advocates manifold vacuum for the VAC but some engines seem to run better with ported vacuum; you might try both.

Even without re-curving the distributor, just bypassing TCS and advancing static timing as much as the engine tolerates without pinging under load will improve performance and reduce temperatures.

Also, some of these cars came with a 195 degree thermostat. Changing to 180 degrees will also help.

UPDATE: See also Duke's last post; we were typing at the same time...

[Douglas C.]

Re: 73 L48 tune up

Thanks all. I have points dwell at exactly 30 degrees. I put in NGK UR4 plugs, I like extended plugs in these old engines. I'll use my dialback light to check timing and can change springs to bring all in sooner. It's a 4 speed and the VAC is a B25. I think that is 16 degrees. I can use my Mityvac to check movement inches. One thing is I find info that the stock distributor only has 14 degrees centrafugal. Stock calls for 12 initial so all in would be limited to 26 degrees. Seems odd but I'll check VAC and total timing anyway. Distributor may need a limit bushing change if only 14 centrafugal. I don't want to force too much initial otherwise I'll wind up with towing a boat feeling low end. It's a great car and I'll take the time and proper steps to get nice driveability. Thanks. Doug

[Mark E.]

Re: 73 L48 tune up

Centrifugal advance of 14 degrees would explain high running temperatures. Is the engine also sluggish?

[Douglas C.]

Re: 73 L48 tune up

Centrifugal advance of 14 degrees would explain high running temperatures. Is the engine also sluggish?

Actually runs very smooth with quick throttle response. Although Qjets do work well at low end. Specs I find say 14 degrees centrafugal at 4,200. So normal driving really has minimal advance. Seems GM went overboard on emissions. So I'll check with my dialback and see where it's really at
First I'm waiting on belts from Quanta as car was mostly in storage last five years minimal use at judges events I'm told. Thanks. Doug

[Duke W.]

Re: 73 L48 tune up

Sometimes centrifugal advance is speced in DISTRUTOR degrees, which is half crankshaft degrees. What does the CSM and AMA specs say.

Also, I've never heard of a B25 VAC. An emission controlled base engine converted to full time vacuum advance should have a 15" B22, and a 12" B26 with an automatic.

Duke

[Douglas C.]

Re: 73 L48 tune up

Sometimes centrifugal advance is speced in DISTRUTOR degrees, which is half crankshaft degrees. What does the CSM and AMA specs say.

Also, I've never heard of a B25 VAC. An emission controlled base engine converted to full time vacuum advance should have a 15" B22, and a 12" B26 with an automatic.

Duke

It's a 4 speed. Very well restored and I assumed everything is original or correct items. I cleaned up the distributor weights before the points, condenser. See pic of the B25. 20211019_112435.jpg

[Douglas C.]

Re: 73 L48 tune up

20211019_112135.jpgAother better picture.

[Douglas C.]

Re: 73 L48 tune up

Sometimes centrifugal advance is speced in DISTRUTOR degrees, which is half crankshaft degrees. What does the CSM and AMA specs say.

Also, I've never heard of a B25 VAC. An emission controlled base engine converted to full time vacuum advance should have a 15" B22, and a 12" B26 with an automatic.

Duke

Not sure of exact unit for stock distributor but I did find this chart of GM units.VAC-ADV-CANS-LINE-GRAPH-DELCO-POINTS-Post.jpg