73 L48 tune up

[Mark E.]

Re: 73 L48 tune up

Sometimes centrifugal advance is speced in DISTRUTOR degrees, which is half crankshaft degrees.
Duke

This would explain it.

[Duke W.]

Re: 73 L48 tune up

So the B25 is 16 crank degrees at 14". I'm not sure if it's currently available, but it's close enough to the 16@15" B26 to be within production tolerance. As long as it's reasonably in spec it's okay to use.

It looks like the distributor needs the uniset under the shield, which is okay.

Those centrifugal springs look REAL stiff. What are the centrifugal specs from the CSM and AMA specs?

Excellent chart of the VAC specs! Where did you find it!

Duke

[Douglas C.]

Re: 73 L48 tune up

So the B25 is 16 crank degrees at 14". I'm not sure if it's currently available, but it's close enough to the 16@15" B26 to be within production tolerance. As long as it's reasonably in spec it's okay to use.

It looks like the distributor needs the uniset under the shield, which is okay.

Those centrifugal springs look REAL stiff. What are the centrifugal specs from the CSM and AMA specs?

Excellent chart of the VAC specs! Where did you find it!

Duke

Can't find exact specs but I did find a reference on line that says 14 centrafugal at 4,200. So I'm not sure if that meant 14 at crank or effectively 28. I can check with my light. Yes, springs came off with small vice grips, crazy. I can check initial and VAC can with my dialback for idle vacuum. I want to bypass the TCS
Plus can movement with my Mityvac. I have a bag of springs for the counterweights. I found the chart at the AMC website, those guys have some nice details at times. I mostly have tuning experience with mid 60's and back. So I'm trying to educate myself on my 73, how TCS works, are there other electric connections to the Muncie, etc. Thanks. Doug

[Mark E.]

Re: 73 L48 tune up

Which pre-HEI engines came with internal shielding? All with radio? I ask because my '70 454 with stereo doesn't have it.

[Duke W.]

Re: 73 L48 tune up

Vacuum and centrifugal advance specs are in the 1973 CSM and the 1973 Corvette Vehicle Information package that is a free download from the GM Heritage Web site. You should have both

[Douglas C.]

Re: 73 L48 tune up

Vacuum and centrifugal advance specs are in the 1973 CSM and the 1973 Corvette Vehicle Information package that is a free download from the GM Heritage Web site. You should have both

Yes, I did go to the GM site. I'll check that in detail. Thanks.

[Douglas C.]

Re: 73 L48 tune up

Vacuum and centrifugal advance specs are in the 1973 CSM and the 1973 Corvette Vehicle Information package that is a free download from the GM Heritage Web site. You should have both

Here is the AMA specs page. So 14 crank centrafugal at 4,200 explains the heat, etc. With stock 12 initial that would be all in mechanical of 26 degrees at a whopping 4,200. Combine that with the TCS cutting off advance in first two gears just has to be changed. Problem is the stop bushing has to be replaced to around 24 degrees. Tks. Doug 20211022_174655.jpg

[Duke W.]

Re: 73 L48 tune up

The non-emission controlled antecedent engine to yours is the 327/300. The best centrifugal is 30 @ 3000-3500 with 6-10 initial for 36-40 degrees max WOT advance. That's what you should target.

Duke

[Douglas C.]

Re: 73 L48 tune up

The non-emission controlled antecedent engine to yours is the 327/300. The best centrifugal is 30 @ 3000-3500 with 6-10 initial for 36-40 degrees max WOT advance. That's what you should target.

Duke

Ok, I'll take your advice and experience and follow your guidelines. Tks. Doug

[Douglas C.]

Re: 73 L48 tune up

The non-emission controlled antecedent engine to yours is the 327/300. The best centrifugal is 30 @ 3000-3500 with 6-10 initial for 36-40 degrees max WOT advance. That's what you should target.

Duke

Yes, will shoot for that. But first I want to bypass TCS and get full manifold vacuum to distributor can. See pics, would best be to cap off where I marked letter P, unplug TCS wire, and run vacuum line from letter V. Also pic of green wire is also linked to TCS function? Unplug that?

I can then change distributor weight springs, etc. and fail in Duke's specs.
Tks. Doug 20211024_135018.jpg20211024_135141.jpg

[Duke W.]

Re: 73 L48 tune up

I'd have to look at a good schematic to figure out a good way to disable the TCS and apply full manifold vacuum directly to the VAC. One way is to make a new 1/8" vacuum harness from the choke vacuum break vacuum source to both it and the VAC with a tee, and I believe the TCS solenoid also has a full time vacuum source, and if so, you can use that.

I don't know how Chevrolet limited max centrifugal to only 14 crank degrees. The cam assembly slot limits advance and they are stamped with three digit numbers like 524 and 530 and the last two digits appear to be max advance. I've also seen some that start with 7 like 730, but I could not tell a difference between it and a 530,

You're going to have to pull and disassemble the distributor to figure out what you've got.

Duke

[Douglas C.]

Re: 73 L48 tune up

I'd have to look at a good schematic to figure out a good way to disable the TCS and apply full manifold vacuum directly to the VAC. One way is to make a new 1/8" vacuum harness from the choke vacuum break vacuum source to both it and the VAC with a tee, and I believe the TCS solenoid also has a full time vacuum source, and if so, you can use that.

I don't know how Chevrolet limited max centrifugal to only 14 crank degrees. The cam assembly slot limits advance and they are stamped with three digit numbers like 524 and 530 and the last two digits appear to be max advance. I've also seen some that start with 7 like 730, but I could not tell a difference between it and a 530,

You're going to have to pull and disassemble the distributor to figure out what you've got.

Duke

From my reading I think it would be safe to unplug the TCS switch, find full port, run that to the distributor can. Also disconnect the green passenger side wire that is a temperature sensor that is part of the TCS system. At that point I can check the initial timing, supposed to be 12 degrees. I can then check idle timing and that should be around 27. I would test drive and see if it still runs ok and hopefully runs cooler than the normal 230 range.

If all ok I can then change advance weight springs. Factory stock says all in at 4,200, so try to get that all in at the 3,000 to 3,500. Now, total timing of 36 to 40 degrees I assume would require pulling distributor and having the stop bushing changed to get about effective 24 crank degrees.

My priority is to cool down normal running, so pulling the distributor could wait for now. Trading off best ultimate performance for much improved drivability and engine cooling.

Do you think I'm on the right track? I like to make changes in steps.

Tks. Doug

[Patrick H.]

Re: 73 L48 tune up

No reason to remove the green wire on the passenger's side head. It's just going to fall and melt on the exhaust.

[Douglas C.]

Re: 73 L48 tune up

No reason to remove the green wire on the passenger's side head. It's just going to fall and melt on the exhaust.

Ok, although it does unplug from the sender, figured I would tie it up to firewall clip. Overall I'm told it's a Top Flight car so I want to keep all emissions items in place, just disconnect to improve driveability. If I have it judged I can easily plug things back in.
Tks. Doug

[Patrick H.]

Re: 73 L48 tune up

Ok, although it does unplug from the sender, figured I would tie it up to firewall clip. Overall I'm told it's a Top Flight car so I want to keep all emissions items in place, just disconnect to improve driveability. If I have it judged I can easily plug things back in.
Tks. Doug

Unplugged vs plugged in will have zero effect on driveability if you are running direct vacuum and bypassing the TCS solenoid on the intake.