67 L36 Tuning Advice

[Glenn B.]

I recently invested in a digital timing light and vacuum guage in an effort to make my car run better. Not too happy with what I learned today and am looking for advice.

I have a Late L36 - believed to be stock configuration.
My initial timing is set at 14 degrees @ 600
My vacuum advance is 16 degrees @ 600. I have 15in of avcuum and the can is chattering away at 15 in Hg.

My centrifigal advance (vacuum plugged and subtract initial) is:

- 0 degrees at 900
- 7 degrees at 1275
- 10 degrees at 1600
- 13 degrees at 3000
- 13 degrees thereafter

My interpretation is this is basically that my distributor is a basket case, that the vacuum can is incorrect. The centrifigal advance is way off. The specs I have are:

- Vacuum advance is 12 degrees at 12 in vacuum
- Initial timin should be 4 to 10 degrees
- advance curve should be:
0 at 900
14 at 1275
19 at 1600
30 at WOT

Can anyone confirm my interpretation? Looks like the dist needs a complete rebuild to me. Anyone have any recommendations for a rebuilder?

Thanks

[Duke W.]

Re: 67 L36 Tuning Advice

Good data! The "chattering away" of the vacuum advance control is why I have my "2-inch rule" - the VAC should provide max advance at no less than 2" below typical idle vacuum, so the OE 12" VAC is just right for L-36.

Look at the data stamped on the VAC mounting bracket and let us know the numbers.

A correct replacement is the NAPA VC 1765(B20)or VC1808 (B26) and you can cross this at any other vendor's web site. Both are speced at 0@6", 16@12". (The is no 12@12" replacement available, but the 16@12" replacements are okay.) Essentially all VACs are now made by Standard Ignition Products and carry the same stamped code listed in paretheses. They are sold under several brand names including all the traditional replacement brands and Delco.

The distributor is easy to rebuild if you are handy. A quick check of its health can be done by removing the distributor cap and:

1. Check the amount of shaft end play by pulling up on the rotor. It should be in the range of 2 to 7 thou, but as built, 50 to 120 thou is typical.

2. Wiggle the rotor side to side. Play should be barely suspectible at worst.

3. Remove the rotor and check the centrifugal advance mechanism for proper function. Remove the weights and springs and give it a good cleaning. If it can be freed-up reassemble it and give the mechanisme a shot of WD-40. DO NOT USE GREASE. This mechanism is lightly loaded and just needs a light lubricant and corrosion protection. Grease will collect dirt and clog the mechanism.

Chances are your distributor has never been touched in 40 years except replacement of points, condenser, rotor. Removing it for a thorough teardown, inspection, cleaning, and regreasing of the upper bushing grease is a good idea, along with shimming up the end play. As long as the housing bushings are okay and don't need to be replaced, this is a fairly simple DIY job. If you rev it above 5500 use the 28-32 oz. point set such as NAPA CS89.

The detailed disassembly/assembly procedure is in your COM. Be sure to install the gear with dimple pointing in the same direction as the rotor tip, which is not mentioned in the overhaul procedure.

According to my data the centrifugal curve maximum should be 30@5000. "WOT" has no meaning vis-a-vis the centrifugal advance as it is purely a function of revs, not throttle position.

Big block optimum total WOT advance, which is the sum of max centrifugal and initial is in the range of about 40, which would dictate 10 initial, but if the engine detonates at this setting you will have to back it off until the detonation is eliminated.

As a new user you should also do some reseach in the archives as this subject has been discussed numerous times.

Duke

[Glenn B.]

Re: 67 L36 Tuning Advice

Thanks for the detailed response Duke. What litle I know about timing and tuning has been mainly gleemed from you many informative posts in the archives.

I have to confess that I have the AIM but not the Chassis manual (will get one shortly).

Looks like my problem is probably worse than I though. I have checked almost all the component numbers on my car, but until now not the Distributor. I just checked it and it is clearly not correct. It is marked 111247 Delco Remy, anyone know what the heck this is for?

I believe I need an 111560.

Glenn

[Glenn B.]

Re: 67 L36 Tuning Advice

I jumped the gun.

Looks like the dist is probably correct. I found conflicting posts in the archives on the correct dist # for Late 67 L36. Just found another that indicates 111247 is correct (I missed the first 1 reading the band).

Can anyone confirm 1111247 Vs 1115360?

[Duke W.]

Re: 67 L36 Tuning Advice

1111247 is correct per the NCRS Spec Guide.

1115360 is the original VAC used on that dist. It would be stamped "360 12" on the mounting bracket.

What is stamped on the mounting bracket of the installed VAC?

Duke

[Glenn B.]

Re: 67 L36 Tuning Advice

Yes, I realized I should check my JG. 1111247 is correct.

The date code looks good also 6M 27 (December 27, 1966 - my car is late May).

The can is stamped B25 - it seems to give 16 degrees at about 15.5 in of Vac

Looks like I need a new can for sure.

Can you confirm the 1111247 advance curve? I just found a post on the 1111247 which indicates:

0 @ 900, 8.5@1250, 17@2000, 32@5000 (Yes, I will buy the chassis manual!)

Glenn

[Duke W.]

Re: 67 L36 Tuning Advice

B25 nominal specs are 0@6", 16@14". Tolerance is about +/- one degree and one inch Hg.

According the the '67 AMA specs the L-36/68 centrifugal curve is 0@900, 17@2000, 32@5000, which jibes with whatever source you quoted.

The AMA specs have a lot of excellent technical info. They are included with the complimentory "restoration package" available from GM. Call customer service, 800-222-1020. They will want your VIN.

Once you have your centrifugal corrected to spec, you will need to back the initial timing down to 8-10 max, and detonation may force you to drop it down to as low as 4, which is okay, but run as much as the engine will tolerate without significant detonation.

The original 1115360 VAC is 0@7", 12@12", but since there don't appear to be and 12" replacement VACs available, it's okay to use either of the 16@12" VACs I mentioned previously.

Once you get the distributor squared away the engine should perform very well including idle quality/stability at about 600 in neutral assuming it's a manual transmission.

Once you find the detonation limit on initial timing, you can experiement with lighter springs in the dist. to bring in full centrifugal sooner. The sooner it achieves maximum advance the better the torque in the bottom half of the rev range, but the limit will be detonation if the curve gets too aggressive.

Buy a Chassis Overhaul Manual, too. Even if you don't plan on overhauling the engine it includes overhaul procedures for all the major components like starter, carburetor, and distributor. These manuals will pay for themselves the first time you use them to do a job.

With the AMA specs, AIM, CSM, and COM, you have the documentation you need to do anything from a tuneup to a frame-off resto.

Duke