68 L79 Overheat

[Richard D.]

OK.

Got the message on the vacuum can needed on the L79 distributor (Duke and others), and, after a few tries over the past weekend to make the Mallory "Unilite" (tach drive, no vacuum can) work, I believe it. Got the motor to idle for about 30 minutes, but then it did overheat.

So, what distributor do I get, and where do I get one?

Would like to get one that looks stock, but I would like to go without the points and condenser - Petronix?

Any advise would be appreciated.

Also, my Chevy Chassis and Service Manual said to set initial timing at 4 degrees BTC, and that the vacuum pulled in 15 degrees at idle.

It also said the thermostat opened at 190 degrees, but I see everyone talking about using a 180 degreee thermostat for an L79. What's the way to go here?

Oh yeah, go a GM clutch fan for the car, and will put a new water pump in (might as well while I have it apart) when I redo everything. Anything special I should consider getting for the pump?

Thanks in advance for your advice,

Rich

[Robert C.]

Re: 68 L79 Overheat

You can do all the unimportant stuff you want but your car will probably continue to overheat until you get a new De Witts radiator!

[Jim T.]

Re: 68 L79 Overheat

Richard I recommend trying to obtain a 68 L79 distributor if you can find one. Trying to locate one could take some searching. Other used distributors for high performance Corvettes may do the job as well. I can understand you wanting the electronics in place of the points, however I have had pretty good performance with my 68's original type ignition. In fact I started the 68 today after sitting for months. I injected fresh gas into the vents of the Holley a couple days ago. It hardly turned over a couple revolutions and was running fine. I would have to check the cars maint log book, but it has been very long time since I have changed the points/condenser or plugs. If you have lots of money to spend you could purchase the MSD electronic distributor PN 8572 with tach drive that has vacuum advance. Couple places to try for the original type distributor are the NCRS Driveline and Comtemporary Corvette who lists tach drive dist for $150.00. I use a 180 thermostat and have had a rebuilt water pump on my L79 for an extremely long time when you consider what a rebuilt pump cost such a long time ago. Check out Advance Auto for a rebuilt AC Delco rebuilt pump.

[Duke W.]

Re: 68 L79 Overheat

Any Corvette tach drive distributor will do. Also, understand that the '68 models were subject to exhaust emission control, so the timing map was different from previous L79s. This is also the reason your car has a 190 thermostat, and I recommend you keep it at this level, but changing it to a 180 will cause no harm. Ten degrees doesn't make any material difference. Your engine may have originally had a "ported" vacuum advance that does not provide a vacuum signal to the can at idle. Your '68 CSM should mention this if it is the case. If your car originally had a ported vacuum advance and does not have to be emission tested, implement a full time vacuum advance that uses manifold vacuum at all times, including idle, to signal the vacuum can.

You want the initial plus total centrifugal sum to be 36 +/-2 degrees, so set the initial timing based on the total centrifugal of the distributor you obtain. Previous L-79s had distributors with up to 30 degrees of centrifugal advance, and many sixties vintage 327s had 24 degrees. Later distributors for emission control engines may have as little at 20 degrees. On these the slot may be filled to increase advance. You will also need to tailor the weights and springs to suit your L-79. As a general rule you want to get all the centrifugal in as quickly as possible with detonation being the boundary on the rate of advance.

Regardless of the centrifugal curve, the NAPA-Echlin VC1810 vacuum can matches the specs for the OE L-79 Delco 236 can for earlier L-79s, but it may have changed in '68 for emission reasons.

What specs do you have for '68?

Lack of vacuum advance or a vacuum advance not suitable to the engine's manifold vacuum characteristics is the most common cause of overheating. Implementation of ported vacuum advance on many engines for emission control purposes beginning in 1966 in CA and 1968 nationwide reduced the cooling system margin of the cars when new, so as the cooling system degraded such as radiator deposit buildup that acts as a thermal insulator to reduce heat transfer, these emission controlled engines would develop a tendency to overheat earlier in their service life than previous non-emission controlled engines with the same radiator.

Duke

[Richard D.]

Re: 68 L79 Overheat

Duke, first of all, thanks for all the info. on this problem I've been having.

You put me on the right track, and in fact, I have saved all your previous postings (and others too) regarding getting things sorted out correctly.

You asked what the specs were on my motor - The previous ownner told me he rebuilt the L79 using GM cam and 11:0 TRW pistons. New bearings, etc. done on the rebuild. The numbers match on the motor for my car.

Carb is an Edelbrock 1406 600 cfm with electric choke (choke is currently not connected, left in the full open position).

Manifold is a recently installed Edelbrock (old one, CJB I think is the code, not sure), alumminum.

Radiator was an Autozone replacement (what they sell as a direct replacement).

Distributor is an Edelbrock Unilite with tach drive, no vacuum (as you already know).

All this work (except the manifold) was done by the previous owner, I changed the manifold over as the one that was on there was a no name aluminum job.

The recommendation for the Mallory is to set it at 12 degrees intial, and it is factory set at 24 degrees centrifical.

There is no emmisions stuff on the motor, and the heat riser is wired into a wide open position.

I must say that the engine runs very strong and smooth. No smoke, leaks, or unwanted noises. The previous owner got some things right it seems.

It's also a 4-speed car with a 3.70 posi rear.

That's the specs on the car.

Thanks again for your advise,

Rich

[Duke W.]

Re: 68 L79 Overheat

Make sure you use a full manifold vacuum source to signal the vacuum can when you install the Delco single point. Before you install it. Disassemble it for cleaning and inspection - check bushings for wear and replace if required, regrease the upper bushing grease well, be sure the breaker plate fits snug and does not wobble (a new one is a good idea), use the 28-32 oz. breaker arm tension points, and be sure the dimple in the drive gear is pointing the same direction as the rotor tip.

Parts should be readily available at most chain auto parts stores.

The radiator could also be an issue. Brass replacements typically do not have the same heat transfer capacity as the OE 316 radiator that I believe was used on your car. Add some degradation from deposit buildup and the engine can run hot. If you don't know the history of the radiator, taking it to a radiator shop for a "rodout" many also be helpful.

Duke

[Richard D.]

Re: 68 L79 Overheat

Thanks for the info. Jim.

Mallory also makes a breakerless tach drive vacuum unit where you don't need the special electronics box the MSD does.

I think that I will go with a stock unit though, if I can find one. Much less money, and from what I've read from everyone, the same performance.

Got my waterpump from Autozone, although the GM unit on my car is probably OK still. I'll hold on to it as a spare.

I'll look in my Driveline and maybe give Contempory a call.

Thanks again,

Rich