That's a DeWitts and looks fairly new.
The vac tap on the 4160 should provide manifold vacuum AFAIK. I know for sure that the 4150 does. Check it with a vac gauge if you can get your hands on one.
Looks like the fan blade is about 2" smaller diameter than the shroud. Don't know about 1966, but on a 65, the blade diameter (18") is about 3/4" smaller than the shroud diameter.
Fore-and-aft looks good.......................1/2 in and 1/2 out.

 

Fan measures about 17 1/2 inches as near as I can tell. Yes, there is about 1 in or so clearance between the shroud and the fan blades.
The radiator is new, less than 1000 miles on the rebuilt engine.

 

Take a mighty vac and connect it to your Vac Advance Can to test it. Make sure it advances as you apply vacuum and then holds steady with the vacuum applied.
I suspect that your overheating issue is more ignition timing related than anything else.

 

Reading through this thread it appears you have ruled out the usual suspects.
Next time you park your car hot turn the idle up to 2,000 RPM. If it stays cool you have a flow issue that is fixed with higher RPM as you have driving.
If it heats up faster you have a cooling issue.
A flow issue could be a bad water pump impeller. Positive flow water pumps are sold for these engines and will push more coolant than a stock water pump.
If RPM does not cool your engine try misting the radiator with your garden hose while it is hot. Misting will drop the temp 20 degrees in 30 seconds if all is normal. See what yours does. The misting effect will last a couple minutes after you shut off the mist. The evaporation of the mist continues cooling until it evaporates completely.

Edit: a blockage will cause a flow issue.

 

Clearance is good, so rule out the radiator, fan, shroud and clutch. I measured the gap between my fan and shroud and it measures 1 1/8".

Await your ignition timing numbers when you get them. Here is what you should have (use 327/350..........this is from 1965...........1966 should be the same). I think static timing is 8* BTDC for your engine, check your owners' manual:

 

I had the same problem your having with my L79. Here is how I fixed it. First I bought Eaton clutch from Paragon, and I moved the bias spring CCW 1/2 turn and connected it to the metal tang that is there. Next I bought this 5 bladed fan from LIC's. It is 18" in diameter, and the blades have a little more pitch than the stock fan, and it pulls more air.....but still looks stock. I also now use a mix of 75% water, and 25% anti-freeze.....water dissipates heat better than anti-freeze. I have my initial timing set at 14º, and my vacuum advance is pulling 16º at 750rpm idle. Motor never goes over 210º, and I live in S. Fla.http://www.parts123.com/parts123/yb....Z5Z5Z50000370a

 

Sorry, slightly OT.

Never realized that DeWitts had surge tanks.

Glad they do!

Thanks for posting the PDFs.



-- Joe

 

Thanks to all for the suggestions. I've taken several of the suggested measurements as follows:
(Again, '66 L79) Cam is L79 clone per the rebuilder.

Vacuum advance mechanism starts moving at 9 in Hg
Vacuum advance all in at 16 in Hg.
Vacuum advance marked M6 with 163 16 underneath the M6
This looks like the B1 advance unit, but as I understand it, the B28 should be in the car.

Exhaust Manifold Temp L = 588 degrees, R = 638. Using emissivity setting of 75 percent - manifold has a gray coating about the same color as the aluminum intake manifold. Heat riser butterfly is operational.

Radiator top temperature shows 225 degrees, Bottom shows 203, idling at 700 - temp climbing (it's hard to get an IR gun on the very bottom) This reading is probably about 6 inches off the bottom of the radiator. Ambient Temperature 85 degrees

Vacuum @ 700 rpm idle , vac advance disconnected and plugged = 10 in Hg
Vacuum @ 2000 rpm idle, vac advance disconnected and plugged = 19 in Hg

Somewhere in the garage is my new timing light - MIA. Timing was set to spec recently, but I'm unable to verify without my light.
Dwell sitting right at 30 degrees.

The idle is unstable, suggesting that the carb needs some work.

Some other tests were suggested including misting the radiator - will get to them as well.

Still overheating at idle...

 

Do you mean "advance" instead of "vacuum" ? That would make sense, but later you say that you can't find your timing light. If your post is what you intended then I'm very confused.

If you're reading manifold vacuum, then @700 RPM it is about 5 in-hg too low for the 151 cam.

If you're reading ported vacuum, then at closed or cracked throttle (700RPM) it should be zero.

The unstable idle can also mean a dithering vacuum advance control.

 

Thanks for the reply:
The vacuum reading at 700 rpm is 10 in. Hg. I recall testing this a few months back and IIRC it was around 15 in. so I'm going to check for possible vacuum leakage. My carb is a Holley 4160 which feeds (non-ported) manifold vac to the distributor advance can.

Testing of the advance can was with a Myte-Vac. My new timing light arrived this afternoon, so I'll be able to get a good reading of the advance at various rpm. (That also means that I'll find my old timing light within a day) Been studying Lars' (and others) papers on distributors and carbs.

I realize I'm going to have to find out what is going on with the low vacuum before anything else is done.

 

Normal idle vacuum for a real L-79 cam is 14-15" @ 750. The VAC must meet the Two-Inch Rule.

Check for a thread started by me about a year ago - tuning seminar. It explains how to set up a spark advance map.

If the "L-79 cam" was made by Comp Cams it's not a real L-79 cam, clone or otherwise, and the low idle vacuum is because it has a lot more overlap due to narrowing the LSA, which costs a lot of low end torque without any improvement in top end power.

A real L-79 cam needs a B26 VAC. Based on your reported idle vacuum whatever cam is installed needs a B28.

Duke

 

OK now we're getting somewhere. So you have the right cam installed. First try another vac gauge. Your readings taken a few months back correlate to what you should have. Your vac gauge may be FUBAR.

Did somebody mess with your valve adjustment. Maybe overtightened to zero lash and was too zealous with a few. This should also partially explain your overtemp condition.
After you check vacuum with a known good gauge let us know if the reading is steady or fluctuating. Here's something useful to help you understand how to interpret vac gauge readings. Go through all the scenarios listed (especially #10) and report back what you have:

http://www.secondchancegarage.com/public/186.cfm

Once we finally get your idle vacuum verified..........or corrected at/to around 15-16 in-hg @ 750, then we can go from there.

 

Thanks Duke,

The engine rebuilder has left the area and the work was done almost 10 years ago. I was told it was an L79 cam clone, but that's all I know.

I would expect more lope than I'm seeing at such a low vacuum.

 

I verified the vacuum gauge this evening. Will have to map the vacuum at various engine speeds. I had seen the second chance garage article. Will go through the scenarios. Don't know about valve adjustments. Probably worth doing the adjustment procedure. Maybe next week when it cools down.