I have been struggling for years to get my car to run cool and am once again looking at the timing to see if it can be the issue. Timing at idle is 25 degrees and total advance is 55 (w/vac adv plugged). Problem is the car runs hot on the highway. Starts fine, turns off fine, etc. Just can't drive it for any significant distance because it gets too hot and will boil over. I have tried lots of things in the past including: jetting, new rad core, water pump, various thermostats, calibrated gauge, fuel, fan clutch, belts are tight, etc. The only thing I have not tried is timing. 98 point car, .060 over with flat tops otherwise stock.. I have read lots of posts and am lead to believe that 55 is too much total advance. How can I reduce it if you think this could be a problem? Thanks for any help..
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Re: 66 L72 Timing
There's something terribly amiss here! For one thing, L-72 has ported vacuum advance from the factory (but I recommend changing it to full time). Without judicious modification of the carburetor, conversion to full time vacuum advance will be obvious if the VAC signal line is teed into the choke vacuum break hose.
Download the '66 vehicle information kit from the GM heritage Web site, and tell us what you find in terms initial timing recommendation and the centrifugal curve - or look in the '66 CSM. Self discovery is a much better learning tool than me telling you the answers.
If your initial timing is REALLY 25 degrees without vacuum advance and the total WOT spark advance is 55 without vacuum advance the thing should be detonating like a m...r f...r, so I have to question your data and/or your test technique.
Duke -
Re: 66 L72 Timing
Just as an aside to Duke's comments, have you VERIFIED that the timing mark on the balancer is aligned with the "O" on the timing tab at true top dead center? Take all the plugs out and using a piston stop in place of the #1 plug, rotate the engine with a wrench (DO NOT ATTEMPT TO USE THE KEY) until the piston hits the stop. Mark the point where the "0" on the timing tab meets the damper. Rotate the engine manually backwards until you hit the stop again. Your factory timing mark should be exactly 1/2 way between these points. If not, the outer ring of the damper is slipped and the damper should be replaced. If the mark is correctly placed you can remove the piston stop and at least have the knowledge that you have an accurate starting point.Bill Clupper #618Comment
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Re: 66 L72 Timing
Check the vaccuum can to make sure it is advancing
DougComment
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Re: 66 L72 Timing
Louis,
Let's time your Corvette without a timing light or any other aids and see if this will cure your problem. It will help because 55 total with the vacuum advance plugged just ain't right.
Over 40 years ago an old mechanic showed me how to time my BB Chevelle and I have used this method on every vehicle I have owned and it has never let me down.
This involves driving your Corvette and setting the timing so it will not knock and getting maximum advance for performance.
1. Your Corvette should be just like you are going to run it on the street. Vacuum advance connected and working. Weights in the distributor mechanical advance working perfectly.
2. Loosen the distributor so that it can be turned but no loose enough to turn itself.
3. Get in your Corvette when warmed up drive about 20 to 30 miles per hour in fourth gear and push down on the accelerator. If the Corvette pings stop the car and retard the timing 2 degrees. You won't need a light.
4. If the Corvette does not ping when going 20 and pushing down on the accelerator advance the distributor until it does ping.
5. When it starts to ping back it off a couple of degrees until it stops and then tighten the distributor.
6. You have just set your timing for maximum advance.
7. Now check the timing back at home with a timing light and write it down.
This method takes in everything, octane, curve, advance, engine characteristics etc. etc. Try it, you will like it!
JRComment
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Re: 66 L72 Timing
He says he has "flattop" pistons. If so, using premium fuel with this SOTP timing method can result in too much advance without detonation.
The Chev. Power Manual says BB total WOT timing should be in the range of 36-40. I don't know if that is with open or closed chamber heads, or both.
In any event, I would not exceed 42 degrees, even it if doesn't detonate, but get it all in ASAP.
Modern dial back timing lights are about 50 bucks. Everyone should have access to one.
DukeComment
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Re: 66 L72 Timing
Duke:
I did as you suggested and went to the GM Heritage website and recorded the timing data. It indicates 8 degrees initial timing, max 28 @4000, van advance begins 7, and max degrees @in hg 12@12. So my understanding is I should have 8 degrees BTC at idle and max of 28 degrees at 4000. Perhaps my balancer has slipped and the timing mark is not correct. I also have side exhaust and can't hear it if it is detonating. It starts well, doesn't diesel when shutting it down, and takes the throttle well.. so for argument sake could we assume my timing mark has slipped and my initial timing measurement of 25 is actually around 8? My total advance is still too much as I have 30 degrees of total advance..Comment
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Re: 66 L72 Timing
No. at idle, plus the 28 to give you 36 at 4000. The 28 is additive to the initial to get total advance, and for general reference 36 total is about the Ballpark for any Chevrolet V-8 in general.Bill Clupper #618Comment
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Re: 66 L72 Timing
Good start. Now get out the timing light and set the initial to 8-12 deg. at less than 900 RPM, which is were the centrifugal starts. (Verify this with the timing light) As you reduce initial timing, idle speed will drop so you may have to readjust idle speed as you bring the initial back to the 8-12 range. Disconect and plug the VAC, but check if it actually has vacuum. In OE configuration it should not. If you have not looked inside the dist. in the last ten years, do so, remove the rotor and make sure the centrifugal mechanism is clean and moves freely. If it needs cleaning, do so, but don't put any grease on anything - just a film of WD-40. Grease will attract dirt and clog up the mechanism. It's very lightyly loaded and doesn't need grease.
Once the initial is in the correct range go through the idle speed/mixture adjustment procedure with a target idle speed of 900. Then drive the car. and evaluate it's performance. It should be good. If it won't idle or the exhaust manifolds begin to glow, there could be a problem with the TDC mark on the balancer, so use a piston stop as previously suggested to locate true TDC and remark the balancer.
Consider converting the ported vacuum advance to full time, and of course, check the VAC to verify that it functions properly. Search B20 L-71 L-72 ported... under my user name. It's been discussed many times over the years, and it significatly mitigates the tendency of these big blocks to run hot in low speed driving.
BTW, my copy of the '66 AMA specs state that the L-72 centrifugal starts at 900 and the maximum is 30 @ 5000. With your low compression pistons running 12 intial should be no problem on premium fuel, but if it detonates you'll have to back it down a little.
DukeComment
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