Good morning, I have a 1966 Corvette with a 327/350 hp. When I tune it at 10 degrees btdc and at 700 rpms. The car runs but has a lack of power and starts to run a little hot. When I advance the time 6 degrees atdc the car has more power and does not run hot. Can someone help me understand why and I am concerned about running the car with such an advanced timing. Thanks in advance for your help.

 

Your post makes absolutely no sense at all. If you are at 10 Before TDC how do you "advance" to 6 After TDC.

Did you remove the VAC signal line and plug it with a golf tee. Also since the OE centrifugal starts at 700 you must set initial at below 700, but you also need to verify the point of centrifugal start as it may have been modified sometime in the last 50+ years.

The max OE centrifugal is 30 @ 5000, which is very lazy. Installing light centrifugal springs to bring it in at 2500-3500 will render the engine much more responsive, and you can then use the total WOT method to set timing by revving a few hundred above the point of max centrifugal and setting it at 38-40, VAC DISCONNECTED and signal line plugged, of course.

The go through the idle speed/mixture adjustment procedure described in the CSM.

Duke

 

Not to post out of hand as I am a student of Duke but if his balancer has slipped then the readings he is seeing with the timing light would off, perhaps way off. Remember my posts last summer when after using a piston stop to find actual TDC, the timing line on my balancer was way off.

 

We also don't know if the timing cover matches the balancer on the car. The OP needs to find TDC on #1,and see how it lines up to the timing marks.

Doug

 

Is it possible you are confusing BTDC and ATDC? Because if you just reverse those two, your findings make sense. Running around 6 degrees BTDC is probably about right while 10 degrees ATDC will generate the problems you report. When you say 6 degrees BTDC, is the balancer mark above or below the 0 degree mark on the scale?

Of course, SWCDuke is the man when it comes to these things. Take what he says to heart.

 

Perhaps an obvious point, but make sure you disconnect the vacuum advance and plug the hose and the fitting on the carb. Then use your timing light. If the mark is still jumping around, the point about a loose harmonic balancer should be considered. Alternately, you can put a vacuum gauge on the motor and lock down the distributor when it reaches the highest point of vacuum (I want to say something around 15 PSI.) I'd also take a look at your float levels on the carb. You should have just a trickle of gas coming over the threads when to remove the inspection fitting/plug.

 

What the hell are you talking about? Vacuum gage... 15 psi... God almighty!!!

Duke

 

OK Duke, you called me on it, what should the vacuum be for a stock cammed SB?

 

that makes no sense either. you talking about a solid cam or other

 

For OP's 66 327/350 HP, about 14-17 inches of mercury vacuum (IN HG).

Larry

 

Well thank you indeed, at least someone knows "what the hell I'm talking about". Rex, although I have not yet done a Spring tune up on my 66, I just put the vacuum gauge on it. At an idle (750 rpm) it was pulling 16-17 inches of vacuum (327/350hp cam) Consistent with what Larry cited above. More aggressive cams will pull less (the GTO is at 13). Hook the vacuum gauge (Hawk used to make a fairly inexpensive one, probably still do) to the vacuum port on the carb base where the vacuum advance from the distributor goes. You'll get more direct manifold pressure that way. Plug the hose to the vacuum advance. (Do this when the car is warm.) Loosen you distributor and gently turn it while observing the engine vacuum. According to my gauge 14-17 is indicative of "late ignition timing" and 17-21 is what it reads as "normal." Take that with a grain of salt though as each engine is a little different. As you turn the distributor, the highest level of vacuum attained is where your engine is happiest. Tighten down the distributor bolt and test drive it. If you timing feels good and pulls evenly through the RPM range, go on to the next step and look at your carb settings (rich/lean, and float levels.) In the event your motor is pulling less than 10 inches, start looking for vacuum leaks (the four nuts that hold down the carb love to loosen up on mine.) One trick for that is to spray carb cleaner around vacuum fittings, if the motor speeds up , you found your leak. Good luck, and remember everyone had to start learning at sometime.

 

Your post is very logical and the original poster should consider your point.

Old Pervette, I don't agree with your timing procedure but I guess everybody has a different method of doing things. It's probably best to suggest the original poster use a timing light like recommended by GM on his engine.

Typical L-79 vacuum at 750RPM is just shy of 14", that's with the initial timing set at 10* BTDC and a manifold vacuum advance providing an additional 15* at idle.

 

I think it's hopeless.

Duke

 

A buddy of mine came by today. He just bought a '65 Corvette "365HP" convertible from the local exotic and vintage used car dealer. We all call him "Honest John" because his cars are like that fifties vintage Army rocket. You never knew where it would go, and in the end it always blew up. He wanted me to check it out because he said it "idles kinda rough." Gee, I thought so too. So I put my vacuum gauge and on it, and it only read 10" Hg, and it was idling at almost 1000 RPM! I tried lowering the idle speed, but it stalled at about 600. Uh-oh, this doesn't look good. The instruction book with my vacuum gage says low vacuum means there can be a serious internal engine problem like bad rings or burned valves, and I know a good engine should idle at no more than 5 or 600 RPM and should pull at least 15", and 18 or more is real good.

I told him he should return the car to Honest John and get his money back ASAP before the engine blows up. Upon leaving he side-stepped the clutch at 5000 revs, left two 100 foot long black rubber stripes, and wound the tach off the end of the scale in first gear. Gee, it seemed to run good.

Duke

 

Okay, now you're just piling on.....

 

Gentlemen and otherwise, I do not simply use a vacuum gauge when I do my tune ups, it is in there with the tach and dwell, timing light, and spark light. The poster expressed problems aligning timing marks with the timing light. I suggested using a vacuum as an alternative to get it running first; it is an indispensable tool for doing tune ups and trouble shooting. The junior poster came to the forum looking for help and all he got was know it alls expounding their righteous disbelief and offering him nothing in the way of real suggestions. Bullying a junior member isn't what this forum is about.

 

Don't worry about it. There's nothing wrong with using a vacuum gauge to see what the engine actually wants at idle (and at lower RPMs). This method gives actual data on what the engine wants for your particular combination of engine specs, altitude, air temperature, etc. It makes more sense than just pulling a number out of a service manual. Those numbers are generally slightly retarded, to allow one number to safely cover thousands of engines driven in vastly different conditions.

But, as there's no free lunch in life, if you change the low speed/load timing to improve the engine's efficiency, you then need to spend some time and effort checking/modifying the upper RPM/Load timing to make sure those areas aren't overly advanced.

 

Your last sentence explains exactly why it's not a good idea to time an engine with a vacuum gauge. Yes, you will make the engine efficient at that engine speed and load but to the point of over advancing the initial timing which is what happens when you time the engine at or near max vacuum. That's why I would suggest using the timing light.

Maybe you could explain to the poster how to check his maximum centrifugal advance after using his vacuum gauge, being there is no free lunch.

I don't know the mechanical ability of the first poster so keeping things simple seems to be best approach IMO, but I do understand your point and Old Pervette's point.

 

I absolutely disagree with your first statement.

 

I guess we'll have to agree to disagree then..