Ignition Antics

[CPN]

Need some help - been lurking for a long time – time to post. I bought a '64 corvette a year ago. I Love Vettes - My dad had a '63 coupe in the early ‘70s (Powerglide 300 hp car) and a ’74 coupe (L-82, auto) while I was in high school (I’m 35). I was looking for a clean early ‘70s car (‘70-‘74) because that’s what I thought I could afford for a long time when I came across a number’s matching (except the rear end –from a ’67) 1964 coupe. L76, 4speed, power brakes with a respray (resale blue) but not much else. Looks great from 10 feet. Out of my price range, but life is short and the car appeared mechanically solid – no rust, straight, no hit body, all the parts except ignition shielding there. So, I love the car but I never thought it ran right. First problem: wouldn’t rev past 4700 rpm. Pertronix ignitor and new coil fixed that. Next problem: real driveability issues. Stall at stoplights, needed to idle at 1300 rpm, didn’t pull like I thought is should, hard starter… Partial fix was a rebuilt brake booster to correct the vacuum leak. That fixed the stalling. I started reading up about carb tuning here and adjusted the float bowl level, the idle mixture and a few other minor things (ensured pedal to the floor was really WOT). This also helped – but I still did not think I was getting full performance. Oh – by the way, I removed the valve covers for a look around. Guess what I found – hydraulic lifters… <o></o>
The more I read, the more I thought my problems were ignition related. Out came the timing light, off came the vacuum assist. Things were a bit off – I dialed it in to have 36 degrees by 3500 rpm, vac advance can disconnected (running manifold vacuum). Ran better, but now it surged and bucked like nuts and had this maddening wandering idle. Some of you have probably figured out where this ends up by now. Courtesy of this and another forum (some of Lars’ papers) I checked out the vac advance can. Stamped plainly on it – B1… the wrong can. I put a vacuum gauge on the engine and I was pulling about what an L79 should be pulling – that hydraulic cam again, I’ve begun referring to the engine as an L771/2. I put my squeezy vacuum gauge thingy on the can to see where it engaged. The wandering idle was because the engine was sometimes pulling the can open, sometimes not – advanceing the timing, slowing down and speeding up the idle. The last guy (we’ll call him bubba) ramped up the idle speed to get the engine into mechanical advance, cancelling out all the screweyness caused by the wrong can. So, I bought the can for the fuelie / L76 327. That proved to be a bit too much. Next can – the one for the L79. Hot damn did that work. Some carb fiddling, the right vac advance can and correct timing… amazing. No knocking (there was some light knocking under accel in 3[SUP]rd[/SUP] and 4[SUP]th [/SUP]when I had the second can on it) anymore and finally I was getting the wave of top end power these engines are supposed to make. I still suspect the spring that regulates the secondary’s action is too stiff. I’ll tackle that and jetting later. Drove it and was back in love with the car. Then I started getting knocking… and more… back to the timing light, reset to 36 all in at 3500… got it out, drove it, same creeping advance. I am convinced the distributor hold down is tight and have not been driving or working on it for a few weeks due to work and weather, but what the heck? Am I just missing something or is this symptomatic of something else wrong. Just looking for some thoughts before I declare Jihad on this thing again…<o></o>

 

[CPN]

Correction

Folks - where I said 3500 rpm, I meant 3000 rpm, referring to where my mechanical advance is all in.

CN

 

[Hib Halverson]

Try connecting the vac. adv. to ported vacuum rather than manifold vacuum and see what happens.

 

[CPN]

Thanks

Hib, thanks for the advice, I will try that. I am under the understanding ported vacuum was a response to the emissions era as automakers, GM in particular, used ported vacuum to control the amount of unburned hydrocarbons leaving the engine (using the Air Injection Reactor system).

I trust your recommendation, I'm curious as to what this would accomplish - part of the fun of forums like this is the sharing of experience and understanding.

CN

 

[Hib Halverson]

Your original post led me to believe that your spark timing is inconsistent at idle when the vacuum advance is connected but, when you disconnect the vac. adv. to set the initial advance, the spark timing is constant.

I suggested that as a way of determining if, in fact, vacuum advance is fluctuating at idle.

But, to be honest, now that I have re-read your initial post, I'm a more confused and having trouble reaching any further DVI (diagnosis via internet) that's going to be useful. After reading again, I'm less clear on the problem.

Do you feel the timing is fluctuating at idle or are you saying that you can disconnect the vacuum advance, set the initial advance then come back after driving the car for a while, check the timing again and find that the timing changed?

You said the car is "numbers matching" except for the rear end. In fact, it's not numbers matching. What's, also, got me confused is your finding the engine has hydraulic lifters. Do you think the engine is an L76 which was converted to a hydraulic cam? When you say you call the engine an "L771/2", I don't quite get what you mean.

You say the engine is detonating. That doesn't surprise me if you set the timing at 36°, the engine has the high-compression typical of an L76 and the engine may have been fitted with a different camshaft having less duration.

As for the carb's secondary diaphragm spring...is the Holley on the engine the correct unit for an L76?

 

[Vettehead Mikey]

[ Then I started getting knocking… and more… back to the timing light, reset to 36 all in at 3500… got it out, drove it, same creeping advance. I am convinced the distributor hold down is tight and have not been driving or working on it for a few weeks due to work and weather, but what the heck? Am I just missing something or is this symptomatic of something else wrong. Just looking for some thoughts before I declare Jihad on this thing again…<o></o>[/SIZE][/FONT][/COLOR]

Is this what you're asking about- drifting timing?

Nothing to do with vacuum source. Check for sticky advance weights and mechanism.

 

[CPN]

thanks

Hib,

The wandering idle was caused by the wrong vac advance can. The car was not making enough vacuum at idle to pull the advance all the way in, so the timing would advance and retard pretty much on its own.

So not numbers matching - got it, I've come to realize numbers matching means different things to different people. Original engine I'll say from now on. The engine is an L76, but someone, somewhere put in a hydraulic cam, making the engine, in my mind, halfway between an L76 and an L79... hence, L77 1/2.

So I had a few problems, vacuum leak from the brake booster, wrong vacuum advance can, wrong vacuum advance can again, out of tune carb (float level and idle mixture). I worked through them and had the car running pretty good. Then, I started to get a bit of detonation, then more and more, causing me to re-time the ignition. As far as I can tell, the hold down clamp is tight (hard to get to). I've read about distributors "jumping cogs" - hoping that is not the case.

The Carb is a holley and looks like the one in the service manual - I have not taken the time to determine if it is original. Not totally sure what kind of Holley it is.

Mikey -

Thanks for the pointer - I'll pull the cap off and have another look at the weights and springs.