I have a 78 Silver Anniversary Edition with a zz502 BB. Barry Grant Speed Demon 850 carb. MSD 6AL ignition, what appears to be an Accel Super HEI distributor. Headman headers, new flowmaster exhaust (no cats), umm what else... Oh yeah the vacuum advance on the distributor I believe is not working. If I remove the vacuum hose from the carb that leads to the vacuum advance and suck/blow on it, it's like sucking/blowing into a straw, there is no restriction at all. I'm not sure if this is part of my problem, but here we go.

My timing at idle and engine warm, connected to the vacuum advance is 48, my timing at 3000 rpm's no vacuum advance is off the chart. My crank (hopefully that's the right name) only seems to go to 60 and then has some letters after the marks end and that's where my timing seems to be at 3000 rpm's. I live at about 4000 feet above sea level and am pretty sure the carb is jetted properly (it passed emissions, so something is right)

I've been told the zz502 should be at about 8-12 for timing, but there is NO WAY I can take it that low without it dying, even if I up the idle. I was hoping that someone here might be able to help me figure out what my problem is. The car runs just fine, at least I think it does and very rarely it hesitates a little when I turn it off. Can someone with more knowledge than me please help me get this right???

 

I don't mean to offend, but is there any chance you have the timing light lead on the wrong spark plug wire?

Seems like you have that ignition wayyy advanced if you are checking it correctly.

 

I'm pretty sure I'm on plug #1, it's the very first plug near the front of the car on the drivers side. Plus the plug wire has a #1 on it at the distributor. Good question though, I'll check anything.

 

Do you have an adjustable timing light ? How old is the balancer ? What made you start looking at the timing in first place ?

 

My timing light is not adjustable, the balancer is the original one that came on the ZZ502 and is only about 2 years old. I got the car from Chicago and when it got here I just ran tests on everything and noticed that the timing was where Chevy says it should be. I'm sure it could run better if I had everything set properly.

 

Well, for sure the vacuum advance can is bad, it doesn't hold vacuum. It seems the balancer may have slipped, the engine should run fine with 8-12 deg. timing.

 

So can you get it down to around 20 degees at idle, that should be fine with no vacuum advance

 

Now that I think about it, last time I dropped it to around 20, it did not not want to shut of at all or run very well. We slowly adjusted it up and up and up until it ran better and gives little hesitation shutting off, that's why I'm at 48 right now.

 

For a start, I would confirm that the zero degrees mark is at TDC. There's a good paper on Corvettefaq (by Lars, as I recall). Once you verify your zero, a lot of the uncertainties above will go away.

 

If you have run on then the timing is too high, reduce the timing and adjust the idle up at the same time

 

The lower I drop it, the more hesitation on turning off it has. So when checking for TDC, I just make sure #1 piston is at the top and at the same time my 0 degree mark should line up with my timing marker?

 

I found the file by Lars and will try his method tomorrow. I think this might actually be my problem, if not, then at least I know for sure that my balancer is lining up propery with the piston. Any other suggestions meanwhile are appreciated.

 

Can't imagine the balancer is the problem on a 2 year old GM crate engine that is why I asked how old it was in the first place. Would be worthwhile confiming I guess though

 

Something is way out of whack here. You need to start from scratch and adjust the timing. The Lars paper referenced above is a good place to start, but you need to also confirm that the balancer is actually indicating "0", and that you are reading #1 plug and not something else.

 

Since the vacuum advance provides no advance when vacuum is applied, could it be that it is somehow always advanced? When I got my 78, the PO had put an edlebrock 750cfm carb on it and attached the advance to the always on port of the carb. At idle it showed about 60 degrees advance. When I moved it back to the gated port on the carb it read correctly at idle and then advanced when the rpm increased.

 

vetteguy -
Do the TDC verification first as outlined in my paper. I've seen balancers slip and read incorrectly even when near-new. The TDC check will eliminate one possible problem.

Once verified, replace your vacuum advance control unit with a good unit. If you're running a GM HEI-type distributor, use NAPA/Echlin part number VC1838 for a 4-speed car or VC1862 for an auto. If you have an MSD distributor or a GM points-type distributor, use VC1765 for a 4-speed or VC1810 for an auto. Hook up the vacuum hose to direct manifold vacuum on the carb - not ported vacuum.

Set timing as outlined in my Timing paper - e-mail me for a copy if you don't have it (V8FastCars@msn.com). The ones posted on various forums are not current. You want to set your timing up for total timing (with the vacuum hose disconnected) of 36 degrees max'ed out at 2300-2500 rpm. This should produce an initial timing (optimum) of about 18 degrees. Fix your advance curve length to achieve a number close to this. Once set, hook up your vacuum advance. This will pull in an additional 16 degrees on top of the 18 initial, and will give you about 34 degrees timing at idle. With the full centrifugal max'ed out (36) and full vacuum advance on top of it (16), you'l end up with a combined total maximum timing of about 52 degrees. This will produce a perfect timing curve for your application. Contact me if you need assistance.
Lars

 

Ok, I've verified that my balancer is correct. Exactly on top of the compression stroke it lines up with 0. I also took the distributor cap off and verified that the roter is pointing at #1. I decided to adjust it to 12 degrees BTDC and I can up the idle and it seems to run just fine, but hesitates very badly to shut off, so I bumped the timing to 20 (this is all at idle and no vacuum connected) and it seems to run about the same, maybe a little better and the first time I shut it off, it didn't hesitate at all, so I took it out and drove it for about 10 minutes, but when I got home, it hesitated for about 3 seconds to shut off. Last time I adjusted the timing I just adjusted it up and up and up until I got the least hesitation and that's how I got to 40 (this is a correction from my original post, it was actually at 40, not 50).

Any other ideas?

 

What are you using for a distb? Factory, msd, mallory? The mech adv. may be messed up, weak spring, broke spring, etc. Some distb's mech adv slides on plastic bushings under the plate. they may be worn or mia. I also agree your vacc can is gone. good luck robert

 

It's a new Accel distributor, so I would assume it's not the problem, but you never know. I was also just told by a local speed shop that I have to get a holley dashpot cylanoid and bracket. He guaranteed that I would be able to set the timing at 12 and it would diesel (as he called it). Anybody had experience with these? I'm waiting on hold for Barry Grant right now to see what they have to say. I'll call Holley next.

 

I was just told by Barry Grant that my 850 carb is downleg and should be annular. He told me that it's pumping way to much gas into the motor, so he told me that's why the more timing I give it, the better it will run, but he recommends boxing it up and sending it to them and for about $85 they will convert it to annular. Just thought I would let you guys know what they told me.