First of all, I tried doing a search and could not duplicate my exact question but there were alot of posts already on ignition timing. First let me explain this condition is in my 70 396/350HP car. I have noticed that when I am cruizing at a constant speed (say 60 MPH or so) there is a noticeable hesitation on the car almost like a big gust of wind has temporarily hit the front end and very subtly changed the speed. There is no noticeable change in RPM on the tach.

A mechanic that I know(owns his own shop) that drove the car indicated that it might need some more timing or carb work to get better performance but did not say anything about the "hesitation". He only took it for a short test drive after the new clutch was installed)Based on his feedback I decided to check out the ignition sytstem.

* Plugs were all in good shape w/gap about .038-.040 so I reset them at .035. when I say good shape all had the tanish brown condition with no carbon or oil evident
* Checked dwell and found this to be at.035 so I dialed it in at .030.
* Checked condition of points on dwell meter after this and this indicated they were ready for replacement so I replaced with new and set the dwell at .030 again
* Replaced Rotor and Cap
* Wires in good shape
* I went to check the timing (4 degrees BTDC according to manual w/vacuum disconnected). I disconnected the vacuum line on the carb and noticed that there was no change in the idle. I went to put a golf tee to plug the line anyway and noticed that the vaccum line has a plug in it about 3/4" from the end ... enough to allow carb hook up. I put the timing light on it at about normal 1000 RPM idle and the mark was about 12:00 O'clock on the Damper. Realizing that I was with the vacuum advance plugged but probably only mechanical advance for some reason this did not surprise me. I dialed in the timing light to move mark to 4 degrees and the dial read 35-36 degrees. I raised the idle to 2000 RPM and dialed it in again to 4 degrees and the timing light read 40-41.

First of all my question would be, Does this timing seem about right for a distributor that is set-up for mechanical advance only?? Why would someone disconnect (plug) the vacuum and run mechanical only?? They would have had to change the weights and springs from stock??? How do I tell? What else do I need to check on the timing to ensure it is set correctly??

One other important point that may be a factor is the prevoius owners pulled off the Rochester and put a dual feed Holley on the car. This is the only thing that I know of that is not stock.

If the ignition seems right with this set-up I will start looking at the carburator unless I get feedback otherwise.

Mytmouse

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67 SS 396 350HP 4 sp 3:55 Posi, Butternut Yellow w/Black Vinyl Top
70 SS 396 350HP 4 sp 3:55 Posi, Black on Black
68 RS Z/28 302 350+Hp? 4sp 4:10 Posi, Euro Red on Black
Mytmouse: A.K.A. Robert Stacho

[This message has been edited by Mytmouse (edited 09-18-2000).]

Hesitation, or surging at a cruise speed generally indicate a lean mixture. But you may need more timing at cruise, when your at cruise speed; you have high manifold vacuum which should increase your timing. Say your putting around at 2000RPM, but your motor only has 12 degrees of timing, you'll get a bog or hesitation.

Someone may have disconnected the vacuum advance because they couldn't set the timing right. Sometimes it is a solution if your total timing makes your initial timing too high with the vacuum hooked up.

I had a 69 396/350HP motor, and this is what it enjoyed: 14 degrees initial timing, all of the advance was in by 2800. I ran vacuum advance, connected to ported vacuum. Total timing was 38 degrees.

Unless your mechanical advance is coming in quickly, you'll probably benefit from having your advance properly hooked up. I would try re-setting everything from baseline, atleast 8-12 degrees of inital advance, and top it off at 37-40. Connect the (unplugged) vacuum line to ported vacuum and give it a whirl. Experiment until you think you've got the timing as perfect as possible, and then fix the carb if your problem persists.

Wes

Mytymouse. I had to reread the post a few times to understand. Your timing is completely off, OR your timing mark has shifted. But the surge at cruise leads me to believe your timing is way off.

If I am interpreting correctly, if you had continued to increse your rpm, you would have found nearly 55 degrees of total advance. If I'm reading this correctly, then, as follows...

Bring your idle up to 1500-1800. Set it there. Then dial back the timing to 12 degrees advanced ( BTDC) No vacuum in any of this yet. Reset the idle to normal.

Hopefully its running OK now. Then slowly move the throttle up until the mark stops advancing. Take note. Or have someone do it so you can dial back to 0 on your light. This reading should be in the 35 area. Adjust the distributor again to get between 34-38 degrees at full mechanical advance. Now let the throttle back to idle. Check your timing again. Make a mental note.

Now you'll know, for eg, that you have say 15 initial and 22 mechanical to give 37, or whatever your numbers are.

Most of our mills like between 34-40 total mechanical timing. No way of knowing preference of your. Closed chambered seem to like more than open chambered. But you set it at the total, and then you know where X initial is to give Y total.

Now get that vacuum advance connected properly. See how much the timing jumps when its connected to DIRECT, not ported vacuum. Again, for most of our mills, I think 8-12 is OK, more is not good.

Now you can finally set both your idle speed and mixture.

Take note of the difference. Now you'll know your total mechanical, total vacuum, and exactly where to set inital. You'll want to experiment with the initial ( actually you're seeking the correct total)so change it, run it and see where your mill likes it.

This is not to say that you have a "correct" curve in your distributor. But here's where I punt and hire the likes of IgnitionMan or The MagShop to do it correctly with years of knowledge and a Sun machine.

Good luck.

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Gene #62gold/ACES
67 SS396/(427L88) (http://www.chevelles.com/showroom/chaas00.jpg)



[This message has been edited by Gene Chas (edited 09-19-2000).]

Gene & Wes,

Sorry about the confusion. http://www.chevelles.com/forum/confused.gif I knew what I wanted to say but probably wasn't saying it correctly. What is clear is that the vacuum line coming off my distributor and going to my carburator has a plug in it so I am not getting any vacuum to the distributor. When I put my light on it with the dial(on light) at "0" the mark on damper was running about 12:00 o'clock or way way off the timing mark plate. I thought that maybe when motor was rebuilt and put back together 7 or so years ago they might have put in lighter springs and weights plugged the vacuum line and ran only a full mechanical advance timing. ( I think I am saying this correct) Is this possible? If so what should my timing be at idle... 12 degrees?? And then increase RPM until the mark stops moving and then dial it back on the light and read the dial ...somewhere between 36-40. Any way that was what I was trying to say in my first post...but I know I try to get too detailed and then I trip on my tongue.

Now having said all that. I have checked inside the distributor and it appears that the vaccum advance is hooked up correctly. I compared with my 67 and it looks the same.
Now when you say hook vaccum line to "port", Do you mean directly to fitting on back of my manifold?? I have a fitting on there with three "ports" and two of them are capped off. Hook up to there and cap off port on carb???

I am getting into this old car game 30 years after they were new cars to me (although I never owned any as nice as I have today back then)but when I set the timing on my 67 I did like I remembered way back when. This was to pull line off carb and plug it. Set the timing at about 4-6 degrees BTDC which is generally a little rough until you put the line back on the carb and then it smooths out and the time jumps to about 12 or so. This is what I attempted to do on the 70 but became confused when I realized I had no vacuum going to the distributor. Gene, if I am reading what you are saying in your reply, I will eventually get to this point but I am going to work backwards on this starting with getting it running decent with only the existing mechanical and then work to towards getting about 12 degrees with vaccum hooked up to port. Is this correct??


Thanks bunches for your help ... I hope I am clearer this time.

Mytmouse

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67 SS 396 350HP 4 sp 3:55 Posi, Butternut Yellow w/Black Vinyl Top
70 SS 396 350HP 4 sp 3:55 Posi, Black on Black
68 RS Z/28 302 350+Hp? 4sp 4:10 Posi, Euro Red on Black
Mytmouse: A.K.A. Robert Stacho

some people used to plug off the vacuum advance to get that lumpy big cam idle but it hurts fuel mileage and low rpm driving.
here is a interesting link http://corvetteforum.com/ubb/Forum4/HTML/001831.html

http://www.chevelles.com/forum/confused.gif

With a stock distributor, you'll get whatever vacuum advance is in there. No dialing in. I run vaccum off manifold vaccum off carb, not ported vaccum ( "On" only when you get into the throttle.)

But yes, get the car dialed in with mechanical first. Then hook up vaccum.