After driving the Skylark to work 3 straight days this week, I am going to park it until I figure out this "problem." The engine ( 120 miles on rebuilt engine) is a bit lazy through the gears, and at a cruising speed of 45 mph seems to have a stumble of sorts. Not a miss, backfire, or surge, it seems to stumble then get itself together for a minute, then stumble some more. My initial timing is 10 degrees, new carb (Edelbrock 1407) is on top, and the plugs look OK. A vacuum gauge reading shows 18" and a slow needle movement back and forth between 17" and 19". Could my distributor advance bushing be a problem here? I feel the problem should lie in the distributor since it the original. I do have a Pertronix Ignitor 1 in there, too. And an older coil.... I would like to hear the problems associated with a worn distributor if someone can help... Russ Waters 1970 Skylark Convertible 455
Cruise stumble 1. Did you degree the cam? I had a similar problem with my car last spring. I put a new timing chain on and retarded the cam 2 degrees to try to help out my mph at the track. Acted similar to what you are describing. I went back at it and ended up with the cam 4 degrees advanced - runs much better. 2. Check your total advance. Too much lead (usually from the vacuum advance) can give you a cruise surge problem. I lived through this too, ended up with about 20 degrees of mechanical advance in by 2200 rpm, 14 degrees initial, and limited the vacuum advance to 6 degrees (non-ported).
Russ, This is exactly why I took the time to write this post: http://www.v8buick.com/showthread.php?t=63475 What is your total timing, and at what RPM? How much advance is added by the vacuum advance? Initial advance is unimportant here, you need to know your total advance(initial+mechanical+vacuum) going down the road. John is right on the money here, again too much total advance can make the engine stumble. Get the timing light out, and find out, or disconnect the vacuum advance. If the problem goes away, there's too much advance in the cannister.
John, I installed the cam 4 degrees advanced for the same reasons. It seems the engine would like to wind up more, but just can't get going. Larry's post ( Thanks for the info, Larry) explains it well. I have already marked my harmonic balancer and checked it with a timing light as the engine is revved. What makes me suspect a faulty distributor (besides being 32 years old) is the erratic readings I get as I rev the engine up to 2500 RPM. They should be consistent if the distributor is working correctly. I'm going to take it out and check the bushing.... I'll check in on Monday. Russ 1970 Skylark Convertible 455 (1973 engine)
Russ, What springs are in the distributor? Are you sure all your advance is in at 2500?. Can you change back to points, to see if it is the Pertronix? Have you looked at the plugs lately?
One other thing - Make sure that you have a ground wire from the points plate to the distributor body. I thought it wouldn't matter with the Pertronix when I was running one so i took it out. You DO need a ground to the points plate.
I pulled the distributor and found: No bushing so there is a problem. I also notice the shaft doesn't spin as freely as it should. A little inquiring tells me Autozone and Advance Auto offer the same remanufactured distributor for around 32.00. That has to be a good option for me. I'll install it as is, run the car a while, then change the points set-up to the Pertronix I. It seems a thorough way to troubleshoot this. BTW, I checked the coil, and it's resistance was 6.7k. This is well within the 4-8k specs given by my repair manual. I'll check in soon. Russ
Russ, I was always under the impression that there should be an advance stop bushing on a stock distributor. I got this from a GS EXtra article I read about power timing a Buick V8. Jim Weise pointed out that he had never seen one on any original distributor he had taken apart. I looked in the Chassis manuals that I had, and they make no mention of a bushing. While a bushing is a good way to limit the mechanical advance, the absence of one doesn't necessarily indicate a problem. If you put very light springs in the distributor, you can easily check exactly how much advance you have in the distributor. If I was you, I would go back to points, and disconnect the vacuum advance. See if you can get the symptoms to go away. A quicker advance curve coupled with a stock vacuum cannister will give you surging, pinging, or both.
