I believe that my vaccum advance problems are a result of the rotor being out of phase. http://www.jalopyjournal.com/forum/showthread.php?t=306636 I just ran my car without vacuum advance and it runs fine, with vacuum it's horrible. did some research and I believe that my rotor is out of phase with the cap, I have a MSD rotor with an Accell cap. I plan on cutting the cap ( it is old) to check the position of the rotor. I've read quite a bit and I have found the same symptoms under "rotor phasing" search with google. My question is how to you change the relation of the cap and rotor on a chevy, do you just move the dist a tooth at a time til it is right?
I put some "uni-points" in a Chevy distributor that had had separate points and condenser and when the vacuum came in when no longer accelerating it started backfiring violently. It was exactly what you are describing. I guess the "bumper" on the uni-points was made just enough off relative too the distributor plate it attaches to compared to where the other points were, it fired so far around the cap that the next cylinder in the firing order was getting the spark instead of the one intended. I put the correct points in, back in in this case, I just put the old ones back in and problem solved. This is a situation that only has to do with where the points "bumper" lines up/clocks initially on the movable plate relative to the cap. Changing initial timing, or which gear meshes with the cam gear has nothing to do with it.
A footnote...stock Chevy distributors are phased when drilled for the gear at factory, and the gear has a dimple to show which side corresponds to rotor point.
The rotor phasing that I'm familiar with is phasing the rotor to be pointing at the terminal in the cap when the points start to open/fire. The leading edge of the rotor will be at the edge of the cap terminal in it's rotation giving the coil time to unload before it is past the terminal. It doesn't have anything to do with the dist. gear. If the rotor is phased (running well) with no vacuum, it will still be phased with the vacuum applied. The phasing is between the points cam or reluctor and the dist. terminals. If this is an aftermarket dizzy, is the vacuum advance adjustable? If so maybe it is way out of adjustment or an adjustable stop in the advance mechanism has slipped allowing too much advance? I think you need to get that dizzy on a machine to see what really is happening. JMO
Entirely possible. I learned about that dimple from Vizard's rebuild your SBC book...he also covers proper factory type location of the distributor body. From there...if still out...compare rotor with a GM one, and study the pivot. I had a Chevy six that suddenly became undriveable unless vac was disconnected. Turned out to be a thoroughly worn out pivot point on the breaker plate forcing a lot of movement at the ponts when it moved. Mallory used to sell ball-bearing plates to correct slop in Delco type distributors. Sitting here at work I cannot remember how SBC type pivots...most cars use a cheap-and-nasty off center pivot that gets really bad as it wears out rather than pivoting concentrically with distributor.
Stock GM single point dist. Only change is that I added the Crane points eliminator. Yes it has adjsutable vac, and when I adjusted it, the missing became worse. I was thinking of checking the marks, like in your picture, and then using my vac pump seeing what that does in relation to the rotor... I had the dist apart, and did use Vizards book to go through it, but that does not mean I may nit have made a mistake.
I was going to suggest the mighty vac for a visual inspection but without a machine to actually show the degrees of advance, you are just looking for shorts and sloppy linkage. Your eye can't tell you if the vacuum is adding the correct amount of advance. I'd temporarily pop a known working GM point dizzy in there and see if the problem disappears.
A guy who builds racing ignitions and has done prototype work on ignitions for GM told me the following about rotor phasing.It applies to street engines with HEI's but the principal is same for points. When the ignition is ready to fire ,the mechanical advance should be at full travel and vacuum advance should be at mid travel in their advance curves with the rotor pointing directly at that cylinder distributor wire terminal.
A word of advice on something I just learned over the last few years: There are 13 teeth on a Chevrolet distributor gear. Yep, that's right, an odd number. That means if you just put the gear back on, there's a 50 percent chance of the timing being half a tooth off. That's why the dimple on the gear needs to line up with the rotor bug terminal (i've always hated THAT term). Simply put, the gear can be installed where the rotor terminal lines up between two teeth on the gear OR the gear can be installed where the rotor terminal lines up with one tooth. Anyway, back to the question about rotor phasing. This is something I've always done and I've had the "modified" Ford distributor in the bottom of my tool box for many years. I cut a large "window" into it to shine a timing light into there. MSD sells a distributor cap called a Cap-A-Dapt. It uses a large Ford-style cap and an adjustable rotor where you loosen two screws and slightly change the rotor-tip-to-cap-terminal alignment. There's quite a bit of difference in procedure if the distributor is vaccum advance equipped. Check out MSD's web site and try to find their Tech Bulletins. They have a very good one that outlines the entire procedure.
Hah, I have it out now and I had it backwards. I am going to test a points dist and then try the other one again.
WOW what a thread. Assuming ( may get us in trouble ) that the distributor is a delco unit and has (or had points) the ONLY way to have the rotor out of phase would be if a modification to the distributor cam and or breaker plate was perfromed. The drive gear has nothing to do with rotor phase , only distributor placement. The dimple should ( as Bruce stated) be on the rotor tip side , this was done so that all chevrolet engines had the same distributor placement for vacuum advance placement etc. If a car came in and the distributor wasnt exactly where they should be the service tech knew right away someone had either changed the timing or had the distributor apart. If the car runs good with no vac applied and bad with vac applied the contacts are set incorrectly, worn out, or something ( as in this case) in the wiring moves when the plate moves. On the Crane points eliminator i would suspect wiring when the plate rotates or loosing ground when plate turns . There is a small ground wire under the plate that breaks... Let me know if i can help in any way , more than glad to help..
I rebuilt a points dist and it workd fine. Bought kit at a swap meet with good weights, and a B1 can. Added springs to get it all in by 2800 timed it at 36 and it sounds good. I have the old dist out with the crane points conversion and I think i had 2 things going on. All wires are good, but I have an adjutable vac can and it was set up without the limiter so I had about 20 deg vacuum. The plate on that one moved alot more than the plate on the b1 can (used a vac pump) Also think I had the rotor pointing late to the number 1 plug so when advanced it juped to 8 I put the limiter on and i am reluctant to sawp it out again, so I may see if I can get it on a machine to see what happens.
All wires are good, but I have an adjutable vac can and it was set up without the limiter so I had about 20 deg vacuum. The plate on that one moved alot more than the plate on the b1 can (used a vac pump) NOW that, would mis-phase the rotor. Moving the breaker plate too far would cause a problem.
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