If the dwell does indeed change when you hook or unhook the vacuum advance, your distributor is mechanically worn out at the breaker plate and/or breaker plate attachment points. (the breaker plate is what the points screw down to) This could explain a lot of your issues. Dwell should not change at all with any timing change, while dwell effects timing, timing should not effect dwell at all.

How much did the dwell change when you hooked or unhooked the vac advance? (any change indicates worn parts, but a massive change might indicate that these worn parts are all or at least part of your problem.

A big change (really any change) certainly indicates the need for a new distributor. You should be able to get a rebuilt points distributor for cheap. (like $20-$30 for Chevy, Ford might be a little more).

A distributor with a loose breaker plate or worn shaft bushings will cause a very eratic spark, the timing will be so eratic that this could be the root of all of your issues period, it would explain all of your symptoms.

Also, when you set your timing, the vac advance should be unhooked.

 

The dwell changed about 5 or more degrees when I removed the vacuum advance line. So does this mean I should replace just the upper portion of the distributor i.e. breaker plate/points/condenser or the whole shabang? I noticed I could turn the distributor shaft just a tiny bit, just a slight play in it. Is that normal?

 

A slight amount of lateral play in the shaft is usually ok, there will normally be some verticle play in the shaft, don't worry about that.

I don't know if you can buy the breaker plate assembly seperatly, but if you can this should be ok, but in my experience, it is diret cheap to buy a remanufactured distributor.

The 4 degrees of change may or may not be enough to cause all of the problems that you are having, but in any case getting this looseness out will help your situation at the least. This amount of play could cause your points to not fully close at times, and therefore this will cause a misfire. Additionally, if the points are not closed long enough you will have a weak spark.... again, a possible misfire....

http://www.autozone.com/selectedZip,.../selectZip.htm

AutoZone carries two distributors, you probably need the top one in this link, it is less than $50.00 + core, and you have a core.

 

Well that's not too expensive at all so I might as well do that. I'll get back to you later with an update.

 

Today, I replaced the distributor. I set the points gap at 0.017-8 (dwell about 29 degrees) and used a timing gun to set the timing at 6 BTC (with vacuum advance hose off). My vacuum pressure is still low at around 16 and when I remove the vacuum hose on the advance, the dwell changes as well as idle speed. The dwell changed about 5-8 degrees with the hose off. This was with a new distributor! I'm happy at least that the distributor and points are no longer a mystery to me, nor is timing and such.

EDIT: I checked again to make sure so starting with vacuum off and plugged, timing is good and dwell is good as well as idle. When vacuum is plugged in, dwell drops from 29 to about 20-21 degrees and idle increases by about 100 RPM's. Should I disconnect and plug all vacuum lines or just the advance?

 

When I time my engine to 6 BTC and then plug in the vac advance, the timing retards about 10 degrees. Is this normal?

 

NO!!!!! There is not suppossed to be any vacumn on the dist at idle. If there is, then the hose is connected to the wrong port on the carb. you want the one with no vacumn at idle.

 

Can anyone tell me if they have experienced dwell changing when the timing is advanced via vacumn advance?

I have never experienced this situation, and everything I have read or studied shows that this should not happen,,,,,,

Sorry Guitlerham, I thought I knew what I was talking about and caused you to spend money for no reason. I was sure I was right about the changing dwell..........

I owe you one,,,, need any tie rod ends? I have brand new ones for a 66 that I will not be using.... I have manual and power ends...

 

Hey JMD, no sweat. You and others have been great help and we learn new stuff everyday. It's just the nature of the beast. I just have a band new dizzy! It's all good!...except I'm still baffled at what could be causing the issue.

jlg2002: you said there should be no vacuum coming to the dizzy at idle and if that is true, then something is slightly off. In the picture you can see which port I use for my vacuum advance.
I hooked up the vac guage to that same port and:
at idle (500 RPM's) it reads about 8 inches
and at 1000 RPM's it reads 15 inches.
The funny thing though is that by pushing the throttle lever forward/off all the way with my finger, the reading would go down to maybe 1-2 inches but the RPM's wouldn't change. So I can kind of play with the vac reading by simply pushing either direction on the throttle lever just enough to move the vac reading but not change the engine speed.
I just had an idea: I have to use two return springs to make my idle come down all the way, maybe this is because the throttle plate is warped or something. That would explain why the vacuum reading can almost disappear when I push harder on the throttle to close it! If the plate wasn't warped or obstructed, then there would, in fact, be no vacuum reading on that port as is required.

This is a problem I must fix, but I don't know if it's the root of my issue though. Could a warped/obstructed throttle cause hesitation and poor in-gear acceleration?


[IMG]local://upfiles/33020/96C4F4FE2F674D72A7D8948DE67FEC59.jpg[/IMG]

 

yep, 8 inches at idle indicates that the throttles are too far open or there is an internal leak within the carb. A stiffer carb spring may do it.