Vacuum advance line disconnected and plugged. Distributor rotated clockwise until maximum vacuum on the non-ported line jumping 18" to 21". Timing light shows that as 40 BTDC. Really?
OK, back off the timing counterclockwise until vacuum gauge jumping 16" to 20". Timing light shows 13 BTDC, close to factory.
Idle feels smooth at either setting. I'd need to pick up a tach to say what RPM that is, maybe a good investment
Engine still shakes the car wickedly at about 1/4 throttle with the vacuum line connected. In this condition the timing light shows 55 degrees. At approx the same throttle position, mechanical advance only shows about 32 degrees and its running smooth.
Joined: Sat Apr 25 2009, 03:08AM
Location: Out West
Posts: 626
Sounds like you might be looking at worn out valve springs, but have a look at these diag demos and see which resembles most closely what you are seeing on the gauge:
Joined: Tue Oct 11 2005, 01:33AM
Location: Calgary, Alberta
Posts: 5893
What a great link on vacuum gauge readings!
When I was a teen my Dad showed me how to use a vacuum gauge to diagnose engine problems and I must admit I had forgotten most of he taught me until the Scenario's discussed brought it all back!
I need to re-run those tests, as I was continuing to play around the vacuum gauge needle was noticeably more stable at idle, so maybe I wasn't warmed up enough the first time.
Got my hands on a timing light with a tach, and that ~55 degrees of advance with engine shaking that I posted previously is happening at around 1300 rpm. That's way too much advance, right? FSM shows max 18 mechanical plus 22 vacuum, by 2400 rpm. (edited to correct to crank degrees consistently).
The needle jumping on the vacuum gauge went away with proper (I hope!) setting of idle speed / mixture screws, so that's good.
With no vacuum advance connected, I'm seeing about 32 degrees total advance all in, so that seems about right (13 initial plus 19 mechanical). So what I'm seeing is another 25 degrees of vacuum. Manual says 15" vacuum should be 11 distributor degrees, that's 22 crank degrees (right?) making a grand total of 54 degrees. So my observed 55 degrees is not too far out of the ballpark, or is my math wrong? (I see over 20" of vacuum on the ported line at higher rpm; is that OK?)
But, the bottom line is, the car runs well with the vacuum advance disconnected up to about 65 mph. But, after that point it seems to run out of steam with a low growling noise. As soon as I connect the vacuum line it runs like crap.
I'm not sure where to go next. I need to verify how these advance numbers look vs. rpm, that might be a clue. But I'm starting to wonder if it's not actually (not only?) an ignition problem.
Is there a problem running with no vacuum advance?
Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
Vacuum advance is there for fuel economy. If it's not connected your gas mileage will suffer.
From the FSM, the vacuum advance is supposed to add 22* (crank) at most. If yours is adding more, I suspect it's the wrong one, even if your total advance winds-up in about the right place.
The fact that the engine starts acting strangely above 65mph even with no vac advance suggests there's still another problem. I presume it used to run ok previously?
Joined: Tue Oct 11 2005, 01:33AM
Location: Calgary, Alberta
Posts: 5893
I have no idea if there are any guys still around, but we used to get our distributor recurved to give the best combination of mechanical and vacuum advance throughout the RPM range. You'd need to find an old fashioned corner service station that still knows about non-computerized ignition systems.
I'm going to check the mechanical curve more precisely. I suspect that I will find that it's all-in too soon. Then I will move on to the vacuum.
@Mike: The car did run well up to 80 mph before, but idled poorly when hot. Out mutual friend did some carb and timing work to get that settled, which is when the problems started. It's going go back in when he has time, but in the meantime I'm trying to learn more myself.
