I have been chasing a problem with my 292 64F100 for several months.   At speeds above 30 mph, I can feel an intermittant miss.  The miss stops if I accelerate.   I have changed points (US made), replaced the ground wire in the dist., no help.  Next rebuilt the carb thinking the propblem was in the intermediate  circuit and the power vaslve opening (upon accleration) was  feeding additional fuel.  No Help.  Today while checking the igntion again, I noticed the dwell dropped by 5 degrees when the vacuum advance opened.   Cap the vacuum line and the dwell remains steady at any rpms.   I haven't checked it on the road yet.  Will do tomorrow.  The dist. is a rebuilt unit about 10,000 miles ago.   I assume the rubbing block is not in the right place relative to the cam surface.  Theoritically, the contact plate should move without changing the point gap.   Am I all wet?   ANybody else seen this condition?  Thanks for the help.

POP

POP

POP

Pop:

Years ago we had a customer with a 390 with the same problem, dwell changing with advance.  He had just installed a new set of points, some el cheapo ones.  We put a set of Echlin points in it and the problem was gone.  I have seen rebuilt distributors, even from NAPA, with the el cheapo points in them.  Maybe, as you said, the rubbing blocks are postioned wrong.  Have your points been changed since the distributor was purchased?

John in Selma, IN

John - "The Hoosier Hurricane"

POP,

      Add to what John said, check the point mounting plate to make certain it's not worn out and moving into or away from the distributor cam. That would change the point gap and therefore the dwell.

Frank/Rebop

Bristol, In ( by Elkhart) 

Curved points or straight?  If the points are straight, then you may have a pre-’57 distributor transplanted in your engine.  If this is the case, then just change out the distributor for one with curved points.  If you’ve got curved points, are you setting the timing with a timing light?  Is the vacuum advance hooked up to ported vacuum or direct vacuum at the carburetor?.  Your description makes it sound like too much ignition timing when cruising.

Lorena, Texas (South of Waco)

Thanks for all the reply.  I was noting in the shop manual, it states the maximum change in dwell should be 6 degrees so I am within that.   The new points I have in now at US made.

Ted, The dist uses the curved points and is the dual advance dist.   The timing was slightly retarded from spec ( 3 verses 6 adv)    I tried after the timing reset and the problem was there with the vacuum hooked up.  Gone when the vacuum was disconnected.    BUT I checked the timing with the motor at idle, pulled the vacuum advance all the way on with a mighty mite pump and it took 25 degrees on my light to bring it back to 6.    25 degrees with the vacuum only??   I increased speed to the point centrifugal had maxed out ( vacuum released and I had about 10/11 degrees.  Pulled the vacuum again with increased rpm and I needed 35/36 degrees to get it back to 6.   41 degrees TOTAL????   I have to check the vacuum to see if it is adjustable inside.  It is a sealed unit.   So Ted, did you hit it on the head?  too much advance at cruising??

Thanks all,  I will get the vacuum advance fixed and see if it cures it,

POP 

POP

I don't know which vacuum pot you've got but....if it has the hex nut at the vacuum port entrance that screws out? If you take off the nut there are some preload washers to set the spring pressure on the diaphragm. Maybe take it off and add some washers to see if you can slow the rate down a bit.

This picture shows the type of vacuum pot I'm talking about.....

 
 Steve Metzger       Tucson, Arizona

Pop, It sounds like you are setting the static timing with the vacuum advance connected. Set the static timing with the vacuum advance disconnected and be sure to cap off the vacuum port while you set the static timing. Your vacuum advance may be adjustable as well. I'm not sure what the proper setting is for the vac advance but I have mine set so that it is just barely advancing at idle - maybe 1/2 a degree. It also sounds like your mechanical advance may need some attention it should return to 6 at idle. Make sure the weights move freely and that the springs are in good shape.



Edit: Oops. I just noticed that you said you have checked the vac advance for adjustability.

Edit 2: Ok, I just read it again and I think I misinterpreted the entire paragraph. Now that I think I understand, I think your vacuum advance is working alright but I think your mechanical advance should advance more than 10 or 11 degrees. I can't use that to explain what is going on. Do you know what RPM the engine is turning when this occurs? Could you be lean and the accelerator pump richens it enough to stop it during acceleration?


Lawrenceville, GA

I found it!...This is the "other" kind of vacuum advance pot that has the screw adjustment buried down inside. Don't believe the accuracy of the graphical "turns versus degrees" chart - but it does alter the total travel a fair amount.

 

 
 Steve Metzger       Tucson, Arizona

AZ28, the vacuum chamber is one of the cheap replacements and is not capable of adding washers.  It may have the adjustable allen screw accessible thru the vacuum line hole.  Don't know yet.

Charlie,  I think my post was confusing also.  I did set the timing at a static condition, vacuum disconnected, idle at 500 rpm.  My centrifugal is showing a max of 11 degrees (shop manual spec says 9 to 12 at 2000 rpm dist.  The vacuum appears to be adding 25 degrees at full stroke which compares to the manual max of 6 1/2.   It appears I have total advance of 6+11+25=42 degrees.  I do not have a tach on the truck yet (part of my new dash) but I suspect the problem starts around 1500/2000 rpm motor.   I can  not detect any ping but keep in mind the condition ceases as soon as you accelerate slightly ( vacuum advance backs off).  All of my advance checks are done with my timing light advance adjustment knob and I have never checked the accuracy of it.

I think I will pull the dist and put it on my Sun Dist Tester.   I have a 312 with 3 x 2 carbs, Isky cam and "G" heads.  I set that dist to have 10 degree initial, 20 degrees of centrifugal in at 2500 motor rpm and 10 degrees vacuum and it runs great.  Thanks for the help.

POP

POP

No worries.  It sounds like your centrifugal advance is not working properly.  You say you had 10 to 11 degrees but your distributor should add 20.  You'll get all that figured out on the distributor machine.  I wish I had one of those.  I don't believe vacuum advance is usually used in the total advance figure.  Doesn't rolling on the throttle increase vacuum?

Show us a pic of the '64 and the engine!

Lawrenceville, GA