I know that too much advance will cause this and I can prevent it by reducing the vacuum advance. What is the real reason this happens? An old instruction book from a 60's CD ingnition system mentions it and I think it said something spark plug gap. Anyone have any thoughts on this? (By chuging I mean the engine jerks and runs unevenly at a steady state of rpm and speed)

 

You are not pulling enough vacuum at idle. You need a different vacuum can. What happens is; car advances and raises RPMs dropping vacuum. When the vacuum advance can doesn't have enough vacuum it kicks out; dropping RPMs. When this happens you have enough vacuum for the vacuum advance can and raises RPMs only to repeat cycle again. And again. Well you get the point.

What is on the car now? There should be a number stamped on the can.

Hope this helps.

P.S. If you raise the idle; it will stop doing this.

 

The idle is fine and this happens under light load such as 40mph in 4th.
The can is the 1810 connected to full vacuum. It is pegged under these conditions. If I limit the travel of the can, reduce the initial timing or limit the centrifical, the problem goes away. I would like to run with the full advance as it runs cooler and has a little more responce. I am looking for a cause. My first thought is that a Hiperf. cam will more likely cause this.

 

Does it do it in every gear? Say 2nd. Try to maintain same RPMs as 40 in 4th.

What is your timing? Spark plug gap?

My first thought is harmonics. Check your balancer; find the keyway and measure from timing mark. As in mark keyway on top of balancer and measure to timing mark. Also measure across balancer (so I will know what size).

Cam should not cause this. The lower the RPMs are the worse a "big" cam will act. If it idles fine this is not your problem.

 

It will do it in other gears and at no particular RPM. Using a timing tape on the 8" balancer I run about 10 degrees initial, 34 centrifical and 10 vacuum. With no vacuum advance there is no problem. If the can is allowed to reach it's full travel. the problem starts. So I limit its travel. The plug gap I'm using is .035. This has always been a problem even though the heads, ignition (MSD at present) and carb have all been changed over the years. The cam is a 60's vintage Sig Erison that has 286 duration (thats what I was told in 71) I think it is close to a 327/350 cam. Except for this chuging thing it runs great at 12 initial 36 centrifical and 15 or so from the can. Lars mentions this in his timing paper but no explination why. When I changed the heads, I checked the timing mark for TDC and its ok. Thanks for your interest.
Bob

 

When the total advance at light throttle goes beyond 54 degrees, the car will "chug & jerk" at light throttle cruise because the pistons are trying to be kicked back the wrong way on occasion due to the early spark timing.

Do 2 things: First, make sure total mechancal advance does not exceed 36 degrees. Then, install a vacuum advance control unit that does not pull in any more than 16 degrees of additional timing. The VC1810 is a good 16-degree can, so check your total mechanical timing and verify 36. You can them hook up the vacuum advance to manifold vacuum and rev the engine back up to verify that the total combined timing (mechanical plus vacuum) does not exceed 52-54. Fix as required...

 

Hello Lars, When I get to around 40 to 45 degrees with the vacuum it starts this "chug" thing. So the rub is that to run say 12 initial and 36 total at 3k. The vacuum can is not doing much when stopped down to 10 degrees or less (yes, it is connected to full vacuum). It runs nice and with less heat at 50 (16 from the can). This most apparent when at 40 mph and 1800 rpm (most used around town speed) or when doing a slow, low gear steady state condition. It's ok at hiway speeds Could my centrifical advance be too quick in the first part of the range?

 

Another "Chugging" feeling you can experience under light throttle is from a bad torque converter in an automatic trans. Some times if the torque convert is starting to fail you can experience a chugging feeling, even enough to see the RPM jump around on your tac, this is refered to as torque converter shuttering

 

The thing about tuning these cars is that it's a trial and error process where you see what the car likes and what it doesn't like. There are tuning baselines to start from, such as the guidelines given in my Timing Instructions, but these are just suggested baselines from which to start the tuning process.

What I would suggest you do is to try a few different things with your setup. First, yank the hose off your vacuum advance and plug it at the carb. See if the chugging stops when the vacuum advance is eliminated. If it is, try running your vacuum advance off of a ported source instead of manifold vacuum. Often, the throttle is cracked so lightly at 40 mph/1800 rpm that the ported source will limit the advance pulled in. Most cars run best on manifold vacuum, but some actually prefer the ported source. You can also try limiting the range of your vacuum advance by installing a stop plate on the vacuum can to limit it to 10 degrees. You can also try installing a stiffer set of advance springs in the distributor, but this will decrease your level of performance under acceleration. But you can try it and see what works. Finally, check your mixture setup on your carb - if you're lean, it will increase the "chug effect." A richer setuop tends to smooth it out a little.

 

I've got a faily good idea about all this timing stuff thanks to your papers and yes, the chug goes away with vacuum can disconected. I don't want to go ported since the cooler running while in traffic would be lost. I may just be nit picking on this. Anyway, could this be one of the reasons that the stock curve reaches max timing at 5000 rpm?
The fun continues.