C2- 67 timing idle question? -I give up!

[Patrick H.]

Duke

Even though it ended up being Glenn's Pertronix unit, any thought on my post above? I asked it not only to get more information to Glenn, but also to help finalize tuning my Dad's 72. My 2 cars are 4 speeds, but his is an automatic and I'd like to know so that I can be prepared whether or not to expect a change in timing based on vacuum when the car is placed into gear.

Thanks,

Patrick

[Duke W.]

Re: Duke

Lets assume that in Drive the idle is about 475 and about 600 in Neutral (or Park). With a 300 HP cam or equivalent the idle vacuum at 600 in Neutral should be 18-20". In Drive at 475 it will drop because of the load - maybe a couple of inches.

I can't quantify the drop, so get me some data!

The key issue here is to maintain enough manifold vacuum to keep the vacuum can plunger pulled to the stop. Since most (pre emission) 300 HP vacuum cans max out at 16" my "two inch criterion" should be meet or at least be close - idle vacuum 2" or more than the value for maximum vacuum advance.

Of course, on your dad's emission controlled engine the above may be irrelevent
because it probably has ported vacuum advance. In this case the total idle timing should be just the initial value either in either Drive or Park.

If you've converted to full time vacuum advance the first two paragraphs apply, but you should convert to a pre-emission vacuum can with about 16 degrees @ 16".
On emission controlled engines with ported vacuum advance the typical vacuum cans were 20-24 degrees @ 18-20".

Duke

[Patrick H.]

Re: Duke

Duke,

It's been converted to full vacuum and recurved, as that was far easier and cheaper on this driver than purchasing all the required emissions solenoids, brackets, etc.

I just have it on occasion where it will quit when dropped into Drive, and ironically enough it has a B1 can like Glenn does. I plan on changing to the more correct B22 can, but there isn't a huge difference in specs between the two other than the fact that the B22 starts a little earlier than the B1 can. My thought was that it might be losing just enough vacuum when shifted into gear that it lost vacuum advance, altered the timing, and quit. Just a tap of the throttle will bring it back to life if I catch it in time. By recollection, the idle will be about 1200 when in park, but drop down to 500-600 when placed in gear. I have yet to be able to get the idle speed lower when in park and still have it be able to idle in gear. It ALWAYS drops rpms significantly when placed in gear.

So, if it was a general rule that engines dropped about 4 inches of vacuum when put in gear, that "might" help explain what I'm seeing if, in fact, I lost my vacuum advance as a result.

Time to get the car out of storage and check it out for sure. Maybe Tuesday.

Patrick

[Duke W.]

Re: Duke

Park to Drive rev drop seems high. One issue could be the idle mixture - too lean. Go through the conventional (non-emission) idle mixture procedure. Do it in Drive and set the mixture for maximum vacuum, idle speed, and engine smoothness with the revs at 450 to 500; 475 is the reommended idle speed for '63 PGs in Drive and the '72 base engine is essentially the same with just a slight change in the valve timing and the larger displacement.

The manifold vacuum in Drive will tell you whether the B22 can would help.

Duke

[Patrick H.]

Re: Duke

Thanks.

I'll see if I can get it out tomorrow to check it.

Patrick