One Tooth Off/Vacuum Calculation?

[Hijinx]

Does anybody know if there is a way to check to see if I am one tooth off on my distributer? I have been monkeying around with the timing and carb for months now and have convinced myself of two things;

1. I need to drop down my jetting to get it to run leaner.
2. I may be a tooth off on the distributer

I say this because I have to run the timing advanced about 1/4 of an inch off of the marker scale to keep it from backfiring up through the carb.

Any thoughts on this?

Also, what is the calculation to figure out power valve vacuum value from manifold vacuum?

 

[cruzer99]

The first question is, have you replaced the cam or timing belt or gears recently? Having the dist off a tooth will just rotate the dist around to where it would be obviously in the wrong position relative to the firewall but not affect the timing mark. The timing mark is for cam to crank timing not dist to cam.......I think

 

[JohnZ]

Having the distributor a tooth off won't affect the position of the index mark on the balancer relative to the marks on the timing tab - it just means the distributor housing position will be incorrect, and the vacuum advance canister will probably hit either the intake on the inboard side or the spark plug wire support on the outboard side before you can set the timing properly. Also, if the distributor has been disassembled/rebuilt, check the position of the "dimple" on the lower section of the drive gear - it must face the same direction as the tip of the rotor, or you'll never be able to get the distributor body correctly oriented (this isn't in any of the shop manuals or the Vizard books, but it IS in the Delco service manuals - not many people are aware of it).

Several potential causes of your index mark being so far off the end of the timing tab:

1. Outer ring on the balancer has slipped - use a piston stop tool to verify that #1 TDC is actually #1 TDC on the balancer and the timing tab.

2. Mismatched balancer and timing cover/tab; they moved the index mark on the balancer 9 degrees counter-clockwise (looking at it from the front) in 1969, and the timing cover/tab position changed to match it; most people don't know this, and using a pre-1969 timing cover with a 1969-up balancer will show the index mark off the advance end of the timing tab. Again, verify TDC.

 

[Jack]

check the position of the "dimple" on the lower section of the drive gear - it must face the same direction as the tip of the rotor, or you'll never be able to get the distributor body correctly oriented (this isn't in any of the shop manuals or the Vizard books, but it IS in the Delco service manuals - not many people are aware of it).

Gear Dimple-to-Rotor Tip alignment ... didn't know that ... but have seen what must be result of not indexed. :BOW THANK YOU JOHN!
JACK:gap