So I'll start this by saying I do not have pertinent information like we all prefer for a post like this.

1968 442
Original 400 - camshaft is unknown, but I have the edelbrock 455 intake and a 1406 carb. Original distributor, points and condenser are under a year old. Plugs are gapped at 35 and my firing order is correct.
M20 for what it's worth.

I've been chasing a low vacuum issue for a while. I'm at 5200 feet of elevation, so keep that in mind with the vacuum numbers. Started with 7 inches, and 18 degrees inital timing going off the original timing mark on the ballancer, because thats where vacuum peaked. Did a compression and leakdown test, and since the numbers were OK, I decided to redo the intake. I also verified TDC, using a piston stop on the compression stroke, then spinning by hand the opposite direction, which showed the mark was off 15 degrees. Put everything back together. Did not immediately fire up, but after some fiddling, got it to run.

Now I have 11 inches, but my timing is at 52 degrees dialing back to the original mark, and obviously further to the tdc mark on the tape.

I've reinstalled the distributor twice using both the timing tape and the original tdc mark, both times with the rotor pointing at cylinder 1. Same results for timing both times to get it happy.

Long story short, I'm hoping my timing light has gone bad but wanted to get the opinion of the masses. I'll be borrowing the neighbors light this evening to verify.

Thanks all!

 

Do you have the vacuum advance disconnected,and pluged?

 

Yep! I knew I'd miss a detail. Vacuum advance is still disconnected and plugged. PCV is connected and my vacuum gauge is on the manifold side.

 

hmmm I'm curious what you find out.

 

"the mark was off by 15 degrees" please explain. The balancer spun?

Is the timing light a variable advance type? If not don't see how it could be off.

Keep us up to date it's peaking the curiosity.

 

I’m confused.
If you followed the CSM, did you first establish DWELL @ 30 degrees at prescribed RPM? After you establish DWELL, you disconnect distributor vacuum advance
plug vacuum advance. It is only after you reconnect vacuum advance can you make adjustments to carburetor A/F idle mixture screws to achieve highest available vacuum readings - one A/F idle mixture screw at a time. Your description sounds confusing. Hope you re-established vacuum advance prior to A/F mixture screws adjustments. DWELL effects timing, timing does not effect DWELL. A/F mixture does not effect either DWELL or timing. The ORDER is explicit: DWELL>Timing>A/F Mixture.

 

^^^Yup^^^

 

I thought the ballancer had spun, but I redid the piston stop test and it turns out it's spot on. 10 degrees timing but back to 7 inches of vacuum at altitude. No vacuum leaks though and drives great!

 

Yes sir! Followed the service manual exactly as above and got stuck at 7 inches of vacuum. So then adjusted timing for highest possible vacuum and backed it off a touch to avoid pinging.

But as an update, I thought the ballancer had spun, so I redid the piston stop test and it turns out the mark on the ballancer is spot on. 10 degrees timing but back to 7 inches of vacuum at altitude. No vacuum leaks though and drives great! Just really low vacuum.

 

I would be looking at your carbuator for over fueling, The last person working on it may have screwed it up. My buddys 1967nImpala with a 327 had the same symptoms low vacuum ran so,so took the carb apart someone had done a hack and slash to it. put a differant carb on it vacuum went from 7hg to 18hg at idle with 18* of base timming.Im in Michigan 902' above sea level

 

Thanks for the info! I have struggled with this carb and it was an open box but unused. Idle mixture screws have no change except when they're too far in, the motor starts to die as expected. Pretty much 1 1/4 turn out and beyond does nothing.