Wondering if I have an issue. 70 SX Conv with 455 and QJet. I eliminated TCS solenoid but all else is stock. Dstributer is a rebuilt NAPA unit with Pertronix conversion. Vacuum for distributer goes to port on carb which is how I saw it in assembly manual. I think some vacuum sources are ported and only come on at certain rpm versus manifold vacuum.

Anyway.....I set timing with vacuum disconnected to 8 btdc. I reconnected vacuum and it still reads 8. Shouldn't it read different? Maybe I have wrong vacuum source or vacuum advance on distributer is bad?

 

If you're connecting and disconnecting the vacuum to ported vacuum at idle, there shouldn't be a big change. Ported vacuum is the small one on the driver's side of the front of the Qjet. This won't make vacuum before you're off the idle circuit, which is advantageous.

 

.. Or not, depending on other factors, such as your cam - use whichever vacuum source makes your car run better.

What happened to the original distributor?

- Eric

 

I have original distributer but shaft bushings were worn and vacuum advance broken. Eventually I want to get it rebuilt and set up for my engine but the rebuilt one was a quick out to get the car going as I was 10 years into the resto and wanted to be done.

Does it sound like the carb port is the factory location for distributer vacuum on thus car?

 

Sounds like the right port to me - about an inch above the manifold, right?

Remember, that replacement distributor probably has different mechanical and vacuum characteristics than the original did, so other settings and connections may have to change.

- Eric

 

If there is not a big difference in idle between hooking and unhooking thats the wrong port.
Hook it to one that is almost touching the intake and it should really idle up then you can adjust the idle down.

 

This is not correct.

The '70 SX came from the factory with the vacuum advance connected to ported vacuum (the higher barb on the carburetor).
Ported vacuum is not active at idle, so you should notice no difference in engine speed at idle, whether it is connected or not, if it is working properly.

As I mentioned above, the decision whether to run ported or direct vacuum is yours, and depends on what (if any) modifications have been made to your engine, and how well it runs with each setup. Either way may be better for you, but it is definitely NOT true that if you do not see a change in how the engine runs, you have it connected wrong.



IF he wants to run his advance on direct vacuum rather than ported vacuum.

- Eric

 

Most engines in this era had the vacuum advance hooked to ported vacuum. People have been arguing over running it on ported vs manifold for years, we have many threads on it. Some engines like it some don't. As Eric stated, it's a personal preference depending on your engine, there is no right or wrong way.

When you installed the Pertronix unit, did you bypass the resistor wire?

 

Well sure it is that why you immediately changed to full vacuum at idle for better performance!

Find a port that is pulling vacuum at idle hook the advance up to it and idle it down.
Or hook to ported vacuum and constantly fight bad idle and rising water temps from a surging engine and hard starting.

 

Oh goody. We're having the "Ported / Non-Ported Vacuum Advance" argument again.

My car is running its advance on direct vacuum because it works best with my setup.
I know because I tried both.
Your mileage may vary.
There is no absolute correct answer, only what works best for you.

- Eric

 

No were not because I couldn't care less what someone else does after I give advice.
My cars I have had ran horrible if they didn't have full vacuum and I was a non believer for years until I tried it and got it all lined out within minutes!

 

Amen to that

"Find a port that is pulling vacuum at idle hook the advance up to it and idle it down.
Or hook to ported vacuum and constantly fight bad idle and rising water temps from a surging engine and hard starting."
=================
DOUBLE AMEN to that
high compression 403 last year
went to mechanical HEI, not computerized.
Overheated extremely and immediately
poor performance
would not idle down right
advance enough to run right then it kicks back and won't start easily.

Changed to manifold vacuum, set idle and everything back to proper.

Easy start
superb throttle response
NO overheating, using the 307's radiator.

This was my experience.
Whatever works for your car.

 

on a 1970-72 455, what should the vacuum range be?
Joe

 

That depends entirely on your cam.

- Eric

 

what should be the timing be set on a 70 olds 455?

Specifications Intake
RPM Range: 1800 to 6000 Valve Lash: Hyd.
Valve Timing: 0.006 Duration: 274
Lobe Separation: 110° Duration @ .050" Lift: 230
Intake Centerline: 106° Valve Lift: 0.52
Lobe Lift: 0.325
Valve Timing @ 0.006 Lift:

 

I would set it @34-36 deg btdc @3-3500 rpm with vacuum advance disconnected and plugged. Then raise the rpm and make sure you have no further advance at higher rpms. The initial timing at idle will be what it will be. Then you need to double check with your vacuum advance connected to ensure you don't exceed 50 deg btdc at the same rpm's.

 

The problem with bypassing the TCS and hooking the distributor directly to PORTED vacuum is that you no longer have the benefit of the TVS that switches to MANIFOLD vacuum if the car starts to overheat at idle. Personally, I'd use straight manifold vacuum, but if the car does run better with ported, at least get a TVS in place as a safety measure.

 

thanks for the info. ill give it a try.