timing by vacuum gauge problems

[48 New Yorker]

Having some difficulty setting my timing using a vacuum gauge on my 48 New Yorker 8 cyl…..and could use some guidance.  I’m trying to find the timing “sweet spot” rather than just rely on the book specs.

 

I’ve attached my vac gauge to a non-ported vac source at the carb base…(it usually reads about 17-18”)… then adjust my  idle as low as I can get it…about 350 rpm or so….to keep the mechanical advance from kicking in.   Did not disconnect vacuum advance because it is not supposed to activate at low RPM. 

 

Next, the procedure is to advance the timing until you get the highest rpm...just before it drops of.  The problem is the more I advance, the more RPM I get…until I can’t turn the distributor any further (about 12-15 degrees (which I know is way too far). 

 

What should happen, is that at some, point the RPM should decline when the timing is too far advanced…but it doesn’t.  I would expect the RPM “fall off” to show excessive advance, but that’s not happening….(FYI, points are set to specs and read about 28 degrees of dwell).

 

The manual calls for 2 degrees ATDC, but with modern fuels I’ve been told most people run about about 2-4 degrees advance;  I’m currently at 2 degrees BTDC based on my timing light.

 

Can anyone tell me what’s going on?

[DJ194950]

I believe that method is to achieve the highest vacuum not highest rpm's ??

 

DJ

[greg g]

If your engine is stock and in reasonable condition your vacuum at idle should be 19 to 21 inches.  If you are only pulling 18, there might be a slight leak some where. Is there a spot where you source manifold vacuum rather than carb ports.  I use my wipers motor port but the Chrysler probably has electric wipers.  But I'd there is a tap in the manifold see what your reading is there.

[Dodgeb4ya]

On all my 1946-50 straight eight cars I run 2-6 degrees advance.... no issues. I do not use a vacuum gauge for timing.

I like the engine to just barely make the slightest ping when hard accelerating from 30 MPG. Idle speed 5-600 RPM M5/M6 trans.

[48 New Yorker]

Thanks to all who responded.  Just to clarify, (I should have worded it differently), I'm pulling vacuum directly from the intake manifold; the fitting is in the manifold  under the base of the carb, not the carb itself.   

Re: vac @ 17-18 rather than 19-21,  I may have an intake leak...I've noticed some "oil moisture" around the bottoms of the intake manifold runners where they attach to the block.  I figured this was due to my adding a closed PCV system to minimize some blowby vapor at idle.  I've tried using a propane torch to find the leak... (unlit ) as well as soapy water....haven't found anything yet.

 

Again, what I'm trying to figure out is why my idle is not dropping (as it should) when I go beyond the timing "sweet spot"..... the RPMs just keep climbing with more advance....not what I expected to see or want to happen. 

 

I think I've got my timing dialed.... it runs quite well at 2• BTDC , for a total of 4 degrees advance over what the manual says (2•ATDC).  The odd results of the vac test have become more of a matter of curiosity as to why I'm not getting the results I expect with the vac gauge.  I'd like to be confident that I'm doing it right, so I can use this method in the future.  

[Tom Skinner]

I just have a 250.6 Six (C-38). So I may be off on some of the Numbers here however my Manual shows a C-39 (323 Eight) as follows:

1. Set Points at 18 Thousands

2. Check Dwell @ 27 -30

3. Start - set Timing 2-6 BTDC (Lean toward 6 because of modern Gas)

4. Use Vacume Gauge to set Carburator Mixture @ 19-21 Deg of Mercury (Or as high as you can get it)

5 Set RPM Last (usually in the 450 range at Idle for a six.

6. Use a decent Tach and Dwell with a RPM Gauge to set things up correctly

No run good - something wrong - Conduct tests - Compression Test Cylinders - Vacumm Leaks with Starter fluid around Intake Manifold Gaskets etc etc

[greg g]

Is it possible your throttle plate is not completely closed at idle?  Are you throttle plate pivot bushings worn?

[kencombs]

4 hours ago, 48 New Yorker said:

Thanks to all who responded.  Just to clarify, (I should have worded it differently), I'm pulling vacuum directly from the intake manifold; the fitting is in the manifold  under the base of the carb, not the carb itself.   

Re: vac @ 17-18 rather than 19-21,  I may have an intake leak...I've noticed some "oil moisture" around the bottoms of the intake manifold runners where they attach to the block.  I figured this was due to my adding a closed PCV system to minimize some blowby vapor at idle.  I've tried using a propane torch to find the leak... (unlit ) as well as soapy water....haven't found anything yet.

 

Again, what I'm trying to figure out is why my idle is not dropping (as it should) when I go beyond the timing "sweet spot"..... the RPMs just keep climbing with more advance....not what I expected to see or want to happen. 

 

I think I've got my timing dialed.... it runs quite well at 2• BTDC , for a total of 4 degrees advance over what the manual says (2•ATDC).  The odd results of the vac test have become more of a matter of curiosity as to why I'm not getting the results I expect with the vac gauge.  I'd like to be confident that I'm doing it right, so I can use this method in the future.  

Low vacuum, non - factory PCV.  Try blocking the line to the pcv and see what happens to vacuum.  It's very possible that the valve you chose is passing too much air at idle.