Jump to content

Vacuum Advance question


az_rat210

Recommended Posts

Weber 32/36 DGEV on an A15.  The car runs great using only mechanical advance but my MPG is suffering by about 4-5 mpg city.  With the vacuum advance connected to ported vacuum on the weber the car stutters and starts missing around 3500 rpm and up.  When I check vacuum on the port of the weber I am still pulling strong vacuum at 4k rpm.

 

 

Wondering if I might need to switch this over to a shared manifold/port vacuum setup or not.  Ideas/suggestions as I would prefer to keep getting 36 mpg city vs 29 mpg.

 

Note if I did have to pull manifold vacuum it would not be off the plenum as there is not port there. Rather I would have to pull it off towards the rear of the manifold where the AB Valve activator use to connect up.

Link to comment
  • Replies 19
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Vacuum advance is load dependent. The more the throttle is opened the less the vacuum. At full throttle under load the ported vacuum should be almost zero. No way to really test this unless you are riding along under the hood reading the vacuum gauge or have a line into the cab.

 

Always set your timing with the advance hose off and be sure idle is 800 or below to avoid any stray mechanical advance sneaking in.

Link to comment

When I check vacuum on the port of the weber I am still pulling strong vacuum at 4k rpm.

Sounds like the wrong port on the Weber. Although at 4000 rpm with throttle almost closed it should have a strong vacuum. Vacuum goes away at large throttle openings as Mike explained.

Link to comment

That is the port I have been using (the front one, the back one is EGR).  The car bucking at 3500 rpm up to 4k with the ported vacuum connected is weird.  As for being under load, at curb side revving to 4k the engine is still having to work (just not as much) so I should not have vacuum at that point should I?

Link to comment

I had to limit the amount of vac advance on my l20b to get it running right. Not familiar with the a series engines, but if it had a EGR valve that has been removed, there could be a chance that you getting too much vac advance.

 

On my L it was as simple as loosing the 2 backing plate screws in dizzy, removing vac advance screw and pull it out quite a bit to take up slack and until base plate rotates 15-20*, then push vac advance back in leaving a 3/16" gap or so, (the screw that hold vac advance to top plate will be against the grove in bottom plate) tighten backing plate, shove vac advance back into place and put screw back in. Simple.

 

Edit. Totally missed the part that said you were running EGR. My symptoms were from removing the EGR, but I suppose yours could be plugged...

Link to comment

I had to limit the amount of vac advance on my l20b to get it running right. Not familiar with the a series engines, but if it had a EGR valve that has been removed, there could be a chance that you getting too much vac advance.

 

On my L it was as simple as loosing the 2 backing plate screws in dizzy, removing vac advance screw and pull it 3/16" or so out, tighten backing plate, shove vac advance back into place and put screw back in. Simple.

 

Ah, I had been thinking that there might be a way to adjust the diaphram.  Is it adjust the distance the connecting rod  from the diaphram pulls?

Link to comment

That is the port I have been using (the front one, the back one is EGR).  The car bucking at 3500 rpm up to 4k with the ported vacuum connected is weird.  As for being under load, at curb side revving to 4k the engine is still having to work (just not as much) so I should not have vacuum at that point should I?

 

Disconnect the EGR first and see if that helps.

 

Vacuum advance is load related and nothing to do with RPMs. The more the throttle is opened the lower the vacuum .You can't check this at the curb because you can't put a load on it.

Link to comment

Ah, I had been thinking that there might be a way to adjust the diaphram. Is it adjust the distance the connecting rod from the diaphram pulls?

Yes. Doing this limits the amount of travel in vac advance. Like I said not familiar with your dizzy. On my L dizzy the screw that attaches vac advance to top plate extends into and butts up against the cutout in the base plate. This will change the static timing so you'll have to back your timing off a bit to get it back to spec.

Link to comment

If your plug wires are damaged, the engine can miss or lightly backfire at light loads with advance. Yet still run good at heavy throttle. Inspect them for burns on the insulation or oxidized ends, and measure the ohms of each cable. If most are 7,000 ohms while one is 400 ohms, replace the bad wire.

Link to comment

Had not thought about the diaphragm leaking as its only a year old, but still possible.  Thanks for the reminder on that.

If vac advance is leaking to the point where it does not move there would be no difference if it was connected or not. The tiny ported hole inside carb in not going to flow enough air to make a significant difference in A/F ratio

Link to comment

If your plug wires are damaged, the engine can miss or lightly backfire at light loads with advance. Yet still run good at heavy throttle. Inspect them for burns on the insulation or oxidized ends, and measure the ohms of each cable. If most are 7,000 ohms while one is 400 ohms, replace the bad wire.

Light loads require the least amount of energy to jump the plug gap... Heavier loads = higher cylinder pressures

 

And BTW when plug wires fail they typically have higher resistance or show open with ohmmeter. Not less!

Link to comment

Nevertheless, I have fixed many cars that way. I'm not as big on theory as what actually happens. Seen a lot of bad plug wires, the common graphite core wires fail in several ways.

 

I've never seen a bad advance cause symptoms at 3500-4000 rpm. Although at off-idle it can cause small changes.

Link to comment

Might also double check that rotor is pointed at a terminal when points are just opening, or if EI, when rotor and stator are aligned. If it's between terminals on the cap the extra vac advance could be enough to cause it to jump to another plug.

 

It's pretty common for the plastic bearing cage to break between top and bottom plates in dist. Top plate could be getting pulled off center and screwing up dwell?

 

When it's cutting out, is it at light or heavy throttle?

Link to comment

Nevertheless, I have fixed many cars that way. I'm not as big on theory as what actually happens. Seen a lot of bad plug wires, the common graphite core wires fail in several ways.

I've never seen a bad advance cause symptoms at 3500-4000 rpm. Although at off-idle it can cause small changes.

Right on. Just wanted to clear up any confusion. Wires typically do not get better continuity when they are bad. Carbon cores are usually in the 7-10k ohm per foot range....

 

Didn't want him looking for a 400 ohm wire as the bad one and thinking the ones reading OL are okay.

Link to comment

 The tiny ported hole inside carb in not going to flow enough air to make a significant difference in A/F ratio

 

 

The vacuum port is above the throttle plate so it has no effect on air/fuel ratios. As the throttle opens and swings up to the port an increasing amount of vacuum is felt by the distributor. As the throttle continues to open manifold vacuum has less and less effect.

 

Besides the vacuum advance in the distributor is sealed so there would be no flow of air from it into the carb.

Link to comment

Most 30+ year old Datsun vacuum advances are no longer sealed. I've taken a few part to see why and found the rubber cracked. A one-year old unit should be OK.

 

Idle a/f mixture effects from idle to above 1600 rpm (in a carburetor the idle circuit and cruising circuits overlap). It's adjusted with the idle mixture screw but vacuum leaks affect it too.

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.