Vacuuming Tuning a Carb

[ChrisMinelli]

Hello,

 

I have a 52 Plymouth with the stock original flathead 6 and the Carter B&B carb.  I have been tuning it by ear, but I’d like to use a vacuum gauge.  
 

My shop manual says you can do it, but provides no details.  A secondary book I have suggests using the vacuum port to the windshield wipers, but I have electric wipers.  I don’t see any vacuum lines except the metal distributor advance line, and that seems not useful for finding manifold vacuum.  
 

is there a port on one of these things I’m missing?  Where do you hook up your vacuum gauges to?  I have a modern gauge I use on Holly carbs (the manifold line between carb and intake).  Do I need any special adapters?

 

Thanks in advance!

[Sniper]

There is a port in the intake on top of the cylinder 5/6 runner that is plugged on my intake.  I removed that plug and screwed in a barbed fitting to connect my vacuum gauge.

 

I do not have a good pic of it though.

[DonaldSmith]

My vacuum gauge is somewhat permanently attached to the cowl with magnets, between the hood hinge and the heater hoses.  That leads me to a naughty trick at car shows. 

 

I'll look under the hood of my car like a stranger gawking at the engine.  Reaching for the vacuum gauge, I'll say, "What's this?" and dislodge the vacuum gauge.  Juggling the gauge in my hands, I'll act flustered.  Then I'll tell the onlookers that it was OK, I'm the owner of the car.  

 

I know, I'm bad.

 

 

[ChrisMinelli]

Well I’ll be darned... I looked at my intake manifold and sure enough there is a port.  Is this what you are talking about?

 

Now how to get it open without breaking something....

[DonaldSmith]

Here is my setup, for the nefarious vacuum gauge and for the PCV system:  (The port is on the intake manifold, just below the carburetor.) 

 

[DonaldSmith]

Here's a slightly better photo of where the vacuum port is:  The vacuum gauge hose goes in place of the short hose with the end plug. 

 

[Sniper]

20 minutes ago, ChrisMinelli said:

Well I’ll be darned... I looked at my intake manifold and sure enough there is a port.  Is this what you are talking about?

 

Now how to get it open without breaking something....

 

 

You got it.  I find that slightly tightening before you loosen works well. 

[greg g]

The procedure using the vacuum gauge is usually used to set the timing. Once the gauge is hooked to the intake manifold fitting, loosen the dist so you can turn it by hand but leaving it snug enough that it hold it's place.  Start the car and set the idle speed to 450 /500 rpm. Now adjust the idle air screw to achieve the smoothest idle.  If the idle speed increases or decreases, readjust as need to aquire the idle speed again to 450 500.  Now watching the vacuum gauge rotate the distributor in small increments to achieve the highest steady reading. Should be apx 21 inches of Mercury. If you have multiple carbs or a non stock cam grind, these will cause a lower reading in the 16 to 19 inches range. If during this procedure the idle speed changes, adjust the idle speed screw as necessary to 450 500 rpm. Now test drive the car listening for ping or spark knock during hard acceleration or climbing a sharp grade in third gear. If it pings retard the timing till the ping disappears.  Return to shop readjust idle speed as necessary and snug down the distributor. At this point you might want to check the timing marks with a timing light to verify a reading for future reference.  If done correctly you should be in the TDC to plus 6 degrees advance.

[wagoneer]

The carter bb-1 has a port at the base that is a 3/8 NPT Thread connection. You can connect a plastic 3/8NPT with a barb that you can find at any local hardware store to connect it solidly to your vacuum gauge. You'll also want to connect a tachometer, dwell meter to your engine so that you can first set the correct idle, and then set the air/fuel mixture  based on the vacuum. See videos below on how to connect them.

 

Read up in the MTSC the background info and instructions, and how to interpret the vacuum readings and your valves/cylinders.

 

Story of the Carburetor http://www.mymopar.com/downloads/mtsc/010.pdf - page 22 

Interpreting the results on your readings can be found here - http://www.mymopar.com/downloads/mtsc/027.pdf - page 14

 

Steps:

 

1. Set the initial air/fuel mixture to all in minus 2 whole turns

 

     Online there is a bit of confusion about setting air/fuel mixture vs idle but the confusion stems from starting points. You should setup your initial air/fuel mixture by turning the air/fuel mixture all the way in (don't do it too hard, just till it's snug), then turn it back two turns. 

 

2. Set the idle per factory manual -- Chrysler C38 manual says 450 RPMs. 

 

  This may need to be higher depending on condition of your pistons/valves, and/or carburetor.  

 

3. Set the air/fuel mixture for maximum vacuum . 

  You’re looking for a solid steady vacuum, or if you can’t get solid then maximum. Swinging readings can mean valve, piston, timing, or carb issues, and a whole lot more fun time on the weekends.

 

     At 450, I get about 21 inches of vacuum, but at 600 RPMs I was getting as high as 24 inches of vacuum!

 

 

 

 

 

How to connect a 12-volt tachometer / dwell meter to a 6-volt positive ground system. Key is you have to halve the RPM reading to get the actual reading. 

 

 

 

I also like this video

 

and one from our own here on the forum Keith.

 

 

 

Also there is an interesting point about setting idle under load that Tony brings up, that is important for automatic/semi-automatic systems. To compensate for a problem with stalling when I stopped under load, I performed the above steps while the car is in High and with brake engaged (rear wheels elevated)!

 

Probably start with just at normal idle to start, and see if you have problems.

 

I later realized I may have a carb leak that I was compensating for.

Edited January 31, 2021 by wagoneer
Minor correction

[greg g]

Maybe we need some better pictures of the vacuum sources as there may be some confusion as to which vacuum signal is tapped into.  There is manifold vacuum highest at idle and steady speed cruise, drops as throttle plate opens under acceleration or load, then increases as throttle closes.  These taps are in various places on the body of the intake manifold. The other source is on the carburetor, but it is posted vacuum and is an opposite signal, no vacuum at idle, increases as throttle opens.  This is the signal used for vacuum advance usually connected directly from a fitting on the carb directly to the vacuum housing on the distributor. The vacuum gauge connected there will show how much you have and if it's sufficient to operate the vacuum advance normally.

 

In my experience with these cars and trucks going back to 1962 I have never seen the one shown that's under the carb, of the four  intakes I have including the Fenton dual one I am running, all have the port either open or plugged that is the tap for the windshield wipers. There are two that have castings in other places but none have been drilled or tapped.

All thst said for either setting your idle speed and mixture and or timing, manifold vacuum is what you want to hook into.

[James_Douglas]

"Also there is an interesting point about setting idle under load that Tony brings up, that is important for automatic/semi-automatic systems."

 

If the dashpot is correct...then setting under a load is not an issue.

 

On my car, when I changed from the the M5/M6 to three speed with OD while still using the fluid coupling...I went to a later Dodge carb that has a manual adjustment dashpot. A much better system than the electric one.  When I rebuilt the convertible that still has the M6 in it...I used the same dodge carb. The other things about that later carb is that it uses a heat pipe and not the Sission choke. (With respect to my fellow high school classmate Dave Sission who relative I think may have been the inventor).

 

James

[wagoneer]

18 hours ago, James_Douglas said:

"Also there is an interesting point about setting idle under load that Tony brings up, that is important for automatic/semi-automatic systems."

 

If the dashpot is correct...then setting under a load is not an issue.

 

On my car, when I changed from the the M5/M6 to three speed with OD while still using the fluid coupling...I went to a later Dodge carb that has a manual adjustment dashpot. A much better system than the electric one.  When I rebuilt the convertible that still has the M6 in it...I used the same dodge carb. The other things about that later carb is that it uses a heat pipe and not the Sission choke. (With respect to my fellow high school classmate Dave Sission who relative I think may have been the inventor).

 

James

 

 

the more I look at my carb the more I realize something is wrong with it, and it’s time for a rebuild. 
 

For mine, it seems the extra resistance  on the engine from  fluid coupling clutch half not spinning in the m5 transmission when brake engaged requires extra rpms that the carb is not responding to including dash pot and maybe the spark advance. I had to up my idle to 750 for it to not stall out. A few days later I found my carb wet from gas...
 

 

[wagoneer]

19 hours ago, greg g said:

Maybe we need some better pictures of the vacuum sources as there may be some confusion as to which vacuum signal is tapped into.  There is manifold vacuum highest at idle and steady speed cruise, drops as throttle plate opens under acceleration or load, then increases as throttle closes.  These taps are in various places on the body of the intake manifold. The other source is on the carburetor, but it is posted vacuum and is an opposite signal, no vacuum at idle, increases as throttle opens.  This is the signal used for vacuum advance usually connected directly from a fitting on the carb directly to the vacuum housing on the distributor. The vacuum gauge connected there will show how much you have and if it's sufficient to operate the vacuum advance normally.

 

In my experience with these cars and trucks going back to 1962 I have never seen the one shown that's under the carb, of the four  intakes I have including the Fenton dual one I am running, all have the port either open or plugged that is the tap for the windshield wipers. There are two that have castings in other places but none have been drilled or tapped.

All thst said for either setting your idle speed and mixture and or timing, manifold vacuum is what you want to hook into.

Here are the two on 40s models. The later ones have in the manifold itself as pointed out above.

An aside, 10 points for the contestant who can spot two strange “gifts” in the picture from the previous owner.

[Sniper]

Piece of flat stock bolted to the intake port for 5 and 6.  Spade terminal by the fuel inlet. 

[wagoneer]

4 hours ago, Sniper said:

Piece of flat stock bolted to the intake port for 5 and 6.  Spade terminal by the fuel inlet. 

1/4 credit. Those are close and related to one of the items:

1.  Manual Choke instead of the Sisson choke. 

     Somewhere along the way the Sesson must have died and been replaced with the Dodge manual choke. They installed those bars to hold the choke wires, and left the auto-choke electrical connection (The spade) hanging loose.

 

2. Blocked off, or removed manifold heat valve

 

     Look down at the bottom under the carburetor, and they have removed or blocked off (not sure yet which) the flap for the manifold heat valve. Just a square bolt.