• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Vacuum advance and idle speed

Share this page

wkilleffer

wkilleffer

Jedi Knight
Country flag
Offline
After doing several hours worth of work on my carbs, I finally had the mixture and idle speed at what seemed like acceptable levels.

I then replaced the air cleaners and reconnected the vacuum advance for the distributor. Trouble is, the idle speed went up 2 or 300 RPM. I could have adjusted the idle screws, but getting the carbs to breathe the same amount of air had been a tedious operation, and that might also throw off the mixture.

Is that normal, and should I re-do all that work with the vac advance hooked up? I'm willing, but not if it takes until after midnight /ubbthreads/images/graemlins/nonod.gif Is this a sign that the vac advance may be failing? Any suggestions would be appreciated.

Thank you,
 
What type air cleaners? They're restricting air flow to carbs
 
Hi Bill, I always set my carbs with the air cleaners on.

You can do this with this special tuning kit for SU carbs, sometimes offered on ebay for little money consisting of two hollow alum tubes and two bent wires.
 
[ QUOTE ]
What type air cleaners? They're restricting air flow to carbs

[/ QUOTE ]

They're the stock kind. The filters don't have many miles on them. Got the assemblies on ebay back when I was on an originality kick, now, I'd rather have something that was effective but easier to work around.
 
[ QUOTE ]
Hi Bill, I always set my carbs with the air cleaners on.

You can do this with this special tuning kit for SU carbs, sometimes offered on ebay for little money consisting of two hollow alum tubes and two bent wires.

[/ QUOTE ]

I was wishing I had one of those about midnight last night. They seem worth the money.
 
Hi Will,
I believe you have "ported" vacuum advance. This means that the vacuum connection to the advance is "outboard" of the butterfly. With the throttle blades closed or nearly closed at low idle, very little vacuum is applied to the advance. As it is opened, more vacuum is applied to the advance. Also the centrifugal advance starts to come into play at fairly low engine rpm.

What this all means is that after setting the idle, connecting the vacuum advance could well increase idle speed. It is not a problem. This will likely reduce the vacuum advance also - the two interact. Just back the carb idle settings off equally until you have the desired rpm. The mixture may need to be changed a flat or so for perfection at the lower rpm & vacuum setting, but very little. Actually, it's probably better to just set the idle mixture & speed with the vacuum connected in the first place, since this is the way the engine is normally run.

If the air cleaners were causing enough restriction to noticeably richen idle mixture, it would be much too rich at higher rpm.
D
 
[ QUOTE ]
What type air cleaners? They're restricting air flow to carbs

[/ QUOTE ]

Tony, would MGA air cleaners work on my B? They look smaller, sportier, and no less functional. A K&N element is also available. Looks like they'd be easier to work around.
 
[ QUOTE ]
Hi Will,
I believe you have "ported" vacuum advance. This means that the vacuum connection to the advance is "outboard" of the butterfly. With the throttle blades closed or nearly closed at low idle, very little vacuum is applied to the advance. As it is opened, more vacuum is applied to the advance. Also the centrifugal advance starts to come into play at fairly low engine rpm.

What this all means is that after setting the idle, connecting the vacuum advance could well increase idle speed. It is not a problem. This will likely reduce the vacuum advance also - the two interact. Just back the carb idle settings off equally until you have the desired rpm. The mixture may need to be changed a flat or so for perfection at the lower rpm & vacuum setting, but very little. Actually, it's probably better to just set the idle mixture & speed with the vacuum connected in the first place, since this is the way the engine is normally run.

If the air cleaners were causing enough restriction to noticeably richen idle mixture, it would be much too rich at higher rpm.
D

[/ QUOTE ]

The vac advance is hooked up to one of the nipples on the intake manifold. Not sure if that's completely "correct," but I haven't seen any way to connect it to the carbs or anywhere else.

I'm planning to readjust the valves, and then re-set the carbs with the vac advance plugged in. Doesn't seem like it will be any harder. Hopefully, I'll be done by midnight.

Thank you for the advice.
-Bill
 
[ QUOTE ]
[ QUOTE ]
The vac advance is hooked up to one of the nipples on the intake manifold. Not sure if that's completely "correct," but I haven't seen any way to connect it to the carbs or anywhere else.

I'm planning to readjust the valves, and then re-set the carbs with the vac advance plugged in. Doesn't seem like it will be any harder. Hopefully, I'll be done by midnight.
Thank you for the advice.
-Bill

[/ QUOTE ]
Hi Bill,
The distributors up to 1970 used ported vacuum. The later ones used manifold vacuum. Your connection to the manifold should be correct if you have the 72 distributor & vacuum advance. Sorry to get you sidetracked. Hopefully you will be done before midnight. Good luck,
D
 
I've always adjusted the jets and idle speed with the vacuum advance tubing connected. However, I do adjust the ignition timing with the tubing disconnected and plugged.
 
And you know what plugs it great, a golf tee.
 
Just a note on all of this:
Some of these MGBs do not actually have a "vacuum advance" unit (triggered from ported vacuum as Dave explained).
Your car, with it's vacuum pulled directly from the manifold (and not "ported") is probably supposed to have a "vacuum retard" distributor.
I have actually seen distributors that that have both a vacuum advance unit and a vacuum retard unit.
Since many of these cars are over 25 years old, it's possible to have a swapped distributor (and thus, the vacuum connections could be wrong).
Be sure that you have the correct distributor and that it is connected to the correct vacuum source.
Many folks set the mix on these carbs by using a screwdriver to lift the piston a bit while idling. This requires leaving the air filters off. You can also use the little button on the bottom of the carb to lift the piston(if your carbs has this feature), allowing you to leave the air filters on.
 
Back
Top