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MG MGB Technical - Weber question

I am using a Weber down draft due to heat tolerance. S.Us and Mikuni's have proven to be extremely heat sensitive with high under hood temperatures and alcohol laced fuel. The Weber is quite solid in high heat conditions.

I note that the vacuum port on the carb is located above the throttle plate, met below and at idle, demonstrates no vacuum at all. It is only slight on full acceleration.

Should the distributor be connected to the manifold vacuum port instead of using the carb connection? It appears that vacuum is minimal at the carb, so as not to retard ignition sufficiently on acceleration.

I am thinking that PCV might best be routed through the Weber port while the distributor might be most responsive to manifold vacuum.

Glenn Mallory

Glen
I take it that you have a DGV Weber
The vac advance hose goes on the little tube poking out the side , yes, it's above the throttle plate,
It will have 0 vac. at idle, vac at part throttle and 0 or very little at full throttle
If it's working properly you should be able to hold the engine at 1500 revs or a bit more and pull the hose on and off and you will hear the change in the engine sound as you do that
just be a bit carefull where you run the hose back over the manifold, keeping it away from being able to drop and get on the exhaust manifold

If I remember corectly your car has AC fitted, Do you have an idle bumpup to compensate for when the AC is on--If not, another little trick to play is to fit an elctrically operated ported vac. switch with the hose from the carb on one supply port and full manifold vac on the other supply port, then the demand hose to the dist.and wired into the compressor circuit--With it like this,it will run normal vac advance but when you turn the AC on the dist will advance up and give you a faster idle to compensate for the AC load

As far as the pcv goes it will depend on what model your car is and , what other breathers etc are fitted
William Revit

Broadly concur with that. My HSs are zero at idle, rise rapidly to a high level as the throttle starts to open, then fades away as the throttle is opened further.

Under any driving conditions i.e. except at idle the vacuum signal is the same as from an inlet manifold port.
paulh4

William,

I am in the process if adapting a vacuum switch to cut the compressor out on heavy acceleration. This is how I came to question the vacuum level at the port on the Weber at idle.

When the compressor kicks in as I am pulling away from a light, it throws cold water on the whole operation. Once set up to cut out when there is a low vacuum condition, it should allow the car to get out of its own way.

I have installed a large diameter crank pulley from a later car which has the compressor cycling nicely at idle.

The vacuum switch is for an automatic transmission torque converter lockout and is adjustable. in the provisional installation, it holds promise although I do not have it permanently plumbed and wired in. That is the task at hand on this sweltering day. I will connect it to the manifold port and leave the distributor connected to the Weber.
Glenn Mallory

I ended up switching the vacuum advance on the distributor to the manifold vacuum port, along with the vacuum sensor and a solenoid to slightly lean the mixture when the A/C compressor is running.

I had a PCV valve from a Japanese application plumbed into the manifold port and switched this to the carb vacuum port. The result was a richer mixture (as air is now drawn from the canister base, through the valve cover and across the lifter chest) and stronger vacuum to the distributor when the throttle is closed.

The PCV valve has worked well connected to the manifold port but it really should be in front of the throttle, drawing a vacuum at speed when blow by is greatest.

I will try it this way to see how oil consumption, idle and disconnecting the compressor at wide open throttle work out.
Glenn Mallory

Glen, I can see what you're up to, interesting project--have fun
The PCV should be ok on the manifold, that's where nearly all Japanese cars run them
William Revit

The PCV circuit didn't do well ported to the Weber. Oil blew past the oil stick.

Oh well. The PCV is reconnected to manifold vacuum.
Glenn Mallory

I'm confused (not difficult).

You talk about carb vacuum, distributor vacuum, Weber and PCV. Surely two completely different systems?

Whilst the Weber may well have a port for vacuum advance which varies the vacuum according to how wide the throttle is, as far as I'm aware only constant depression carbs like the SU and Zenith/Stromberg have a PCV port which gives a relatively constant low level of suction.

With the carb vacuum port it is the varying manifold depression that results in varying vacuum advance at the distributor, not flow past the port. Even though the port is immediately behind (or covered by) the butterfly when it is closed or at idle, as soon as it starts to open the port is effectively in front of the butterfly, and gives exactly the same signal as a manifold port.
paulh4

Paul,

It appears that the vacuum port on the Weber is not on the high vacuum side of the butterfly. There is virtually no suction at idle and it is comparatively weak throughout the range of throttle position.

I had problems with leaking seals so I adapted a PCV valve from a Japanese engine in order to have a stronger vacuum condition to draw air from the evaporative loss canister and crankcase than I had when I ported the crankcase to the bottom of the air filter plate (the provision that Weber gives for blow by). This was equivalent to early midget and MGA crankcase porting into the air filter cans, not directly into the S.U bodies. I believe that the earlier Bs had a road draft tube vented from the front tappet chest.

The leaking seal problem turned out to be a failed seal (hard as a rock) on the timing cover. I have had success porting the PCV with the Japanese valve to the manifold port (no oil consumption and proper evaporative loss evacuation) so I reconnected it.

I reconnected to the distributor advance to the Weber port but since this is on the atmosphere side and not the vacuum side of the throttle plate, I am thinking that the distributor advance should also be ported to the intake manifold to function properly (or at all).

I will experiment with this over the weekend.
Glenn Mallory

Early Bs had a hose from the rocker cover to the front air cleaner can, as well as the road-draught tube hanging down from the from tappet chest cover, the effect was marginal.

In 1964 they gained a positive system using a diaphragm type valve on the inlet manifold, which cause problems over time, then went to carb vacuum in 1968, which is virtually trouble free.

Can't speak for the Weber, but with the SU vacuum advance port there is no vacuum at idle, it rapidly rises to a maximum with a very small opening of the throttle which puts the port on the manifold i.e. high vacuum side of the butterfly, then progressively reduces as the throttle is opened further. The only difference in manifold and carb vacuum is at starting and idling, for all driving purposes they are the same http://www.mgb-stuff.org.uk/ignitiontext.htm#vac
paulh4

The weber vac advance port works exactly the same as an SU---but- it is a very small drilling in the housing ,only big enough for a vac advance supply
There wouldn't be enough flow for a pcv
Most pcv valves are designed to run on full manifold vac.
William Revit

Yes, I agree that the Weber port was too small to properly evacuate the crankcase. It works perfectly when ported to the manifold. There is no excessive oil consumption. I tried the Moss PCV valve kit, which uses a generic valve from a large U.S. engine application. The system sucked in too much oil and did not work properly. The Japanese PCV valve with the smaller vacuum line to the intake manifold is the trick.
Glenn Mallory

Paul. I believe that the porting on the carbs came in a little later than 1968, perhaps the early 70s. My 68 has the "mushroom valve" on the intake manifold. Such an item is rebuildable and, in my experience, bears looking at every couple of years to ensure the diaphragm is still in good condition.

For the Weber DGV system, I have always run the vacuum to the dizzy from the port at the bottom of the carb and have had my "pcv" take off attached to a port in the lower plate for the air cleaner. This carb was used in a number of applications and the old VW commonly had the factory carb changed our for this model Weber. Have run the line from the tappet cover to the air filter bottom plate on three cars with no problems.

Les
Les Bengtson

Les, I was going by Clausager who dates it to the 18GG/GH/GK engines for the 1969 model year that began in October 68.

As far as I'm aware it was only HIF carbs that had the vacuum port on the bottom because the butterfly opens the other way, HS4 had them on the top. Again from Clausager HIF weren't fitted to export models until the 18V in August 71, UK models didn't get them until the 1974 model year.
paulh4

This thread was discussed between 14/06/2020 and 22/06/2020

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