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MG MGB Technical - 2 B's, 2 very different vac advance pressure level

Trying to find out why my GT doesn't have the 'torque' of my roadster so during troubleshooting I put a manifold guage on the vac lines of each car, they are both carb ported. I realize that vac doesn't add power but I wanted to check it anyway.
Both engines are rebuilds, GT .060 over, Rdstr .030 over, HS4 carbs.

The GT shows 13Hg at idle and 20 Hg at 2000 rpm. Too high?
The rdstr has maybe 1 Hg at idle and 5 Hg at 2000 rpm. Too low?
Can anyone make any sense out of this? Any experiences or suggestions?
Samuel Sullivan

I don't think you should have any vacuum at the carb port at idle. Normally the throttle plate is supposed to cover the hole at idle. Once you get off idle then you should see vacuum rise rapidly. Upon further opening the port vacuum should follow the manifold vacuum.

For some reason the GT the vacuum port is getting vacuum at idle. Maybe you have to open the throttle plate a lot to get proper idle speed uncovering the distributor port. Make sure the throttle plate is seating properly. Check to see there isn't something wrong with the vacuum tap in the carb. I assume nothing else is tied into the vacuum line to the distributor.

If you didn't plug the vacuum port when setting timing on the GT you are getting a lot of vacuum advance which means your base timing is way retarded. You'll really notice it when there is a load on the engine resulting in low vacuum.

That's why on manifold vacuum controlled distributors you have to plug the vacuum tap while timing. When the vacuum is hooked up the the timing advances a lot at idle meaning you may have to dial down the idle screws.
Robert McCoy

Samuel There are several reasons that the vacuum can be different on two cars supposedly the same. The condition and tune of the engine for one, tappets, timing etc. Also the accuracy of the vac port drilling, when they were new they varied a bit, a re bushed butterfly shaft a little off line or even a carb that a PO has drilled himself. With a ported vacuum the butterfly is supposed to just close off the vac signal at idle and a little wear can alter this. As the carb is opened up to higher revs the air squeezes between the butterfly and carb bore and creates a venturi effect. Its not as is said at times that it is then just subject to manifold vacuum, this happens at wider openings. You can see that small amounts of wear and inaccuracies can effect vacuum readings quite a bit, more so in a no load situation. I think you would find the readings a bit closer under load.
DENIS4

As Robert says there should be no vacuum from a carb port at idle. You don't give the years, but the USA changed from carb source to inlet manifold source in 1971, but some people have drilled the carb bodies to provide a carb port instead. It's possible this has been done but incorrectly, making it more like manifold than carb vacuum.

Nevertheless in itself it is irrelevant to your problem, as carb or manifold only makes a difference at idle and just off it, at any useful acceleration or cruising they are both the same. But again as Robert says there must be no vacuum advance applied when you are setting timing to the book specs.

Manifold vacuum at idle is typically around 15 in. Hg. (with carb vacuum at zero), so 13 is a bit low, but 20 at 2000 rpm is about right. OTOH only 5 at 2000 on the roadster is very low, maybe the vacuum capsule diaphragm is punctured and leaking. You can check this by removing the distributor cap and sucking hard on the pipe to the distributor. You should be able to move the points or trigger plate clockwise, and keep it there by sealing the vacuum in with the tip of your tongue. If you do that and the points plate gradually returns, or you can continually suck air from the the capsule, then it is leaking.

Whilst vacuum advance is primarily intended to improve cruising economy it does also make part-throttle acceleration a bit sharper.

But apart from all that there are many characteristics of engines, distributors and carbs as well as condition of these components and what they are set to that can give a very different driving experience.
Paul Hunt

Paul,
The measurements were taken directly from the port on both carbs so it's basically why is the vac amount so different between the 2 carbs. They are both factory ported as far as I can tell.
Samuel Sullivan

Like I said, a leaky vacuum diaphragm could cause it. But if you put the gauge on the port *in place of* the pipe to the distributor (you don't say), that would be eliminated. It may also be that one of the butterflies has been modified in some way, or the carbs were not originally intended for the MGB, I have read of some applications that have a different vacuum signal to the MGB. Because of the design of the SU it gets its 'constant vacuum' designation, which means the space between the butterfly and the piston is at a more or less constant level of vacuum through the range of carb operation, possibly getting a bit higher as the piston rises and compresses the spring as more vacuum is then required to lift the piston any further. This can be measured on the *breather* ports, if your carbs have them. It's possible that the roadster has its vacuum port connected to this space, rather than being under the butterfly in the throat. A thin wire poked up the port (don't use one so thick it gets stuck!) should reveal this. If it's in the correct place, then maybe the port is cracked or leaking.
Paul Hunt

This thread was discussed between 12/09/2008 and 13/09/2008

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