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MG MGB Technical - vacuum cannister

Got a question for the experts...The vacuum can on my '74 mgb dizzy (163-670/model 41491) aint what it used to be......and a new correct model is very expensive...and the car runs/idles perfectly...(I've rebuilt/replaced everything in sight) and before buying one...I was wondering should I bother, since I long ago removed all pollution controls (including soldering the carb (hif4) poppet valves)...or just leave well-enouth alone?...Not sure if this is good question...but I noticed the 41491 vac can has unique values very different than other years and concerned I am adding a component designed for conditions I removed...Oh well...thx for comments...


pete

Hi

The vac advance only affects the engine at part throttle conditions. it also operates differently depending on where the take off is situated, the best place being just after the throttle butterfly valve. It advances the ignition when it is subjected to a high vacuum leading to better mpg and emissions (theoretically! )

I may be wrong but I think your dizzy has the vac takeoff on the manifold rather than the carb and thats the reason for the differing values, as the vacuum is different depending on where you measure it.

If you have a leaking capsule and don't intend to replace good practice says remove the pipe and block up the hole. or you'll get erratic timing and small air leak

Simon
S S Tew

thx...excellent/salient point!...the vac is on the intake manifold...going to get new one asap...Running out of things to do on this car...just polished the dashpots to a chrome-like finish...
pete

I think its a good idea to replace the capsule as it provides ignition advance during light throttle conditions. This makes a significant improvement to fuel economy.
Iain MacKintosh

There is very little difference caused by whether the take-off is from the carb or the inlet manifold. North American engines, distributors etc. including the move from carb vacuum to manifold in 1971 changed very frequently as it was a continual fight to meet emissions limits. By contrast UK cars kept the same engine and distributor for the whole of rubber bumper production, even when the take-off moved in 1976. The only difference is that manifold vacuum gives lower emissions at idle.

Vacuum advance very definitely gives improved cruising economy and part-throttle acceleration, my wife noticed the difference when changing from a 5 degree back to the correct 10 degree even though I hadn't told her anything about it.

If the vacuum capsule is actually leaking this will significantly affect mixture through the throttle range - you will have to richen the mixture at idle to compensate, then at higher throttle openings it will be over-rich and you will be using even more fuel than simply not having a working vacuum advance. Suck on the end of the pipe coming from the capsule and if you can draw air through it is leaking. At the very least leave the pipe disconnected from the manifold or carb, and cap off the port on manifold or carb, and this will restore the mixture.

Paul Hunt 2

Thx to all "across the pond"...for your kind comments...I have a new capsule ordered...and will report back after installed...
pete

Well...well...a splendid improvement!!!...(and I felt all was good before)...the new capsule is working very well...and the old original was certainly shot...Although the HIF4's needed to be leaned out a bit...as I suspected and you all predicted...
It's much sharper throughout the throttle range now...I must confess I've had this car for 30 years...and I like tinkering with it more now than driving it, at times...and I'm leaving the driving up to my wife...Regards to all...
pete

This thread was discussed between 19/06/2007 and 27/06/2007

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