Vacum gauge

Post by **mpg529d** » Wed Sep 10, 2003 6:24 pm

Hi All

I have got hold of a vac and oil gauge (riped from a metro), The question I have is is it possiable to fit the vac gauge in the line from the carb to the dizzy, I have looked and think that the presure (or lack of it) should be the same as at the manifold.

I awaite to be told I am wrong.

I have no problems with the oil gauge.

Thanks Tony

Post by **Kevin** » Wed Sep 10, 2003 7:13 pm

I have seen them fitted both ways but I dont know if one is preferable to the other or more accurate.

Post by **lowedb** » Wed Sep 10, 2003 8:09 pm

If it's a straightforward pipe (early cars it is) then you tee into anywhere in that pipe. The flow is so small (if any) there's virtually no pressure difference. Later engines have a 'blob' in the pipe. I don't know for sure what it is, but I expect it's a thermostatic device, which limits the advance until the engine is warm. I'm guessing that as it's bolted on top of the thermostat housing. You'd need to be on the manifold side of this 'blob'.

Post by **ianedwards25** » Wed Sep 10, 2003 9:21 pm

The Vac gauge you have is best taped into the inlet manifold as close to the carb as poss. to give an even vacuum you will also need to reduce the rubber hose to stop the gauge from flickering as was the problem when it was fited to the metro. As for fitting in the line from the carb to the dist it will read wrong as the pipe that goes to the dist comes from the base of the carb which is throtel dependent to work the advanc of the timing

Post by **lowedb** » Thu Sep 11, 2003 7:57 am

The whole of the manifold is throttle dependant. The air being sucked past the throttle which restricts it, gives the partial vaccum. The vaccum is used to adjust the timing as it's a way of determining the load condition of the engine. At wide throttle openings and low engine RPM, the vacuum is quite small and the load high, so the timing is retarded to reduce detonation. At small openings and higher speeds (like cruising) the timing is more advanced.
There are some carbs where there are more than one tapping. 1 is manifold side of the throttle blade, the other is between the throttle blade and the piston. I assume this is related to the difference betwen the throttle blade position (driver demand) and whats actually happening (piston opening), but I've never looked into it.

Post by **Kevin** » Thu Sep 11, 2003 12:49 pm

A guy a work who has a kit car has just fitted one to his (dolomite engine) it was a Tim guage and came with fittings to mbe fitted in either position and he used the carb to distributor line as it was easiest and said it works OK, but a local branch member has one fitted to the manifold and it did not come with the other option, so it appears the are different fittings, but which is best ?

Post by **Cam** » Thu Sep 11, 2003 2:08 pm

I have used both in the past and they both work, it's just whichever is more convenient for you.

But that is assuming normal carbs with one vac take-off (to the dizzy) and not the one between the throttle butterfly and the piston.

Post by **mpg529d** » Wed Sep 17, 2003 12:23 am

Thanks to one and all

Have not yet fitted vac gauge, still working on it.

Once again thanks