3 questions on vac advance
Discussion
Hi
over from S series forum here because there's more RV8 discussion on here.
1) What is the usual vacuum pressure at idle at the nipple off the top of the plenum which the vac advance pipe normally attaches to? I can barely feel any suction at all with my thumb over the end of the nipple or the open end of the vac pipe when attached.psi if anyone has used a pressure gauge or just a general 'feel'method.
2) I had the vac advance unit off today and couldn't suck hard enough to make the arm pull in the slightest but read on here a sharp suck should be enough to move it. It seemed really stiff when pushing the arm down into the bench and it move on the spring about 1/4 inch til it hit the stop. Is this normal?
3) One way valve seems to be labelled back to front, ie "carb" labelled side wont suck but "dist" side will which is opposite to what you want?
Help appreciated.
Rob
over from S series forum here because there's more RV8 discussion on here.
1) What is the usual vacuum pressure at idle at the nipple off the top of the plenum which the vac advance pipe normally attaches to? I can barely feel any suction at all with my thumb over the end of the nipple or the open end of the vac pipe when attached.psi if anyone has used a pressure gauge or just a general 'feel'method.
2) I had the vac advance unit off today and couldn't suck hard enough to make the arm pull in the slightest but read on here a sharp suck should be enough to move it. It seemed really stiff when pushing the arm down into the bench and it move on the spring about 1/4 inch til it hit the stop. Is this normal?
3) One way valve seems to be labelled back to front, ie "carb" labelled side wont suck but "dist" side will which is opposite to what you want?
Help appreciated.
Rob
Dont think you will feel much suck on tick over because the hole is the wrong side of the throttle to see vacuum. Its only when the throttle is opened slightly that it will be introduced to some vacuum. Its only a tiny hole so always a good idea to check its not blocked.
My vac pipe works the same as yours and yes initially you suspect it is not working. I can however blow through it from the distributor end so i have assumed its not blocked. Upon researching this myself i have read that you shouldnt be able to just suck the vac advance on otherwise everytime the throttle is opened and vacuum introduced there would be a sudden spike in advance, which isnt wanted. Therefore to stop the sudden advance spike, it has that valve thingy which only allows advance through after a more prologed period of vacuum acting on it, ie when on cruise, which is when you want it for economy. This is how i understand it but await to be corrected.
If you do a search there are quite a few threads on the subject as many state they are unable to suck through the valve thing.
My vac pipe works the same as yours and yes initially you suspect it is not working. I can however blow through it from the distributor end so i have assumed its not blocked. Upon researching this myself i have read that you shouldnt be able to just suck the vac advance on otherwise everytime the throttle is opened and vacuum introduced there would be a sudden spike in advance, which isnt wanted. Therefore to stop the sudden advance spike, it has that valve thingy which only allows advance through after a more prologed period of vacuum acting on it, ie when on cruise, which is when you want it for economy. This is how i understand it but await to be corrected.
If you do a search there are quite a few threads on the subject as many state they are unable to suck through the valve thing.
SILICONEKID340HP said:
Why not remove the dizzy cap and give it a good suck , you should see the base plate move ..
Or suck on the end of the vac pipe leading to the dist and you will hear the idle rise noticeably while this is happening if you stick your tongue over the end the idle should stay high until you release your tongue if it don't then the vacuum capsule is probably knackered 
Hi chaps. Thanks for replies there.
Just to answer about the vac advance itself, I actually took it off the dizzy so was just sucking thru it with end free and still it wouldnt move. But it wasnt totally jammed cos pressing it Into the bench it moved but felt pretty stiff. So thats why I just wondering whether i should be able to do it. And the base plate span freely once vac unit was unhooked.
Cheers for clearing up where the vac hose was, ie upstream of throttle plate. Couldn't quite figure out for certain without taking whole plenum off.
Just to answer about the vac advance itself, I actually took it off the dizzy so was just sucking thru it with end free and still it wouldnt move. But it wasnt totally jammed cos pressing it Into the bench it moved but felt pretty stiff. So thats why I just wondering whether i should be able to do it. And the base plate span freely once vac unit was unhooked.
Cheers for clearing up where the vac hose was, ie upstream of throttle plate. Couldn't quite figure out for certain without taking whole plenum off.
Gassing Station | Chimaera | Top of Page | What's New | My Stuff