Vac Advance Pipe
Discussion
fitted in the vac add pipe, plem to dist, on my hotwire 400 is an in line none return valve, its fitted so the dist CAN NOT pull advance !!!! should it be there as standard ?? if not, why would someone do that ?? I removed it last night and Rubin sounds like he likes it better with it off. Surely the dist needs to pull advance ?? if not why not just remove the pipe and fix the dist plate. Or am I missing something here?? 
Yes, it's correct. In fact it CAN pull advance, but the amount of vacuum is more than you can suck! 
The idea is to prevent advance coming on as the engine is revved; it has to see a certain amount of vac for an extended period (ie cruise - at least I think I have that the right way round)
The idea is to prevent advance coming on as the engine is revved; it has to see a certain amount of vac for an extended period (ie cruise - at least I think I have that the right way round)
badredwedge said:
fitted in the vac add pipe, plem to dist, on my hotwire 400 is an in line none return valve, its fitted so the dist CAN NOT pull advance !!!! should it be there as standard ?? if not, why would someone do that ?? I removed it last night and Rubin sounds like he likes it better with it off. Surely the dist needs to pull advance ?? if not why not just remove the pipe and fix the dist plate. Or am I missing something here??
Strictly speaking it is not a non-return valve, it is a vacuum delay valve. It should be very restrictive in one direction and flow freely in the other. The valve is supposed to be fitted so that if you disconnect both ends of the pipe and suck hard on the end that connects to the plenum, the pipe is very very restrictive. Suck on the 'distributor' end and it should flow freely.
The purpose of the valve is to stop the vacuum advance from coming in transiently when you open the throttle from idle. In practice, I think the valve is more trouble than it is worth because if it is blocked, jammed or fitted back to front the ignition timing will go to pot. Mine was removed years ago and I haven't noticed any problems.
Mine has the valve fitted, it flows the correct way, but I am not sure if the advance mechanism is working on the dizzy. Checked the timing with a strobe light, it was a long way out so I adjusted it to about 6* BTDC. When I revved the engine, the timing marks never moved. I thought they were supposed to. However, the pinking has stopped and I get a lot more pops & bangs out of the exhaust
Was fiddling with the timing again, trying to stop the awesome pops and bangs happening on overrun. Although the "valve" thingy on the vacumn pipe is the correct way round {its idiot proof, it's labelled which side where}I decided to take GV8's advice on older thread, Take it off!. As soon as I replaced the pipe the vac advance worked on the dizzy. So the valve was not working prpoerly. It is now in the bin. Havn't stopped all the pops&bangs but at least I know the advance mechanism is working now. So my advice would be, Take the valve off if in any doubt at all
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