BIS vacuum advance

[Gadge]

Just a quick question concerning the advance mechanism on the BIS. For a good while I was running the vacuum pipe to the manifold by mistake. Not so long ago I learnt that it actually goes to the base of the carb so duly swapped everything around the way it should be but the other day I wanted to regap the points and set the timing so a static timed it and disconnected the vacuum - described in the manual - plugged it up with a screwdriver then stared the engine and set it with a strobe light. All was ok until I removed the screwdriver plug and noticed that there was no suction from the pipe? In fact the engine note never changed and it was only when I reved it up near the top end of the rev range that it did finally start to suck. Is this normal?

[Gadge]

Got a new issue now, I just tried to drive it and it won't tick over. If I Rev it up it sputters and then struggles to idle. The timing seems to be spot on so it looks like a fuelling issue perhaps a blocked jet but it can't be a coincidence that there's no vacuum at the pipe so it either needs a new carb or a strip down.......and all before the big festival Italia show too 

[Pete126]

The vacuum advance only operates when you have high manifold pressure I.E when you’re accelerating from start or putting your foot down when overtaking, easy way to check is blip the throttle when you’be got a timing light connected and watch the timing advance and retard, as the other issue make sure all the vacuum pipes are connected, check for air leaks around the inlet gasket

[Gadge]

Thanks Pete it doesn't move when up blip the throttle only when the revs go really high up. When it's idling it doesn't make a difference if the pipe is on or off. If you put your finger over the pipe hole there's no suction

[elanman99]

The vacuum advance only operates when you have high manifold pressure I.E when you’re accelerating from start or putting your foot down when overtaking, easy way to check is blip the throttle when you’be got a timing light connected and watch the timing advance and retard, as the other issue make sure all the vacuum pipes are connected, check for air leaks around the inlet gasket

I'm currently trying to solve a carburation problem and suspect a leak on the inlet manifold but your posting has me more confused than I was previously.

As I see it there is never (positive) pressure in the inlet manifold the highest it can be is ambient atmospheric about 14.7PSI pressure when the engine is not running. When the engine is running the inlet manifold is under vacuum conditions with highest vacuum at idle when the throttle plates are nearly closed.

Ian

[Pete126]

If you suck on the vacuum pipe you should see the base plate move and it should hold the vacuum, if it doesn't  the problem is in the carb area,

[Rusty's Uncle]

The vacuum advance only operates when you have high manifold pressure I.E when you’re accelerating from start or putting your foot down when overtaking, easy way to check is blip the throttle when you’be got a timing light connected and watch the timing advance and retard, as the other issue make sure all the vacuum pipes are connected, check for air leaks around the inlet gasket

I'm currently trying to solve a carburation problem and suspect a leak on the inlet manifold but your posting has me more confused than I was previously.

As I see it there is never (positive) pressure in the inlet manifold the highest it can be is ambient atmospheric about 14.7PSI pressure when the engine is not running. When the engine is running the inlet manifold is under vacuum conditions with highest vacuum at idle when the throttle plates are nearly closed.

Ian

No the vacuum should be minimal at idle unless you have a blocked air filter. The maximum vacuum comes when you bury the noise pedal and the engine is trying to gasp for more air/fuel mixture which is the principle function of the carb to mix and control supply. Vacuum gauges used to be very trendy once to help drivers drive with economy. Funny I was just looking at a Veglia one this morning.
On the air cooled cars we have just the centrifugal advance on the distributor but with the Bis you have the addition of the vacuum advance to take the advance a bit further 

[Pete126]

Ian, at idle the pressure in the venturi is equalized and there is no vacuum advance its only operates when the butterfly is open, the airflow velocity increases past the vacuum take off in the side of the venturi that creates the vacuum and in turn operates the diaphragm on the distributor   

[elanman99]

Not sure whether terminology is confusing the issue here but the vacuum value presented to the distributor on the Bis seems to be dependent on where the actual drilling/port is located. I am sure I am correct in saying that the maximum value of vacuum 'in the inlet manifold' is at idle. I remember when some cars had 'econometers' and thought they were plumbed in to the inlet manifold.

Google found this description of vacuum advance systems but I dont know how it applies to the bis.

One source for vacuum advance is a small opening located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. This is called a ported vacuum. The effect of having the opening here is that there is little or no vacuum at idle, hence little or no advance. Other vehicles use vacuum directly from the intake manifold. This provides full engine vacuum (and hence, full vacuum advance) at idle. Some vacuum advance units have two vacuum connections, one at each side of the actuator membrane, connected to both manifold vacuum and ported vacuum. These units will both advance and retard the ignition timing.

Ian

[Gadge]

If you suck on the vacuum pipe you should see the base plate move and it should hold the vacuum, if it doesn't  the problem is in the carb area,

I think you may well be right, I just tried sucking the pipe from the carb end and the vacuum mechanism worked perfectly.......I also got black on my lips  the way it runs suggests a weak mixture so I'm suspecting some sort of blockage in or around that vacuum port as it is close to the mixture screw so ths off with the carb tomorrow and I'll go from there

[Pete126]

Try the opposite blow down the vacuum pipe to the carb, see if it’s blocked

[Gadge]

Well I’ve been hard at it this week trying to sort out this issue ready for brands hatch and it’s been semi successful. So by blowing down the vacuum hoses I found there to be no blockages and the advance mechanism worked so I convinced myself that it was an electrical problem so I cleaned up the plugs, used different leads and used a different (but serviceable) distributor and managed to get the car started but it wouldn’t tick over and had a stutter on half throttle. Removing the vac tube did nothing to alter its running so I decided that the carb was duff. In the back of my mind I have thought this before as the carb isn’t in the best of shapes but luckily I purchased a good one off eBay a few months back so I swapped it over. Once I fiddled with the idle and mixture screw the car ran beautifully in fact I haven’t fully apricated just how bad the other carb was! It starts first turn of the key the choke works really well and the fast idle mechanism works (a first for me) and a road test was even better showing better touque in third and fourth and it didn’t bog down when the cooling fan kicked in BUT all is still not well. There is still a small stutter at half revs, at idle if I remove the vacuum advance pipe there is still no change and using my gas analyser it’s showing -0.7 O2 so is running weak but no mater how much I turn the screw it doesn’t move either way ?

So in some ways great news as it’s now ready for festival Italia but still not perfect

[Gadge]

Just to tidy up this thread I've finally sorted the problem out. Although the "new" carb was better, it seems as though it was blocked up with crud and the remedy came from a good old familiar name which is often looked on as a gimmick these days - REDEX. I've been using it for years on my other cars in the past (just because I'm old school  ) but last week I put a quarter of a bottle of this stuff in a full tank and blasted it to tramways in Derbyshire and back and when I did get back it was running very lean and crucially the misfire at high revs that I interpretated as both an issue with the vacuum advance or an electrical fault was all gone. Just ran it up now and tuned the carb up to a steady 1.5% +\- 0.3. I've always wanted to belive that REDEX wasn't just snake oil and now I'm sold! ........REDEX 1 - sceptical youth of today 0