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Not enough vacume
Posted 04 July 2006 - 11:44 AM
Posted 04 July 2006 - 01:06 PM
Revving the engine won't tell you how much ported vacuum the disty sees, as it is too transient, and the engine isn't under any load. Not a good test.
As I recall, the Holley 5200 is a US version of a Weber, without any ability to tune it properly. I suspect it is running lean and causing the ping. Bigger jets if available, or perhaps the accelerator pump can be adjusted to give a bit more squirt, if it is only a transient thing that disappears as the flow in the carb gets established properly.
Start with the stock advance setting (8 deg. ? I am not sure, mine is SPFI), and play with the carb. You might just have to live with it. I have seen worse problems than a bit of ping, with the Holley 5200.
Posted 04 July 2006 - 06:03 PM
My Weber is OLD, no vacume barbs at all. so I ran the disty hose for advance to a barb on the manifold, hoping, just hoping it was ok.
It seems to run ok, but my valve rattle a bit too.
So no I see sombody saying it's bad to run advance vacume off manifold.
I'm pretty ignorant. What's the difference where you get vacume for the disty?
Posted 04 July 2006 - 06:24 PM
Posted 04 July 2006 - 06:31 PM
At idle, disty is running about 10 deg. advance.
Open up to accelerate, and the maniifold vacuum drops as the throttle opens. The disty still sees low vacuum and remains retarded, the engine is under heavy load where the advance is not needed, and all is well. THe only advance is due to increasing RPM. As the engine "catches up" with the throttle, and the load drops, the vacuum increases, and the advance does too, just like it supposed to.
At idle, the vacuum is very high, and the vacuum advance is maxed out, 10 deg. + vacuum advance, 20 deg. total maybe?
As the throttle opens, the vacuum drops as the load comes on, but the disty takes some time to roll off the vacuum advance, as the signal takes some time to propagate through the tubing and the canister takes some time to move the disty where it belongs. The engine will be running more advance than it needs for a second or 2 after hitting the throttle, and it will rattle until the disty gets to where it should be.
This isn't off-topic at all, as desertsubaru figures it works the other way round. So do lot of other folks. Education is always useful.
To try and get rid of the rattle, make the vacuum hose as short as possible. It might help a bit, as the vacuum can will respond quicker.
Posted 04 July 2006 - 09:38 PM
The hose is considerably longer than it needs to be, I will shorten regardless.
I remember being baffled with with what to do with the darn disty advance hose when I was first trying to install the weber, what with NO vacume barbs whatsoever. I take it this is somewhat unusual.
And the "ported barb" should be where exactly on the carb? And what makes it "ported"? I mean, is it just a hole in the carb somewhere?
Posted 05 July 2006 - 10:35 AM
I believe it is called "ported vacuum" because it comes from a port on the carb. Pretty obvous, maybe there is another reason for the name?
The port is usually between the primary venturi and the throttle plate. It is just a hole, with the connector on the outside.
Racing applications don't usually bother with vacuum advance. They run WOT all the time, so they don't run into situations where vacuum advance is of any use (no vacuum), and don't care about fuel economy anyway. They claim that the extra linkages etc. make the disty less stable, the timing less exact, esp. at high RPM, so they prefer to do without. So perhaps you have a carb intended for a track application.
If it pings for just a second as you get on the throttle, don't sweat it. If it pings contiinuously as you accelerate or climb a hill, then that is a problem, and probably a different issue, like too much static advance, or the wrong jetting.
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