Not enough vacume

[desertsubaru]

86 gl wagon Ea82 After doing a holley 5200 swap, I had to reset disty because of when i push it hard the valves rattle. Tried setting it at 8,10 and even 20% but still rattles.My car has only three vacume lines and i have the one for the disty hooked to the only barb on carb above throttle plate. When i take the line off the disty and rev the engine i get little pull on my finger.( dont have vac gauge) Could it be that my disty is not advancing enough and making valves rattle? Can i hook the disty vacume to the intake? with out all those electec valves.

[robm]

No. Ping or knock ("valves rattle") gets worse with more advance. Hooking it to the manifold might make it worse, but it won't hurt to try and see.

 

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.

[pyromanic]

No intent to hijack, but this thread got me thinking:

 

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?

 

Thanks,

 

Pyro

[desertsubaru]

Did find another barb. Didnt use it before because it sucked all the time. But under further test ( hooked it to disty) it works fine for advance. Carb is tuned as close as i can get it be by sound, idle and smell. Car runs great gets 33 mpg on highway and likes to cruse at 75 in fith gear. Set timing back to 8% and no ping. Thanks for the help.

 

 

 

 

[robm]

Ported vacuum barb sees vacuum only when the throttle is open. At closed throttle (idle) the disty will be running pretty close to static advance. Better for emissions, doesn't make much difference for economy or power. The advance curve is cut for the ported vacuum, which will be slightly different, and probably less than the manifold, especially when accelerating. That would account for the ping you hear.

 

Examples:

Ported vacuum

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.

 

Manifold Vacuum

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.

[pyromanic]

Thanks robm, for that great explanation. Any way to wind up with a ported barb on the this antique weber? But you seem to be implying that it very well may be all that important in my case.

 

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?

 

Thanks

 

Pyro

[robm]

I have no idea about modifying old carbs. If you had lots of guts (and a spare carb) you could just go at it with a drill...

 

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.