distributor vacuum line

[Zogbert]

I'm slowly fixing the pinging my truck has had since I got it a few weeks ago. '72 C10 with a 250 and a three-on-the-tree.

Timing was quite far off, and adjusting that made a big difference.

I bought a vacuum gauge, and I figured out that the rubber hoses that plug the vacuum lines were leaking. I'm not sure if the distributor vacuum line was hooked to the correct port. When I check the vacuum at the port it was hooked to, there is very little vacuum at idle (maybe 1# at 700 rpm).

It was hooked to the top port on the first picture. I've now plugged off those two ports. The bottom one of the two ports was plugged, but did leak. I've now hooked it to the port in the bottom pic, and the vacuum increased to 14#. This port was also plugged before, but leaked.

With the vacuum line hooked up, the rpm jumps to 1030, and the timing jumps from 4 to 14 degrees. Is that normal?

I've done searches to figure this out, and have looked at the service manual, but am still not sure.

[geezer#99]

The top port is called a ported vacuum source (above the throttle plate). Should show zero at normal idle near 700 rpm.
The bottom port is full time manifold vacuum (below the throttle plate).. Will show anywhere from 10 up to 20 depending on the tune of your motor, your timing, condition of your motor and to some extent your elevation.
When you hooked it up your idle increased which is normal. More timing (to a certain point) makes your motor run more efficiently.
Your timing increased because the vac pot does just that. Adds more timing by advancing the point plate in the distributor.
You can turn down your idle with the curb idle screw on your carb linkage on the side of your carb.
Once your idle is back to 700ish, them adjust the mixture screw on the base plate of the carb to obtain the highest vac reading. This screw is delicate. Don't turn it in tightly. Optimum setting should be about 2 turns out.
Your 4 degrees initial (base) timing can be increased to about 12 with no ill effects on your motor. Plug off the vacuum line from the carb to the vac pot when adjusting the timing. After you reset the timing, check the point gap. It can change when you play with the timing.

BTW welcome to the jungle!!
Do you have some more pics a little further away from your carb?

[Zogbert]

Thanks! Lots of great info there.

So, the distributor vacuum line is supposed to be on the full time manifold vacuum port for normal operation?

I think someone may have messed with the CEC solenoid set screw. No matter how much I turn out the idle stop solenoid screw, the rpm remains the same. Any thoughts on this?

I'll upload some pics in a day or two. Thanks for the friendly welcome!

[geezer#99]

You can use either. Depending what your motor likes. Stock motor usually manifold vacuum.
On your high idle, check your choke high idle isn't interferring. Might just need a little carb/choke cleaner sprayed on the linkage.

[Zogbert]

Thanks again!

[Jack_71C10]

I've always used the ported vacuum.
Some people say to use manifold vacuum which will give you full
advance at idle. I've never found a circumstance where full vacuum
at idle was beneficial.

[Zogbert]

I'll try both and see what happens.

[firedemon]

I used the vacume gauge and timing light on mine got it set great . I used the factory steel line that went to the ported spot on the carb . put the vac gauge on the manifold . set the timing to the factory 4 deg with the light I was getting like 10 inches vac so I adjusted the carb to 700 rpm and adjusted the airflow screw most I could get was around 16 inches vac . then I went back to the timing and bumped it up a little at a time till it got to 20 inches of vac . ended up being 12 deg of advance . runs perfect no pinging power seems way better than it ever did

[Zogbert]

Mine doesn't have a factory steel line that I'm aware of. Is it one of the ones that I have blocked off with the rubber lines? Are you running a stock 250?

[davepl]

Originally Posted by Zogbert

So, the distributor vacuum line is supposed to be on the full time manifold vacuum port for normal operation?

No, it should only be connected to ported vacuum, which is how it was designed. Some older vehicles had them connected to manifold vacuum but in those cases the distributors were set up to expect that - hooking up a ported vacuum distributor to manifold vacuum is incorrect.

People still do it, but it's usually to crutch for some other problem that just happens to like a lot of initial timing.

After all, set up correctly, they expect NO advance at idle, but if you hooked it up to manifold vacuum you'd have a ton of advance at idle, which would be wrong.

Since motors with vacuum leaks and other problems can be crutched to run better with more initial timing, you -will- find people doing it. It's just usually a sign something else is wrong.

No motor that shipped in one of our trucks, properly tuned, "likes" having the vacuum advance connected to manifold vacuum. They only like it if something else is wrong.

Now you could (and in the pre-emissions days they DID) hook it up to manifold vacuum if your distributor was set up to expect it. It's just that ours aren't, so you'd wind up with a butt-load of advance at idle, crappy throttle tip-in response, and so on. So it can be done, but doing it without "inviting the distributor to the party" and adjusting it's mechanical curve, would be a bad idea I think.

[Zogbert]

Thanks Dave. I haven't had a chance to change it back yet. I'll do some testing and update.

[davepl]

Welcome! BTW, I think the "fancy steel" vacuum line is made of steel so it doesn't melt or burn where it goes over the exhaust crossover. That's just a guess or assumption on my part though.

Anyone actually know for sure why it's metal? Or is my guess correct?

[firedemon]

I'm guess ing your right it gets hot there real hot and yes its stock 250 and it should be above the throttle plates

[geezer#99]

Originally Posted by davepl

No, it should only be connected to ported vacuum, which is how it was designed. Some older vehicles had them connected to manifold vacuum but in those cases the distributors were set up to expect that - hooking up a ported vacuum distributor to manifold vacuum is incorrect.

People still do it, but it's usually to crutch for some other problem that just happens to like a lot of initial timing.

After all, set up correctly, they expect NO advance at idle, but if you hooked it up to manifold vacuum you'd have a ton of advance at idle, which would be wrong.

Since motors with vacuum leaks and other problems can be crutched to run better with more initial timing, you -will- find people doing it. It's just usually a sign something else is wrong.

No motor that shipped in one of our trucks, properly tuned, "likes" having the vacuum advance connected to manifold vacuum. They only like it if something else is wrong.

Now you could (and in the pre-emissions days they DID) hook it up to manifold vacuum if your distributor was set up to expect it. It's just that ours aren't, so you'd wind up with a butt-load of advance at idle, crappy throttle tip-in response, and so on. So it can be done, but doing it without "inviting the distributor to the party" and adjusting it's mechanical curve, would be a bad idea I think.

Some of the stuff you come up with is just flat out hilarious!!

[Gmc.guy]

I just replaced the steel line on my 250. It's a 74. It connected to the top port of the two that point right at the valve cover. I couldn't find the right tubing anywhere so I used a pre-flared 3/16 brake line and just cut the ends off and bent it to fit. My original was almost rubbed through in a few spots so I replaced it when I did the 5/16 line from the pump to carb I think it was meant to be a more reliable way to plumb it without having a 4 foot hose flopping around
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[Gmc.guy]

I see your pic now. The top port of the two you have plugged off. The port in the bottom pic, is hooked to egr. The egr used to go through a vac switch in the top of the thermostat housing, then to this port. I deleted that switch about 15 years ago
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[demian5]

Disconnect that vacuum line and plug it off. You don't need it.
Make sure your weights in your distributor are moving freely
Adjust the idle so its at about 6-700 in park/neutral.
Set your timing to a total advance of about 34-38 (adjust for pinging).
Check timing at idle and note it.
Adjust idle with truck in drive so its smooth.
test drive the truck and adjust the timing to where it pings, then back it off to where it does not.
Enjoy driving your truck.

[davepl]

Originally Posted by geezer#99

Some of the stuff you come up with is just flat out hilarious!!

Glad you like it. If there are any special topics you'd like me to consider, just ask!

Disconnect that vacuum line and plug it off.

This works especially well in cars running Dominators, tunnel rams, and a spool.

[davepl]

There is one big caveat I need to add here, I think. When I say the system was designed to operate on ported vacuum, that's true, but there's a "why" in there that might be important.

Emissions controls started to raise their head about this time, and ported vacuum was in part an attempt at helping the A.I.R. injection systems to work by firing the idle spark really late (like 4 BTDC) so that it's still burning (or close to it) when it goes out the exhaust valve.

This has the side effect of causing a very hot idle (retarded timing increases heat), which I'm facing a problem with on my own, actually. It's also (in my opinion) hard on exhaust valves, though I can't prove it.

Hooking your vacuum advance up to manifold vacuum has one big benefit - cooler idle characteristics because you wind up with a lot more timing.

I know my engine would -like- to run at idle with a bunch more timing, but because the distributor advance curve isn't set up to expect it, I worry a bit about tip-in throttle up around 2500 where you've got most of your mechanical (let's say 30 degrees) plus a bunch of tip-in vacuum advance. That's just a lot of advance.

So I stand by my "the way GM designed it to run" comment, but keep in mind they were by now designing for emissions too, and in these early days of emissions control it often came with unwanted side effects.

Vacuum advance is needed for lean or diluted (high vacuum) charges because they take a lot longer to burn, so disabling it entirely doesn't make sense in a street car. But I wanted to soften my stance on the "never run it to manifold" because it's not that simple...

[Zogbert]

This seems to be a ping at higher RPM, even if not accelerating. It starts around 50mph (sorry, no tach in truck) and stays on to 60. It does seem to subside a bit around there.

[firedemon]

lets not forget these trucks were designed to run regular leaded gas that had better octane numbers . I know you should be able to eliminate the pinging with timing and carburation adjustments but usually pinging is on acceleration not a steady speed could you possibly be hearing the valve seats being theres no lead in your gas ? could you be hearing lifter noise these trucks have very little insulation and you can hear the motor much better I know my 250 seems at times to be loud in the lifters mostly when its warm out . it is loud enough that I have adjusted them 3 times .

[Zogbert]

Good info. Thanks. It may be that.

[crazy longhorn]

If you want to know about an ign curve ...play it /play it, then play again, till you find it@! Understand , that all of the 60's cars ran on full manifold vac,as we went into the 70's...they went ported on the vac can....this was for emmitions Go figure.....I don't feel like Gezzer is too damn far off....(set the curve, & let it run)! Longhorn

[RedRoad]

My experience has been that engines with large combustion volumes/chambers need more timing for efficient combustion. Tighter deck heights and smaller chambers which create higher compression ratios run more efficiently requiring less total advance.

Ive had builds that had low compression/large volumes run hotter than higher compression/tight quench engines. Sounds counter intuitive but as mentioned above when emissions came into play compromises had to be made.