pickup's vacuum line

[LoHo]

Fine...that's it for me, too!

[cywelchjr]

Ok, not just to revive a dead thread, but this weekend I decided to test this while checking/setting the timing on my 80G. So, I opened it up and hooked up the timing light, and then watched what happened and here is what I found (which I consider to be at least somewhat definitive.

At idle, the vacuum advance mechanism is adding 0 advance. As soon as the throttle starts to open the vacuum advance starts to add advance, blipping the throttle above idle causes the vacuum advance to back off momentarily and then advance again (of course I'm sure it stays backed off longer under load). If I let off the throttle all the way to idle the vacuum advance turns off right away. This matches how my old 67 chevy did, and how my daughters 66 mustang behaved as well. The port for the advance does not get any vacuum (at least not enough to move the advance) until you come up off idle, then the amount of advance is based on the load (the more load the less advance).

I think that looking at how it's actually behaving is a definitive check on this, and this works just like I was taught it was supposed to in auto shop in high school (yes, I took 1 year of basic auto shop and 2 years of advanced), I remember that with points systems the vacuum advance could change the dwell angle as well if things were not right, but that doesn't apply here. If you look closely at the carb and the vacuum port, at idle it's not really in either the vacuum portion of the carb or the non-vacuum portion, but rather in a transition zone. Just a slight movement of the butterfly though and it's fully into the vacuum zone, which is why you get no vacuum advance at idle, and if you do your having the throttle too open at idle, and that causes other problems as well (including uncontrolled high idle).

Not necessarily my last word, as I'm open to reasonable debate on the issue, but I would have to say if your getting vacuum advance at idle, you have something wrong.

[oseaghdha]

That's what it does

Vacuum advance is designed to operate under light throttle/cruise conditions.
It gives better gas mileage under those conditions.

Heavy load=low vacuum=centrifugal advance only.

[XSokieSPECIAL]

Ok, not just to revive a dead thread, but this weekend I decided to test this while checking/setting the timing on my 80G. So, I opened it up and hooked up the timing light, and then watched what happened and here is what I found (which I consider to be at least somewhat definitive.

At idle, the vacuum advance mechanism is adding 0 advance. As soon as the throttle starts to open the vacuum advance starts to add advance, blipping the throttle above idle causes the vacuum advance to back off momentarily and then advance again (of course I'm sure it stays backed off longer under load). If I let off the throttle all the way to idle the vacuum advance turns off right away. This matches how my old 67 chevy did, and how my daughters 66 mustang behaved as well. The port for the advance does not get any vacuum (at least not enough to move the advance) until you come up off idle, then the amount of advance is based on the load (the more load the less advance).

I think that looking at how it's actually behaving is a definitive check on this, and this works just like I was taught it was supposed to in auto shop in high school (yes, I took 1 year of basic auto shop and 2 years of advanced), I remember that with points systems the vacuum advance could change the dwell angle as well if things were not right, but that doesn't apply here. If you look closely at the carb and the vacuum port, at idle it's not really in either the vacuum portion of the carb or the non-vacuum portion, but rather in a transition zone. Just a slight movement of the butterfly though and it's fully into the vacuum zone, which is why you get no vacuum advance at idle, and if you do your having the throttle too open at idle, and that causes other problems as well (including uncontrolled high idle).

Not necessarily my last word, as I'm open to reasonable debate on the issue, but I would have to say if your getting vacuum advance at idle, you have something wrong.

Thank you cy,

That is exactly what I was TRYING to get across when this whole thing got started.

The MANIFOLD VACUUM is ported so that there is NO VACUUM at idle. When the throttle is moved off idle position the port is subject to MANIFOLD VACUUM and is transfered by VACUUM hose to the diaphragm causing the diaphragm to pull and turn the breaker plate in the opposite direction of engine rotation creating a spark ADVANCE condition.

Thus--- VACUUM + ADVANCE = VACUUM ADVANCE!

It's not freakin' rocket science. Huh?

Thanks again cy for actually checking this out ( as I did long ago ) and posting your results.

Oh yea! I wasn't going to say anything else on the subject.

UUUUUMMMM!!!! NEVER MIND

I hope Eddie got his problem taken care of.

[cywelchjr]

I recall someone in this thread saying to check it out, so I did!

But what I found was not what they were claiming was happening. But then that happens with the scientific method, come up with a theory, setup an experiment to test it, and either verify it or disprove it. In this case I proved your theory, which was what I was expecting as it matched what my shop teacher and mechanical experience told me would happen.

As they used to say on GI Joe, "now you know, and knowing is half the battle".

[randy]

Ok, so I'm breaking a promise, guilty! After much consideration of all the good info posted on this subject and a re-visit to the technical sections in all three of my manuals, I think I am more confused than I was when I first posted my theory.

According to the advance charts in the fiche and in the manuals, the vacuum advance mech is suppose to be at full advance with about a 6in of mercury vacuum applied to it. I think that's what got me into my line of thinking.

When I synch carbs, it's routine to see 15-20 in of mercury on the guages, so I don't understand how it's possible to have 15-20 in at the boot nipple and less than 6 in at the advance nipple. I'm NOT saying anyone is wrong or right, all I'm saying is that I don't understand it.

Now I'm no carb engineer, but I've been wrenching on engines since before I could see over a fender (Dad was an Olds Dealer mechanic) and it frustrates me that I don't get this

[dbeardslee]

Randy - I get a reading of 20-22 on mine at idle - but it's centimeters, not inches. Could the conversion be the problem? 6 inches is 15.24 CM. JAT

[cywelchjr]

Ok, so I'm breaking a promise, guilty! After much consideration of all the good info posted on this subject and a re-visit to the technical sections in all three of my manuals, I think I am more confused than I was when I first posted my theory.

According to the advance charts in the fiche and in the manuals, the vacuum advance mech is suppose to be at full advance with about a 6in of mercury vacuum applied to it. I think that's what got me into my line of thinking.

When I synch carbs, it's routine to see 15-20 in of mercury on the guages, so I don't understand how it's possible to have 15-20 in at the boot nipple and less than 6 in at the advance nipple. I'm NOT saying anyone is wrong or right, all I'm saying is that I don't understand it.

Now I'm no carb engineer, but I've been wrenching on engines since before I could see over a fender (Dad was an Olds Dealer mechanic) and it frustrates me that I don't get this

If you look at where the port is in the carb, at idle it's not really in or out of the vacuum area of the carb, and as such it's not getting significant vacuum at idle. As soon as the butterfly moves it's now clearly in the vacuum part of the carb, and as such participates in the vacuum just like the carb boot. Another way would be to put a vacuum gauge on it and read how much there is at different levels, as I'm a firm believer in observation of actual function to help learn.

[motoman]

Less confusion for you Randy............Those ports are vacuum ports.....highest inch vacuum at idle. The port used for the vacuum advance is port vacuum.....no vacuum till throttle plates are open.......and thanks XSokie, as I was beginning to think my many years of building and dialing in high performace and off-road set-ups, I was missing something!............NOT, and if your advancing the timing(distributor/ timimg plate) you are actually retarding firing time! Used to applying the basics to big high performing V-8's, and gotta remind myself that these scoot engines are no different.......X amount volume of air mixed with a "controled leak" X amount of fuel ignited at correct X time hopefully will make X amount of power at the correct time and will correctly scavenge(collectors/headers) so as to create a better inhaling of hopefully correct mixture of air/fuel for next cycle.

[XSokieSPECIAL]

and thanks XSokie, as I was beginning to think my many years of building and dialing in high performace and off-road set-ups, I was missing something!............NOT, and if your advancing the timing(distributor/ timimg plate) you are actually retarding firing time! Used to applying the basics to big high performing V-8's, and gotta remind myself that these scoot engines are no different.......X amount volume of air mixed with a "controled leak" X amount of fuel ignited at correct X time hopefully will make X amount of power at the correct time and will correctly scavenge(collectors/headers) so as to create a better inhaling of hopefully correct mixture of air/fuel for next cycle.

Your welcome------------------
[IMG][/IMG]

I------- think?