Another timing question

[enjenjo]

Not to bust you, but why don't you agree? Do you have proof to back you up? Everything there is fact, and can be duplicated with any engine.  And no it's not my article. In the recent past I have fixed several boilers at car shows just bt moving the vacuum hose without any other changes, and it was done right on the spot.

[PeterR]

To expand on Fat Cats thorough explanation of the reasoning behind retarded timing at idle on pre-cat engines, I shall quote from the Ford service manual from that era.

"The effect of retarding the ignition at idle is to reduce the power output of the engine.   This necessitates opening the throttle more to achieve a specific idle speed.   Since the volume of the residual exhaust gas left in the combustion chamber from the previous cycle only changes slightly with the wider throttle, but the volume of incoming air/fuel mixture is significantly increased, there is a greater proportion of combustible material in the combustion chamber.   The overall effect is to promote more complete combustion and reduce HC emissions."

In other words retard the engine so it barely runs at all, open the throttle to compensate, and even though it runs like crap the additional flushing action will lower emissions.

Further, anyone doubting that manifold vacuum allows an engine to run cooler should study the factory plumbing of a port vacuum engine.

Usually there will be a three-legged temperature sensitive valve in the vacuum line.  The valve has one connection to the port vacuum, one to manifold vacuum and the third to the advance canister.

At normal operating temperatures ported vacuum is routed to the distributor, but if the engine water temperature rises beyond a predetermined point, the valve directs manifold vacuum to the canister.     The increased vacuum advances the timing, the engine operates more efficiently, the idle speed increases and the engine temperature returns to a safe level.

What this all means is, stay away from ported vacuum unless you do not use cats and have to meet specific emission standards.

[Mikej]

Questions
Why would the vaccuum canister fill up with gas?
Why does an engine ping going from cruise to just off cruise? (light acceleration)
Could the the canister and line mantain enough vaccuum to keep the advance to high? ( not sensing quick enough) causing ping
Could the high vaccuum at cruise and a sudden WOT (heavy accl) cause the rich fuel mix to be sucked into the manifold vaccuum port?

[rooster]

It appears that un-regulated, continuous Vacuum, supplied through holly carb port best
fits my needs !

For a few day now I have been trying to track down the cause of what I call rough idle
and a hesitation when first giving it the gas. FatCats post  about  Vacuum advance 101
seems to have shed some light explaining why at idle my engine runs rough. What
bothers me is why did this start suddenly after fueling.

The maker of my dizzy requires his customers to supply information about the car's
engine , drive train, exhaust and its way to be used. With that information he selects what
Vacuum advance canister best applies.

It seems I told the maker BAD INFORMATION about my engine. It turns out  the
vacuum canister in the dist is NAPA VC1807.  This applies to engines configured for
stock to mild performance. I told the maker my engine was a stock 350 from a 94 pickup
which originally had TBI but now converted to 4 bll carb.I really don't know any history
of the engine other than that. I did a compression test on the engine today 180+ all
cylinders. My daily driver is a 94 GMC PU 350 TBI.  I did a test on 2 of its cylinders and
got 150.
I now think my engine may be modified and require a different canister.. He offers "For
engines with higher performance levels and lower intake manifold vacuum, the NAPA
VC1810 vacuum canister should be used as the replacement canister. The engine vacuum
level should be lower than 12 in/hg at idle to use this vacuum advance canister" . this
information seems to jive with the data in FatCats post.

I borrowed a vac gauge today from a friend , and I'm going to see where I stand with that
after I put all the ignition stuff back together to run the engine. If the engine has a reading
lower than 12 in/hg  Ill go with the VC1810. For 10 bucks how can ya go wrong!
I have not found any information in the paper work that came with my dizzy of where to
connect the vac can hose.

Thanks for the info!

[Mikej]

I switch my vaccuum from manifold to ported this afternoon. Adjust the idle screws and speed a bit. Ran pretty smooth. Ran it thru the gears. great. Went from cruise to light accel. (ping). Also as Frank said, motor ran hotter. Hotter it gets the more it pings. Normal stuff but I let it idle and come up to temp that my fan comes on at. It ran about 5 degrees hotter than normal. It did seem like much but after I shut it off the temp really started to climb. I left the fan run and It still stayed hot. So I was going to start it back to circulate the water alittle but it wouldn't turn over. The starter got to hot and its a mini. It never done that before. So far what Frank said is what happened.
I have run the Edelbrock on ported vaccuum without all this trouble but it was running a lot richer. ( 11 mpg) I haven't done anything to this new Holley yet. It is getting 13.5 mpg so I'm sure its alittle on the lean side.
 Needless to say I switched it back to the manifold vaccuum.  Also my car is on the very edge of being able to cool so any change can have a bad effect as shown.
 Fun afternoon.

[Fat Cat]

Just one question?  Who is the author?

Don't have that info. I pulled it out of an old post off the previous version of the RRT when I posted it there. I think I got that from a post on the old old RRT. Before I ran it. So I can't say for sure.

[Pope Downunder]

The Demon carb has two ports to hook the vac advance up to. One is ported vac. and the other is full time for testing. Which do I want to hook up for vac advance for the dist.
TOM

As they say: 'more than you ever wanted to know about vacuum advance'
http://www.chevytalk.org/threads/showflat.php?Cat=&Number=741951&page=1&view=collapsed&sb=5&o=31&fpart=1

Try putting an inverted 'U' in the vacuum line to the dizzy.  Fuel should not be draining down there, but that should stop it.

A lot of dizzies have very strong vacuum canisters.  This was most of the problem on mine; once we limited it to about 8 degrees, and optimised the mechanical advance it worked like a charm.

[PeterR]

Well, I could not resist reading the thread in the link above and of course the inevitable appeared. "Fact is that any engine will run better with the spark timing as far advanced as can without pinging."

I would have thought in this day and age that ancient belief would have been well and truly dispelled.