Hard starts when warmed up...

[76LongBox]

The flying donkey is pretty funny.... 

I've seen the timing checked before, just never done it myself, and I really didn't know what it SHOULD be set to (so thanks for that info).  I should have some time tomorrow to get it figured out.

[rich weyand]

That distributor has a tuneable vacuum advance.  It could be as high as 20 degrees of vacuum advance right out of the box.  So you might want to read what your timing is at idle with the vacuum advance connected first, then read it disconnected.  The difference will tell you what your vacuum advance adjustment currently is.  You want to be about 25* BTDC with the vacuum connected, so 10-12* base timing (vacuum disconnected) could be too much.

What you want your vacuum advance set to is a whole 'nother issue!  I would set it at 8 degrees of vacuum advance, and 17 degrees base timing, which with a stock 350 and regular pump gas should be a couple degrees short of predetonation while giving you the max advance with the throttle open, which is what you want.

[Irish_Alley]

rich isnt that for the manifold vacuum? i know you always preach that over ported (and i agree) but dont recall you saying it

[rich weyand]

Yep, vac advance should be connected to manifold vacuum, as it was exclusively in cars from its introduction by Studebaker in 1930 right through 1967.  In 1968, they moved vac advance to ported vacuum, a.k.a. timed vacuum, to increase exhaust manifold temperatures at idle to work with A.I.R. pumps.  This was to reduce smog at idle in LA traffic jams.

[76LongBox]

Can you educate me more on the timed/ported vacuum vs. manifold vacuum?  Right now, I have the vacuum advance run to the timed port on the carb - this is per the instruction video from Edelbrock as well as the MSD documentation. 

What are the advantages/disadvantages for running the vacuum advance on full manifold vacuum rather than the ported/timed vacuum?

I have no problem changing it if that's what I need to make the thing run right, but I'd just like more info.

I really appreciate you guys taking the time to help folks like me!!

[rich weyand]

Read this:
http://www.corvetteactioncenter.com/forums/c3-technical-performance/60830-ported-vs-manifold-source-vacuum-advance.html

Note Edlebrock diagram below.  Do you have an A.I.R. pump on your engine?  If not, use manifold vacuum.

[76LongBox]

Wow Rich....what great information.  I will definitely be changing over to manifold vacuum for the advance. 

I currently have the vacuum manifold port on the front of the carb hooked up to some large canister that is beside the radiator on the driver's side.  From that canister, a rubber hose runs to a hard line that goes across the truck under the fan shroud, and it looks like it turns back down the inside of the frame and runs toward the back - maybe to the gas tank... (I can't really get under the truck right now to trace it fully.)  Maybe this shouldn't be hooked up this way at all?  Please advise if you can....

The rear manifold port is running to my brake booster.

I do have a threaded port (currently plugged) in the manifold that is easily accessible as well.  It's up front beside the water inlet/thermostat.

And regardless of which port I need to use for the vacuum advance connection, I assume I would just need a rubber cap for the timed port on the carb, right?

Thanks again for such great info.

[rich weyand]

You can probably just swap the two lines.  The fuel vent will be fine on timed vacuum.

[76LongBox]

Thanks Rich....I'll give it all a try and report back.  Might be a day or two...and again, THANK YOU for the help!!

[76LongBox]

Ok guys....here goes....

I checked the timing (that really was easy), and without the vacuum advance hooked up (unhooked and plugged the line and the port on the carb), and initial timing was set at 4*.  So, I went ahead and bumped up the initial timing to 12* and locked the distributor back down.  Not fully understanding how all of this works, I went ahead and hooked up the vacuum advance to the ported/timed side of the carb (as it was before), checked the initial setting and it made no difference.

So, then I swapped the vacuum hoses as suggested above, and rechecked the timing with it hooked up to manifold vacuum.  BIG difference.  Initial timing now reads 30*, which if I read the info above correctly means that I have about 18* of vacuum advance.

While my distributor is "adjustable", it's apparently not as simple as it used to be on this model - now instead of using a 3/32" Allen wrench through the port, you have to take the distributor apart and put some "stop" in there turned a certain way to get it set to what you want.

That said, I'm not necessarily afraid to do that, but before I do (since it seems like a pain), I would like to know if that's really what I should do at this point.

Advice is MUCH appreciated.

[rich weyand]

Yup, your disti right now is set at 18 degrees of advance.  And with the centrifugal advance all in, right now at cruise you will be running 51 degrees of total advance, which is a touch high for my taste.

You can either 1) set the base timing at 7 degrees, so you get 25 degrees of advance with the vac all in and 46 degrees of advance with everything in, or 2) adjust the vacuum advance to a lower level (like 8 degrees) and set the base timing at 17 degrees, so you once again get 25 degrees of advance with the vac all in and 46 degrees with everything in.

I would prefer the 17 degrees of base timing -- performance will be better pulling out of the hole.  So why do they make distributors with so much advance? Because if you are running high-compression heads, pre-ignition (detonation, or knocking) will be a problem, so the higher vac advance allows you to go to a lower base timing and avoid pre-ignition when pulling out of the hole, and still get to target advance at cruise.  With the stock heads of that crate engine giving you about 8:1 compression, pre-ignition won't be a problem, even on 87 octane.

[76LongBox]

Awesome Rich...thanks!  I'll get that little "stop" installed and see what I can get it to do.  Hole shots have been pretty bad with it, so maybe this will fix that.

[76LongBox]

Ok....so I installed the stop at tab "C", which was supposed to be from 8-11*, and it resulted in 10*.  So, I set the timing to 15* + the 10* vacuum to result in 25* initial with the manifold vacuum hooked up.  I went for a test run, and it definitely has WAY better throttle response from a dead stop - no question - throttle response was better all the way through, but especially noticeable from a stop. 

However, the other issue I was having with the labored turn over when it's hot is now worse.  It didn't take many of the slow revolutions to get it started after sitting for a few minutes, but the revolutions were very labored.  I do have the new battery cables on the way, but wondering if that was to be expected by advancing the timing so much.  I guess ultimately I went from 4* to 25* initial timing, which I would think is a fairly significant swing, and with my limited knowledge, I wasn't sure what to expect for this part.

[rich weyand]

15+10 is perfect.  That'll also leave you a little more headroom against pre-ignition if you get a load of cheap, off-spec gas.

The advance makes no difference on cranking effort, except it should catch easier with the higher advance.

The throttle response is from more advance and having the vac advance on the correct vacuum port.

The cables or the starter are dying.  Hopefully it's the cables.

[76LongBox]

Once again, thanks Rich.  I put a new starter on when I did the engine/transmission swap around a year ago give or take a few days, so hopefully it's not it.  I have heard of people having trouble with them when they get too hot, but I did install one of the Mr. Gasket heat shields on it to protect it from the exhaust manifold.

Anyway, when the cables come in (hopefully early next week) I'll try to get them changed out and see if that helps.