Timing / Idle Mixture / Vacuum question

[74 C-10 Shorty]

All "GM" units have a number on the arm(only way to tell how many degrees it pulls), adjustable units don't have a number because, well they're adjustable,lol. The rod is hooked to the diaphragm, by turning the little allen screw puts more or less tension on the diaphragm which extends or reduces travel of the rod and controls the amount of degrees it pulls, I like the adjustables because you change them a degree at a time if you want where the non adjustable I believe are 5*.

GM HEI distributors had a bunch of different vacuum advance curves depending on the year and vehicle.  77-78 light trucks (i.e. mine) had the least at 5 degrees @ 12-14 in Hg.  The most I know of in a stock application is 15 degrees.

That's mostly because of the emissions crap over the years, but the curve is in the mechanical not the vacuum advance it only pulls x amount of degrees after the mechanical is all in, when properly setup that is..

[74 C-10 Shorty]

Very interesting read, thanks guys!!  I bet the majority of poor mpg's and performance issues stems from mix-matching parts over the years.  I guess I have never really thought about it and was lucky the combo's I've had or have had the right "stuff" to make them run fine, pass emissions and get acceptable mpg's.  Folks can't understand that I have averaged 15 mpg on my 85 C20 with a 454.  All I can say is that it is untouched, original and that it just does...

I manged to squeeze 15 in my 86 dually running empty, got 12 with a camper and towing the race boat, now my 73 only got 13 but had a heck of a lot more power than the 86, couldn't complain nothing's free, lol..

[bswilson80]

seems that I may have spoken too soon. I don't have a permanent tach, so I threw on a temp one yesterday because after switching ports to full vacuum the truck sounded like it was idling too high. It went from about 850 in park (ported vac) to about 1200 (full manifold), and then I checked the timing advance, and in park at idle the advance was up around 55-60, this seems high?

That's normal, ported vacuum you have no vacuum at idle, full manifold you have vacuum at idle and it's pulling the vacuum advance at idle, vacuum advance units come in different amount of degrees they pull and they are marked how many degrees on the arm, I like the adjustable ones myself. Go back and read the article I posted it explains how this system works.

Distributor centrifugal advance sounds modified; the stock one is 20 degrees all in and you have 24.  That said, 14* BTDC should be OK.  You might back it off to 12* BTDC so that it is 36* BTDC when the centrifugal advance is all in.

A fellow in an adjacent thread found his carburetor step-up pistons and rods were gummed up, affecting performance.  Pull the rods in the Edelbrock and make sure the step-up pistons, the cylinders they sit in, and the rods are all clean.  They gum up within about a year of normal operation.

While you have the rods out, use a magnifying glass and a strong light to read the tiny numbers stamped on the shaft.  Should be something like "6852".

The distributor is just this one: http://www.amazon.com/gp/product/B005GRGOGG/ref=oh_details_o04_s00_i00?ie=UTF8&psc=1 . It's brand new, the only thing I changed was one of the springs for one lighter one as the all-in timing wasn't coming in until too late. Would this cause the extra degrees?

The metering rods are stamped 7547 and everything seems relatively clean in there.

I can't find the spec on the centrifugal advance on that distributor, but it has an adjustable vacuum advance canister.  Not sure about the vacuum advance arm.

7547 (.075" on cruise and .047" on power) sounds lean on cruise and rich on power to me.  Do you have the stock intake and exhaust manifolds, or is it an aftermarket intake manifold, or headers, or both?

I've tried adjusting the vacuum advance, it seems to be the same amount of advance regardless of if it is all the way closed, or 12 turns open, I plan to get a non adjustable one, would you suggest a particular one?

The intake seems to be stock, if it isn't it has been a long time since it had one (I've only had the truck 6 months, and the guy I bought it from inherited it, but not the knowledge of what was done to it.) as it is painted the same as the engine and you can tell that was painted awhile ago. The exhaust manifold seems stock as well.

[rich weyand]

What you are adjusting is the vacuum when it comes in, not the amount of advance.

I don't know the exact needles to use for stock manifolds, but 7547 seems off to me.  I would probably get a set of 7052's (a bit richer on cruise, a bit leaner on power) and see if performance and drivability improve.  http://www.jegs.com/i/Edelbrock/350/1452/10002/-1?parentProductId=

[bswilson80]

What you are adjusting is the vacuum when it comes in, not the amount of advance.

I don't know the exact needles to use for stock manifolds, but 7547 seems off to me.  I would probably get a set of 7052's (a bit richer on cruise, a bit leaner on power) and see if performance and drivability improve.  http://www.jegs.com/i/Edelbrock/350/1452/10002/-1?parentProductId=

I see. So is 55 degrees BTDC at idle too much in your opinion? Or is it fine and I should adjust the Tranny Vacuum modulator to shift earlier, as right now I probably get up to 3500-4000 rpm before it shifts to third, and its a really rough shift

Sounds like a plan, although I was looking towards rebuilding a Q-Jet to replace the Edelbrock, unless this is just a stupid idea... I'm eventually going for a very stripped down stock-ish look to the engine, and the Edelbrock is anything but that.

[rich weyand]

That sounds like a LOT of vacuum advance to me.  Like 40* worth!  Yowzah.  The stock disti on the 77-78 350 trucks was 5 degrees of vacuum advance, on top of maybe 16 degrees of base. 

Have you adjusted the TH350 cable properly?  That will set your shift points.  You can also set it a bit off the standard setting to move the shift points around.  Takes about 10 seconds.  http://www.novas.net/forums/showthread.php?t=9025

The Qjet has little passages and air bleeds that need to be set up properly for your induction, heads, exhaust, etc.  Great carburetor -- probably the best -- when it's set up properly.  This is not that easy to do, and I wouldn't attempt it without an air/fuel ratio meter.  There are some good books on how to do it (one by Roe, one by Ruggles), but it is involved.

The other option is to get a Qjet all adjusted to your setup, which is pretty common.  Sean Murphy Induction offers Qjets set up to spec -- there is no standard product off the shelf with them, you tell them what you are running for manifold, heads, exhaust, and they build it up for you from their copious notes on proper tunes for different setups.  Sean was previously with Jet, and has probably tuned the Qjet to more different setups than anyone else alive.  For about $50 more than a new off-the-shelf one you still have to tune, you can have a properly tuned one from Sean.  http://www.smicarburetor.com/products/sfID1/28/sfID2/9/sfID3/32/productID/18

[bswilson80]

That sounds like a LOT of vacuum advance to me.  Like 40* worth!  Yowzah.  The stock disti on the 77-78 350 trucks was 5 degrees of vacuum advance, on top of maybe 16 degrees of base. 

Have you adjusted the TH350 cable properly?  That will set your shift points.  You can also set it a bit off the standard setting to move the shift points around.  Takes about 10 seconds.  http://www.novas.net/forums/showthread.php?t=9025

The Qjet has little passages and air bleeds that need to be set up properly for your induction, heads, exhaust, etc.  Great carburetor -- probably the best -- when it's set up properly.  This is not that easy to do, and I wouldn't attempt it without an air/fuel ratio meter.  There are some good books on how to do it (one by Roe, one by Ruggles), but it is involved.

The other option is to get a Qjet all adjusted to your setup, which is pretty common.  Sean Murphy Induction offers Qjets set up to spec -- there is no standard product off the shelf with them, you tell them what you are running for manifold, heads, exhaust, and they build it up for you from their copious notes on proper tunes for different setups.  Sean was previously with Jet, and has probably tuned the Qjet to more different setups than anyone else alive.  For about $50 more than a new off-the-shelf one you still have to tune, you can have a properly tuned one from Sean.  http://www.smicarburetor.com/products/sfID1/28/sfID2/9/sfID3/32/productID/18

I'm guessing then, that the vacuum advance is broken or malfunctioning, guess I'm headed to NAPA. The reason I would lean towards the Vac Modulator would be that the truck shifted perfectly prior to me switching to full manifold vacuum advance, which leads me to believe that maybe it just isn't getting enough vac now.

I've got the book by Cliff, and trust me, I don't head into this rebuild lightly, I know it will take some serious time and learning, but that's why I got the truck, and if it doesn't work out, I know the Edelbrock will always be there to throw back on. When I have more fun-money, I will keep Sean in mind.

[74 C-10 Shorty]

I'm assuming you had the vacuum on full manifold when you adjusted the vacuum advance right, and at idle you still have the same degrees of advance no matter no matter how far you turned the adjuster?, if so the diaphragm is ruptured. Check the Modulator you may have an adjustable one like the vacuum advance, you adjust it the same way and it changes the shift points.

[bswilson80]

I'm assuming you had the vacuum on full manifold when you adjusted the vacuum advance right, and at idle you still have the same degrees of advance no matter no matter how far you turned the adjuster?, if so the diaphragm is ruptured. Check the Modulator you may have an adjustable one like the vacuum advance, you adjust it the same way and it changes the shift points.

Yes, it was on full manifold when adjusted, and I adjusted mech timing with it plugged, and mech timing was 14 degrees at idle, when I add the vac adv it would jump to 55 degrees at idle and not change. I'm going to buy non-adjustable, at least until I'm better at fine tuning.

I don't really want to mess with the Modulator just yet (it is adjustable) as it shifted perfectly before the vac adv (read: I) messed things up, I'll replace that and then see where my shifts are at.

[74 C-10 Shorty]

I'm assuming you had the vacuum on full manifold when you adjusted the vacuum advance right, and at idle you still have the same degrees of advance no matter no matter how far you turned the adjuster?, if so the diaphragm is ruptured. Check the Modulator you may have an adjustable one like the vacuum advance, you adjust it the same way and it changes the shift points.

Yes, it was on full manifold when adjusted, and I adjusted mech timing with it plugged, and mech timing was 14 degrees at idle, when I add the vac adv it would jump to 55 degrees at idle and not change. I'm going to buy non-adjustable, at least until I'm better at fine tuning.

I don't really want to mess with the Modulator just yet (it is adjustable) as it shifted perfectly before the vac adv (read: I) messed things up, I'll replace that and then see where my shifts are at.

That's right, only one change at a time.

[rich weyand]

And don't forget to adjust the transmission's throttle cable.  It's quick and free.

[74 C-10 Shorty]

And don't forget to adjust the transmission's throttle cable.  It's quick and free.

I believe all he has is a TH350??, that cable is just a kickdown and don't do anything for shifts.

[rich weyand]

If it's set wrong, it can screw up your shifts, by calling for a kickdown at part throttle.

[74 C-10 Shorty]

Only if you're in high gear, if you're in second it's already there and won't do anything in first, I've tried it before just to see what would happen..

[bswilson80]

If it's set wrong, it can screw up your shifts, by calling for a kickdown at part throttle.

actually, it MAY (I say may because I haven't driven it since, so I haven't really studied when it happens) be doing this, I'll check it just to be sure. I can't mess anything up doing this can I (if done correctly, obviously)?

and, yeah, its just a regular th350.