[bascom123]

Community, I.m looking for some help with my W72 1977 TA.

What I know is this, Stock intake, and carb. Dizzy looks stock, but have not researched it (HEI) and motor was refreshed (lower bearings pistons). Unknown CAM with stock rockers. Runs great no smoke.

I have set initial timing to 18 degrees per the instructions with my fancy new timing light courtesy of Harbor freight (set to zero on dial). Disconnected and capped the vacuum advance line and let her warm up then set it to 18 degrees (mark on the near end of the indicator on the timing cover - towards ground).

I set the idle in drive to about 600 rpm and park about 750-800 hard to tell for sure. Did this by a small adjustment on the idle screw on the stock quadrajet.

It runs nice without the vacuum advance hooked up. however when I hook up the vacuum advance line to the the dizzy it starts ok and idles however I get a bouncing effect on my rpm's as i give it gas. Almost like its cutting in and out as the rpm goes up. If I disconnect and plug the vacuum advance I get smooth acceleration as I would expect.

I'm at a loss and could use some guidance from you Guru's.

Appreciated

[78w72]

where is the vac advance source? is it manifold or ported vac? or still using the stock TSVV or whatever its called?

[bascom123]

All the emission stuff is disconnected carb has a electric choke on it.

I tried at the rear of the carb on the "tree" and it was bad, tried on the front of the carb right side top and it was not as bad but still did it constant.

[77 Canamman]

[bascom123]

thanks 77 Canamman, mine is a 77 auto, I had it hooked up at the tree port as indicated in the top image.

[78w72]

if you dont have the original tvs valve... then a ported source (above throttle plates on carb body) is best to avoid bucking as you described. i experimented with ported & manifold vac on my 78w72 4speed which is more sensative to bucking & found that ported was best by far.

cliff who rebuilt the q-jet also suggested ported. once i used the ported vac i played with timing & idle mixture/carb idle settings & it runs great now with no signs of bucking or or bouncing effect you describe. you may want/need to back off a little from 18, especially if you dont know what carb it has.

[bascom123]

its the stock carb, i confirmed from the number on it side of it. The correct carb for the a 77 w72.

[78w72]

Originally Posted by bascom123

its the stock carb, i confirmed from the number on it side of it. The correct carb for the a 77 w72.

thats good, but the stock emissions stuff is gone so you need to play with tuning after finding the best vac source. ported should work best.

best fo luck!

[77 Canamman]

The original carb calibration was very lean. This will contribute to a very unhappy engine, especially when timing is thrown at it.

[78w72]

Originally Posted by 78w72

if you dont have the original tvs valve... then a ported source (above throttle plates on carb body) is best to avoid bucking as you described. i experimented with ported & manifold vac on my 78 w72 4speed which is more sensative to bucking & found that ported was best by far.

cliff who rebuilt the q-jet also suggested ported. once i used the ported vac i played with timing & idle mixture/carb idle settings & it runs great now with no signs of bucking or or bouncing effect you describe. you may want/need to back off a little from 18, especially if you dont know what carb it has.

sorry, meant to say since you dont know what CAM it has. & that a 4speed is more sensative than an auto trans as far as bucking or bouncing.

[allenga]

I have the same car and 18 was too much. Down to 12 initial.

[bascom123]

im going to play with it tonight figure out the amount of dizzy mechanical advance I have using my dialed timing light. finger crossed i find a sweet spot.

[74Grandville]

Original carb calibration is configured with egr in mind. If you disconnect the egr, you need to fatten the calibration or you’ll be way to lean.


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[pippintook]

On my 78 w72 4speed quadrajet carb (# on carb is 17058263), I am pretty sure the drivers side upper port is ported vacuum . I run this source for my distributor vacuum advance.

I seem to recall the factory hei vacuum can was 25 degrees. It had 25 stamped on it - I think I still have the original one around somewhere. I have been using an older crane adjustable vacuum advance with my hei.

Maybe let us know your quadrajet carb number? If you can post a pic of front of
77 carb and what ports you have. I can get pic of front of 78 #17058263 and compare.

[bascom123]

yes pippintook the driver side upper port is ported. I confirmed that yesterday with a vacuum guage. What I have learned so far is this:

I set my initial timing to 12 and repeated my tests and had the same issue. No matter what I set the timing to it bucks when i add vacuum advance around 1750 rpm ish.

So I measured and determined that I have around 23 degrees of mechanical advance but it peaks way to early like 1700-2000 rpm so i think the initial, plus the mechanical and vacuum are tipping the scales and giving me way to much advance all at one time. If i don't use the ported and use manifold the engine dies (too much advance).

Next up I have a Crane vacuum advance limiter and a set of weights and springs coming. I plan to play with the springs/weights to delay mechanical advance to peak around 2.8-3.0K rpm and then see if i need to limit vacuum advance. I'm hoping that all I need is to delay mechanical advance but worst case I can limit vacuum. I opted for the limiter as my can is brand new.

I can honestly say i understand this system much better now but it is a PITA to get right. Once its right i don't expect any issues.

I confirmed it is a 17057266 carb (1977 TA 400 W72 auto).




More details next week when parts arrive


I think this will make it such that they don't overlap and compound the advance.

[allenga]

Originally Posted by bascom123

yes pippintook the driver side upper port is ported. I confirmed that yesterday with a vacuum guage. What I have learned so far is this:

I set my initial timing to 12 and repeated my tests and had the same issue. No matter what I set the timing to it bucks when i add vacuum advance around 1750 rpm ish.

So I measured and determined that I have around 23 degrees of mechanical advance but it peaks way to early like 1700-2000 rpm so i think the initial, plus the mechanical and vacuum are tipping the scales and giving me way to much advance all at one time. If i don't use the ported and use manifold the engine dies (too much advance).

Next up I have a Crane vacuum advance limiter and a set of weights and springs coming. I plan to play with the springs/weights to delay mechanical advance to peak around 2.8-3.0K rpm and then see if i need to limit vacuum advance. I'm hoping that all I need is to delay mechanical advance but worst case I can limit vacuum. I opted for the limiter as my can is brand new.

I can honestly say i understand this system much better now but it is a PITA to get right. Once its right i don't expect any issues.

I confirmed it is a 17057266 carb (1977 TA 400 W72 auto).




More details next week when parts arrive


I think this will make it such that they don't overlap and compound the advance.

It is a PITA. I am putting in a custom curved DUI distributor today.

[bascom123]

Great read btw:

HEI Tuning Secrets

[pippintook]

If you can, I would try and get some stock hei weights and springs. A lot of the aftermarket weights and springs are not the best. Cliff R. might say they are crap - Cliff usually tells it like it is...

Some of the aftermarket springs are too light and bring mech advance in too quick. The stock factory springs are pretty heavy duty and dont do this.

Factory weights are usually advised by Cliff too...I have a bunch of hei factory weights and springs. Some I scooped in wrecking yards...some I got on ebay from old hei "parts/broken" distributors. Again, it seems like there can be issues with aftermarket hei weights and how they bring in the mech advance.

For many combos, Cliff usually advises to run around 12 initial mech advance and say around 22? centrifugal mech advance - for around 34-36? total mechanical advance. And then add in say 10/12 ported vacuum advance.
Though, it is advised to tune what runs best for your car. You may need different numbers for your combo.

[bascom123]

it was a hot sticky weekend here in west central Florida. I did however get the distributor just about dialed in.

First off i was setting up to install the Crane vacuum limiter and after having done that it would not start. So I double checked everything and that's when I noticed one of the wires of the pickup coil was broken. I got a replacement at Autozone and installed it which meant I was taking the dizzy fully apart. While apart i cleaned it up well removing the 40 years of cruddy lube on the plates and springs and I then re-assembled it. I ensured the vacuum advance was set (limited it to about 6 degrees). I turned the key and she fired right up.

I reset initial timing to 12 degrees, measured total timing to about 35 degrees. Added the vacuum advance to the equation and on acceleration there was no more bucking. I did notice around 2500 rps when popping the gas peddle to hear the engine roar that I was getting some backfiring coming from the back of the car. So i moved the initial timing back to 10 degrees and the backfiring was much improved.

Took it for a test run and it pulled like a freight train.

Didn't mess with the vacuum limiter, I just left it at 6 but I'm debating bumping it up till the bucking returns then backing it off maybe next weekend.