72cutlass350rocket

Hi I am looking for some help on how to properly time and set up my carb on the 350 olds

do you set vacuum pressure for the carb before doing the timing?
timing is at 20 now with an hei dist putting me at an assumed 38 total
do I connect the vacuum gauge at manifold vacuum or on the carb?
and what is a good number I'm looking for?
what rpm should I be doing all of this at?
carb is a demon and seems to be running rich as it's idles Smokey and burns the eyes.... I have a dial timing light, vacuum gauge and a tach

Schurkey

What model-year engine? What compression ratio? What intake manifold? What camshaft? What vehicle?

Don't "assume" 38 degrees total timing. Measure it, and verify the RPM that it's "all in".

Vacuum gauge goes to manifold vacuum. Can be a port on the manifold, or a manifold-vacuum port on the carb--either one. Actual vacuum reading depends on a bunch of different factors, but in general--more is better.

Idle mixture and initial timing are done at curb idle. Worth trying with automatic in neutral, and then in gear, wheels blocked, park brake "on".

If the exhaust is eye-burning, look for signs of misfire. Even lean misfire will smell "rich".

72cutlass350rocket

1972 cutlass 350 with #5 heads and unknown cam but supposedly mild and edel intakes
I just played with the timing again I now have it set to 18 and will go see what it reads at full advance
vacuum is around 13

72cutlass350rocket

just checked and you were right dist advances 26° so timing is at 10. I just plugged in my vacuum revved it to 3000 rpm set the light to 36 and put the mark on 0 and tightened the dist and now timing is at 10

Schurkey

Is it DONE advancing at 3000 rpm? I wouldn't count on that. Some OEM springs don't end the advance curve until 4000+ rpm.

Some folks remove the springs and then test at 1500--2000 rpm, enough to get the weights to fling all the way out. Or use very weak aftermarket springs, which also lowers the rpm for full advance, but not as much as removing the springs. Or put the distributor on a distributor machine and test it off-the-car, like I do.

oldcutlass

Set your advance to 34 at what ever rpm it stops advancing with the vacuum can disconnected. Then note what the advance is at idle for your future tune up reference. Then adjust your carb at idle for the highest vacuum you see on the gauge while maintaining the same amount of turns from all the way seated. You may have to play with your idle speed to stay in the idle circuit.

72cutlass350rocket

thanks for the reply ... so to figure out how much the dist is going to advance I need to time it with it disconnected? How will the dist advance with out being connected? In my mind youd rev the engine past whatever rpm the dist stops advancing and time it to 34 to 38 then disconnect the vacuum and get a reading? Thanks again for the help

Fun71

Set the total mechanical advance with the vacuum advance canister disconnected. You want to set JUST the mechanical advance. To do that, as Schurkey said, remove the springs so the mechanical advance mechanism will go to maximum advance at 2000-ish RPM. After that you can re-install the springs and connect the vacuum advance.

FYI, vacuum advance works only at light to medium throttle and since the vacuum signal goes away at full throttle, it is out of the picture at full throttle operation.

Roqetman

While we’re on the topic of timing, I have a question and would appreciate some expert advice. I recently installed a rebuilt points type distributor and installed Pertronics electronic module. The GM timing specs for my ‘72 Cutlass with stock 350 indicate timing should be at 12 degrees BTDC at 1100 rpm. Obviously 1100 rpm is too high an idle speed to set initial timing as mechanical advance is in play at that speed. So... with the vacuum advance plugged, the engine idling at 650 rpm in Drive with AC on, what should the timing be idling at 650 rpm. Should it be at 0 degrees? Then as the rpms are increased to 1100 rpm it should be at 12 degrees per the spec’s, correct? If not, please explain. Thanks, Roqetman.

Fun71

The GM engineers took that into account for the published timing specs. If you want to know what the advance is at 650 RPM, then set the timing per spec then idle down to 650 and see what it is at that RPM.

Roqetman

Yes, that’s what I did and at 650 rpm it is 0 degrees TDC, at 1100 it’s at 12 BTDC. I just needed clarification as I thought there may be a need for it to be more than 0 at idle. On another note that may be helpful to others with timing and/or idle issues, I went back to the stock mechanical advance springs in the distributor which seem to be heavier than some of the aftermarket springs I’ve tried. With lighter springs the timing and idle was inconsistent and variable. The stock springs allowed me to dial it in and now it cranks Up and runs out great. Thanks!

droldsmorland

Heres is my method:
Verify ignition and fuel systems are in good shape and you have zero vacuum leaks.

Hook up a vacuum gauge to a direct manifold vac source.

Adjust timing and air fuel to obtain the highest possible vac reading at curb idle. Go back in forth between the two.

Now check the base timing and see where it is. This will be what the engine likes. I don’t use the 1100 rpm thing, never have.

What most mild Olds engines like on today’s fuel: 12-14* base timing at warm curb idle ~700rpm +/- 50. Then another 8-10* vac advance. Then the centrifugal for the total. Centrif can be customized for rate and total.

Use manifold, not ported vacuum for the vac can. I like vac advance canisters with the travel limit set to 8-10*. Use an adjustable travel, not adjustable rate canister. Plug in the can, now at idle, you should have ~20-22*.

Have the centrifugal brought all-in before 3000 RPMs for a total of 36-40* at 27-3000RPM.
You can use two different springs in the centrifugal advance to tune the rate at which it comes in. You'll be rewarded with burnouts and smooth acceleration.
A selection of weights, springs, and stops can be purchased if you have the patience to super tune. Or send the dist out to get it “curved”.

This is a good starting point for most Olds 330 350 400 425 & 455s in stock or mild cammed engines. At least this has been my experience over the last 35-40 years.

This spec applies to at or near sea level, 91-92-93 octane or better gas, 9.5 to11.0:1 comp ratio max. Adjust according to your ambient, octane conditions and driving habits.

Road test under load and readjust as needed.

Roqetman

Sounds good, thanks for the advice! I do have a question about using full manifold vacuum for vacuum advance. At idle, 650 rpm, the engine is showing ~18-20” of vacuum, timing 0 TDC. At 1100 rpm it’s showing 12 degrees BTDC. Help Me understand how Full manifold vac is better than ported vacuum? The car is Running great with the current set up and ported vac advance. At this point I’m not sure what the All-In timing is as my test tachometer only going to ~1800 rpm.

droldsmorland

Non ported vacuum gives you instant vac advance = better performance.

Bump the base timing to 10* at 650, (verify TDC and the damper and timing tab are accurate). Then try 12-14. See what it likes.

Plug the vac adv into a non ported source and read the timing again. What is it? You are looking for 20-22-24max.at 650 curb idle.

Verify the vac can holds vacuum and it moves the breaker plate smoothly.

18-20" of Hg is good. See if you can achieve 21-22Hg with small timing and A/F tweaks.

Ported vacuum delays the vac advance. If you have it plugged into the stock thermal switch on the intake(?) it will add or subtract the vac can depending on coolant temp.

Before you touch anything, take it for a ride with it hooked up both ways. Then report the road test results.
If you think it runs well on ported wait till you feel unported.
If it spark knocks you need to bump base timing down. Or get an adjustable. The adjustable allows more base and control over the total vacuum advance. = better burnouts from a stop.

Roqetman

Sounds good! I’ll give those adjustments a try And see how it performs. Thank you!

droldsmorland

No prob, welcome.
Once you get these specs in the engines sweet spot...then we can move you on to super tunning the carb.

Still have the OEM QuadraJet?
How fresh is it?

If it's original I can almost guarantee the secondary AVS clock spring needs to be tightened up a skosh It can lose its spring allowing the Secondary Air Valve (AVS) to open too soon causing a rich WOT tip-in bog...aka Quadra-bog.
This is how the Q jet received its bad rap.
Its actually the most refined OEM carb ever built...IMO. Today only GDI, Direct Fuel Injection is its rival...IMO

Several more HP and bl/ft of torque can be found simply dialing in what you have. As well as a mile or two /gal of gold.

Steve