89 305 TBI/carb swap edelbrock manifold/1406 carb. Mallory SF 8362 distributor. Bypassed computer can't get idle to smooth out. Question does the vacuum advance need to be on port or manifold vacuum. Thanks

 

Yes it does. Absolutely.

There is no "universal" "right" choice between the 2. Pick the one that makes YOUR engine run the best. As judged by some combination of throttle response, running temperature, gas mileage, detonation/pinging, power, and so forth, based on what's important to YOU, which only YOU can judge, since it's YOUR car.

 

All the Carb manufactures say this Holley, Quickfuel, Edelbrock, etc.

The Full vacuum port is for power brakes, smog pump, etc.

The Timed vacuum port is for spark advance

see page 6

https://ac17cb7e3cc3506f0d23-7386afb...0319-2rev2.pdf

there is a reason for this, because at WOT you don't need any additional vacuum advance to the distributor because on a carb setup total mechanical timing is dictated by the distributor.. there is no ECM to control timing.

Need to make sure float setting and idle mixture are correct before you can get the idle to "smooth out" with the idle speed screw. and make sure you have no vacuum leaks.

but before we get ahead of ourselves what is your base and total timing set at without vacuum advance?

 

At WOT there IS NO vacuum advance regardless, no matter where you hook it, because there IS NO vacuum. A total non-issue. WOT has NOTHING WHATSOEVER to do with vac adv.

The reason the carb mfrs tell you to use the "timed" vacuum port instead of hard manifold vacuum, is because of emissions. "Ported" vac adv was one of the very earliest emissions hacks as I'm sure you will recall like the rest of us WHO WERE THERE at the time, even before PCV. All it does, is eliminate vac adv at idle, thus reducing NOx emissions under those circumstances. Pre-emissions, there WAS NO such thing as "ported" vacuum; vac adv was connected straight to the manifold.

The CORRECT advice remains the same: hook it to whichever one give YOU the results YOU like best, for YOUR car the way YOU drive it on the fuel YOU can buy.

The questions re. static and total advance however, are VERY valid. Vac adv is the very last of the timing settings to make. Your "total" timing (static plus centrifugal) should be around 36°, and is completely independent (almost) of everything about the motor; it's determined by the fuel. That's how much gasoline needs. If you think of "total" timing as the "base" quantity, i.e. that everything else is compared to, it is the correct timing for max power output etc. at the engine's peak HP RPM. Then, if the RPM is lower than that, the engine needs somewhat less timing as RPM decreases (the centrifugal component); and as load decreases, it needs MORE timing (the vacuum component). Of course the dist doesn't physically work that way, so you have to work it backwards, starting with "static" and adding the correct amount of the other 2; but for understanding what's going on, it helps to think of it that other way.

 

How ironic, this was in the mail last night.
Take the time to read it as it offers some additional insight.

My advise is to plug the vacuum ports on the carb and distributor and tune the carb/engine without any vacuum advance. This will make sure the initial and total mechanical advance is correct.

You may have to tune the distributor accordingly to get this right.

simply put; low compression engines need alot of initial timing.

Then play with the vacuum advance if you want.

 

the ported vs. full manifold vacuum has been discussed many, many times.

the best advise i got was, "try it both ways and see what you like. it only takes a minute to switch the lines around."

full manifold has made my car run 10 degrees cooler and idles a lil smoother.

i do agree about tuning everything else before worrying about ported or manifold vacuum.

 

You will be amazed that by just advancing the distributor (initial timing) a few degrees can greatly improve the idle quality.

 

Thanks it took several attempt but I finally got it

 

so what was the issue?

 

Timing was off just enough that I had to reset distributor to keep vacuum advance housing from hitting the firewall. Finally got carb set right.