Little ditty about manifold vacuum vs ported vacuum advance with carburetors and big cams.

Now we all know the difference between the two types of vacuum advance and I'm not going to add to those comments. What I'm talking about is a little trick used to get a carburetor big cam motor to idle without altering the carb's throttle body. Adding manifold vacuum advance will increase idle speed and even with a huge cam this trick will help get idle speed up. And, this trick is used frequently to speed up carburetor installs (even by pros). But (and there's always a but), while this does increase idle speed allowing you to close the throttle plates to improve transition throttle response, it carries with it a bad side effect (at least with big cams). And, you may not even be aware of it, you may have recently had a carburetor installed or purchased a vehicle set up this way.

The problem with using manifold vacuum to stabilize idle, on hard deceleration the motor will often die. This can be disastrous in some situations. The cause is simple, on deceleration the vacuum can not build fast enough to advance the timing to correct idle speed and the motor dies from insufficient throttle. And, if you have never in countered this it will most likely leave you scratching your head.

Fixing it is not difficult, it requires you do what should have been done in the first place, TUNE the carb and use ported vacuum advance. Yes, I know most of you will say what are you using vacuum advance for on a high performance motor anyway. Simple, on street motors it helps with cruising economy and cools the motor. And, most of us driving on the street with high performance motors are just wanting to enjoy the extra performance and have some fun. A little extra economy and a cooler running motor on the street is a Win, Win.

So why am I even talking about this, to save someone from wasting their time troubleshooting and possibly prevent someone from getting hurt by bad advice. I totally understand the advent of the original vacuum advance used manifold instead of ported source. And yes, on stock or mild motors it works well using manifold sourcing in most cases. Yes, this trick is posted on the internet on several sites, but without any caveats as to the problems that may result.

So, when using carburetors and large cams, well, don't waste your time with tricks, tune your carb and distributor properly, it only takes a little more time to do it right. And once done, you can enjoy Safely!