Hot Rods Now what???

Just did a complete tune-up on my '69 318. Rebuilt the carb, fired it up and it ran great. Good idle, took it out for a ride, no problems. Good acceleration, no hesitation, no pops or misses. Looking at it today and I noticed that I forgot to connect the vacuum advance hose. Hooked it up, nice smooth idle, but when I gave it some throttle it started popping, surging and missing like crazy! Take my foot off the gas and it smoothed right out to an idle. Disconnected the vacuum advance hose and it ran great. Didn't have any problems with the advance before the tune-up.

Any thoughts?

 

Me? Hell, I'd block the vac off and run her for a while more. But the logical conclusion is you're getting to much suction? Recheck your timing and advance........ or.... plug that hose and go, man! GO!

 

Your Distributor needs checked. Should be giving the proper enough vac per RPM. The vac will decrease as the RPM increases. Look for tables on the distributor vac specs for your 318 engine.
Fist thing id do is check to see if your distributor advance weight springs are missing or broken.

Dont know what year but here are some starters.

 

Tempting, but don't I need advance for higher rpm's?

 

A broken spring is what I am thinking. Tempted to change it out for an electronic distributor. I've gotten spoiled with my last few cars with HEI.

 

no vacuum advance helps at idle and part throttle for economy
I added a crane points eliminator kit to a sbc dizzy and it did the same thing, left it unhooked.
I am running a dual point with no vacuum advance right now but i just got a pertonix ignitor III and i'm going to run that

 

According to the mopar specs Ive read. All engines lose vacuum with higher RPM on manifold connections
Manifold and ported vacuum hook up behave differently.
Manifold full vac at idle. Ported no vac at idle.
Is your vac hose connected to manifold or carb below the venturi??
The vac decreases as you increase RPM on manifold connections
Your spring loaded weights play a part in this.
Heres a good article.
http://www.dodgecharger.com/forum/index.php?topic=14888.0

 

How initial much timing are you running? Is the distributor hooked up to ported or manifold vacuum?

 

It's connected to the carb below the venturi.

If it doesn't hurt to run without it, I may just leave it that way for now, and spend my money on something else I need. (Just ordered a mechanical fuel pump today.)

 

Sounds like it's over advancing, any chance you have the vacuum line hooked to ported vacuum? If you do, you might try hooking it to manifold vacuum and see what it does.

I always try to get my initial timing and total advance set where I want it, then play with vacuum advance. If it doesn't work out, I leave it unhooked.

You can get an adjustable vacuum advance unit to put on the distributor. Something else to play with.

 

My engine builder says vacuum advance is not needed. Leave it disconnected and go.

 

Roger that! As you can tell from my "signature", economy is not that important to me...

 

Yup......... "If it works...... don't fuck with it!" If that fails.... then ask for help.

 

On high compression engines, the vacuum advance can be used to allow for less advance while cranking the engine for easier starting.

I assume you have checked initial timing and how much advance you get as the engine speed changes. It's possible that the mechanical advance isn't working properly.

 

One thing, it could be that the vacuum advance diaphragm is defective and there's a big ole' vacuum leak.

Or, it's advanced way too far. Neither is good reason to ditch the whole shootin' match. Unless you're a drag racer running full throttle all the time, you want vacuum advance, not just for economy, though usually 15% or more. Distributors made for drag racing don't have vacuum advance because they spend all their time balls to the wall, so there's no point to it. For everybody else, it's a good deal and will help keep temperatures down among other things.

Vacuum advance is load dependent only, and independent from mechanical advance. Cruising down the highway at steady speeds from 30 to 70 takes very little power and consequently the fuel mixtures get very lean, and manifold vacuum very high, so the vacuum advance pulls in more timing when load is low and RPMs relatively low. Lean mixtures need the fire lit early, moreso than the mechanical advance will provide.

Usually the routine is to disconnect and plug vac advance while tuning the initial/mechanical part of the distributor. Once it's dialed in, THEN reconnect and tune vacuum advance. It will run cooler in hot weather and have better part throttle driveability.