Finding the Correct Vacuum Balance

[projectmidnight74]

Morning Gents. Working on tuning this SBC 350 I have. Has a mild cam, nothing crazy. Motor has less than 10o miles. We have to get the fuel pressure down and find the correct timing. But now, I am more concerned with the correct vacuum balance. I have a gauge I got from harbor freight. From my research, with this Edelbrock carb I am running, the guru's say to keep it steady in the green. Well, after about 30 minutes I found that magic green area, but I feel like idle is too high. When I adjusted the idling level, it dropped to the late timing section of the gauge. So, I danced with that until I said **** it. Any help would be great. I gave it a drive, and it drives fine, just rich on fuel and I think around 1100 rpm at idle. HELP!

 

[fast 99]

With a "mild" cam vacuum might be lower. If you're from Mississippi doubt altitude has anything to do with it. Motor should be in the 16-18 range possibly higher. Cam overlap has a direct effect on vacuum.

I wouldn't trust a HF gauge too much unless verified with a known good gauge.

Being rich should be fixed right away. Depends on how rich but that alone could kill the motor.

 

[Ricko1966]

Okay a few things here check your fuel pressure,Edelbrocks don't like too much fuel pressure. What year truck are you working on? And what fuel pump are you running? Are you running a return line. Adjusting the carbureator does not change your timing.Too fast of an idle will change your timing. It's ignition first then carbureation when you are setting things up. Have you set your timing? Post back these answers and we will try to uncover the mystery.

 

[75gmck25]

I have had bad luck with diagnostic equipment bought at Harbor Freight. The electronic meters have been unreliable and their return policy for electronics means you pay a restocking fee even if the meter is bad. I gave up after two faulty dial-back timing lights, and bought an Innova instead.

Timing is very important, so I suggest you check your timing before you start using a vacuum gauge for tuning. Base timing should be in the range of 4-12 degrees BTDC at about 650 rpm, with the vacuum advance line disconnected and plugged. When you then rev the engine up to about 3000 rpm you should see about 18-20 degrees more timing (max of about 30-32 degrees). Using the Mityvac pump to pull vacuum on the vacuum advance will add 18-20 degrees more timing. Just check to see that it works.

Then buy or borrow a MityVac or other common vacuum gauge and get an actual vacuum reading at idle. A gauge with only red, yellow green is useless.
With a mild cam you should be running about 18"-21" of vacuum. Then the common way to set idle mixture is to gently bottom out the mixture screws, turn them about 1 1/2 turns out, start the vehicle, and then gradually turn them out about 1/4 turn at a time and look for the highest vacuum reading. This should get you idling smoothly.

If it's an automatic transmission I usually set the idle to about 850 in Park and then it will drop to about 650 when you put it in Drive.

 

[Ricko1966]

I'm still waiting on a response from the O.P. I'm going to assume since he bought a HF vacuum gauge,he probably doesn't have a mighty vac. So here is an alternative to check for working vacuum advance after setting your timing,vacuum advance disconnected and plugged. Now timing is set,connect your vacuum advance to manifold vacuum your timing should be advanced from where you just set it.

 

[projectmidnight74]

Okay a few things here check your fuel pressure,Edelbrocks don't like too much fuel pressure. What year truck are you working on? And what fuel pump are you running? Are you running a return line. Adjusting the carbureator does not change your timing.Too fast of an idle will change your timing. It's ignition first then carbureation when you are setting things up. Have you set your timing? Post back these answers and we will try to uncover the mystery.

1974 c10. donor 335 machined and slightly bored over with flat top pistons. I'm not a motor guy so I'm giving what I remember lol. Timing was set. I initially felt like it was wrong so my guy is coming back tomorrow to get it back on track after my tinkering. Holley red electric fuel pump. Return line is in, but we are installing a better fuel pressure regulator. Fires up fine, just rich. can smell the fuel in the exhaust and its smokey. Hope this answers all the questions.

 

[Ricko1966]

Okay you need to measure the fuel pressure. I asked about timing because you mentioned timing,if it was set and you didn't move the distributor,than it's set. Make sure vacuum advance is hooked to manifold vacuum,and check for vacuum leaks. Vacuum leaks will cause a fast idle. Read your carb manual or find one at jegs,summit etc. Find your fast idle screw and throttle stop screw see if those are in too far. You can't use a vacuum gauge to set idle mixture,if you have a vacuum leak or the throttle plates are open. Also you can't set timing correctly if the idle is too fast. Sorry not more help but giving you the basics. Get your fuel pressure down to 4or5 lbs,measured. Get your idle speed down,then check timing,make sure engine is warmed up and choke full open. Then you can set mixture.

 

[fast 99]

Smokey exhaust?