Too much timing?

[George Bongert]

Originally Posted by TJ's GMC

Clarke:
That does sound normal. I didn't have a gauge on mine, but mine does the same. Give it a bit of throttle and the advance arms starts to move....give it more and to a point it stops. And since I'm running a 4 barrel that was on Ported Vacuum. I do need to get another video up. lol Been sick and busy so I haven't gotten around to it.

Greetings TJ!

Sorry to hear that you've been a bit under the weather. And I'm sorry to "rain on your parade" so to speak, but you should have full vacuum at idle where your vacuum advance is connected to. That's why it is recommended when setting your ignition timing that the vacuum advance hose be disconnected from the advance and plugged (temporarily) with the butt end of a drill bit, or some other object to prevent the engine from having a rough idle while setting the timing. If you have vacuum, and your vacuum advance is working properly, you should notice a perceptible increase in engine idle RPM when the vaccum advance is reconnected. You definitely should not have to increase your engine RPM to get the vacuum advance to "move." You should also have no less than 15 inches (PSI is a misnomer here) of vacuum at the fitting that your vacuum advance is connected to. And not to make anyone sound dumb here, but the mistake is made by many people refering to a vacuum reading as "PSI." This was one of my Automotive Technology teacher's pet peeves--refering to vacuum as "PSI" instead of the proper terminology of "15 inches of vacuum."

[TJ's GMC]

I REALLY need to get another video up. LOL.
My 4 barrel carb has two vacuum advance ports. Ported and Manifold. Ported allows the engine to idle at initial timing and Manifold vacuum allows the engine to idle at full vacuum advance. I believe I Did post a video on this thread showing the differences. I hooked the advance pump to Manifold advance and the idle rpm increased noticeably and the engine was Much smoother, but I did notice some hesitation coming off idle. Now that probably could have been fixed by retarding the timing a bit. Where I have the motor currently is 10 degrees initial with the advance pump hooked to Ported timing. No advance at idle, but full vacuum advance while cruising down the highway. I may try manifold advance again and retard the timing back to factory specs and see how it likes it, but so far it's been happy.

[Clarke]

Thanks George, for the clarification on the proper terminology. As I was catching up on messages, I realized I was making the same mistake, and know better.

[George Bongert]

Originally Posted by Clarke

Thanks George, for the clarification on the proper terminology. As I was catching up on messages, I realized I was making the same mistake, and know better.

No problem Clarke! I think you will find that your engine will run much smoother and operate more efficiently when your vacuum advance is working properly!

[Clarke]

Update:

With the steel vacuum line (for vacuum advance) disconnected from the back of the carb, I connected my vacuum gauge to the back of the carb and here are the results:

0.8 inches @ 500 RPM
2.0 inches @ 800 RPM
5.0 inches @ 900 RPM

When I tested my vacuum advance the other evening, it started to move at 5.0 inches.

My vacuum reading off the back of the intake manifold (at idle) is consistently 19 inches.

Based on what I provided above, sounds like I'm losing vacuum in the carb?

[George Bongert]

Originally Posted by Clarke

Update:

With the steel vacuum line (for vacuum advance) disconnected from the back of the carb, I connected my vacuum gauge to the back of the carb and here are the results:

0.8 inches @ 500 RPM
2.0 inches @ 800 RPM
5.0 inches @ 900 RPM

When I tested my vacuum advance the other evening, it started to move at 5.0 inches.

My vacuum reading off the back of the intake manifold (at idle) is consistently 19 inches.

Based on what I provided above, sounds like I'm losing vacuum in the carb?

Hello again Clarke!

What you are describing would be the "ported" vacuum connection that TJ has referred to. On some carbs there are multiple vacuum ports, and most notably on GM (Rochester) carbs, there is a vacuum port on the throttle body, and that is the one that the vacuum advance is connected to since at that location you will have full intake manifold vacuum. Since the GMC Big Block V-6's don't use (for the most part) GM Rochester carbs, your best bet is to tap into the intake manifold to get full vacuum to the vacuum advance. Check your carb to see if there is a port in the throttle body that you can hook up to. You mentioned that your intake vacuum is 19 inches at idle which is good, and if you can get that full vacuum to your vacuum advance, your engine will purr in appreciation!

[Clarke]

Great idea George!

I'm still running the Stromberg carb and I didn't find any other ports to tap into, so I will follow your suggestion and tee into my intake port. It will be interesting to see how different it runs at low RPM.

Thanks a lot George... I learned a lot with your assistance.

And, thanks TJ for bringing this topic up.

[TJ's GMC]

There should be two ports on your carb Clarke. One ported and one manifold. My original stromberg has two.

[Clarke]

TJ, I looked again and only saw one port. I attached two photos... I guess you have a different carb?

[TJ's GMC]

Boy that's interesting. Mine does have two, but maybe it wasn't what I thought. My 305E is a 67 series from what we were able to tell and it had the governed points dizzy on it...two vacuum ports on the vacuum pump....one in the front and one in the back. How it was supposed to work I'm not sure. lol I'll get some pics of my carb for ya later today. The two ports on the carb were both hooked to the pump.