timing quetion

[fireguyfire]

I have a fully rebuilt 383 that I put a mild comp cam and a Edelbrock performer 4 bbl carb. I am getting a low vacuum problem (at best 15) and the needle on the gauge jumps a lot at that measurement. The engine is running like a top, and right now I have timed it to 10 BTDC ( I live at altitude so I was told to go earlier than the 7.5 degrees recommended by the shop manual).
At idle with this engine, and the vacuum advance disconnected, what do you guys recommend I set the timing to to get great performance, and good vacuum?

[Cooter]

Gotta vacuum guage?
How 'mild' is cam? What's the duration at .050??
All this will be needed before hand.

[fireguyfire]

I do have a vacuum gauge, that's how I know my current manifold vacuum at idle. I will have to look up the cam number for you, but it is a very mild cam for the 383 from comp.

[firefighter3931]

If it's making 15in of vacuum at idle that is pretty good. I'd be happy with that 

How does the timing mark look at idle with the light on it ? How does the timing mark look at a steady 2000-2500 rpm ?


Ron

[fireguyfire]

Cooter; the duration at .050 is intake 224, exhaust 230. Does this help?
Firefighter, I'm not sure what you mean when you ask how the timing mark looks; it is strong and easy to read when setting and right now is dead on 10 degrees BTDC at idle. I dont often have an "assistant" so I dont know what it is reading at 2000 to 2500 RPM. Would this be important to know?
I am also a bit freaked out by the rapidly bouncing needle, but in previous threads I see you can fix this by adjusting the timing also. My engine has no miles on it after a pro rebuild, and I have just done the break in on it.

[firefighter3931]

Fireguy,

The cam you have in there is the Comp XE268H which is a nice street cam. The 15in vacuum at idle is about right. 

It sounds like your timing is steady at idle based on your response. I like to check them at several different rpm's to see what the timing mark looks like. This requires a helper to hold the throttle steady while you aim the timing light. 

A vacuum leak or sticky valves can cause erratic vacuum readings so check your vacuum lines and carb base gasket.


Ron

[Cooter]

Cooter; the duration at .050 is intake 224, exhaust 230. Does this help?
Firefighter, I'm not sure what you mean when you ask how the timing mark looks; it is strong and easy to read when setting and right now is dead on 10 degrees BTDC at idle. I dont often have an "assistant" so I dont know what it is reading at 2000 to 2500 RPM. Would this be important to know?
I am also a bit freaked out by the rapidly bouncing needle, but in previous threads I see you can fix this by adjusting the timing also. My engine has no miles on it after a pro rebuild, and I have just done the break in on it.

15" vacuum is about all your gonna see with that cam. Might squeeze a couple more, but wouldn't count on it. A fresh engine will still need about 500-1000 miles for everything to seat properly so that will effect vacuum readings as well. Yes, total timing is needed. Many mild builds like between 34-36* total.
this means initial at crank, mechanical weights.

[fireguyfire]

Thanks fellas; looks like I have a vacuum leak to track down. Not surprised as the lines are very tires; I had to cut back an inch or so off several ends that had split in the headlight hose system.
How can I test if I have a leak around the base of the carb?

[firefighter3931]

How can I test if I have a leak around the base of the carb?

Spray a mist of carb cleaner around the base of the carb with the engine running. If the idle speed changes....you have a vacuum leak 

Ron