Timing adjustment using vacuum
Junior User
Joined: Apr 2002
Posts: 62
Likes: 0
From: Screven, Georgia U.S.A.
Timing adjustment using vacuum
Hi everyone, on a 1975 390 I've set the timing at 20 BTDC to get the truck to idle close to smooth.Any less timing and it's rough. Using a vacuum gauge hooked to the intake at highest reading and vacuum is at 20 and timing at 25 BTDC. Any advice? Thanks
Posting Guru
Joined: Jul 2001
Posts: 1,613
Likes: 2
From: Gainesville, FL
Timing adjustment using vacuum
Skeese,
There are a few things to consider when answering this.
1. Is the motor stock?
2. Is the factory specified timing chain on it?
3. What's your compression? Does it burn oil?
Does your vehicle start warm OK? Does it ping? If there is no problem with the way it runs, then don't fix it!
20 inches is high, in my opinion. Many say that's OK. I think running around 16 inches is better. Set your timing according to factory and then set your vacuum to about 16 inches. Then enrichen your idle circuit to get a nice idle. I start my idles screws at 1-1/2 turns and enrichen each equally until I get the idle I want. If the idle is too high at 1-1/2 turns then close your throttle down to get the right idle. If this happens remeasure your vacuum and report back. I'm curious to see what you end up with.
KingFisher
There are a few things to consider when answering this.
1. Is the motor stock?
2. Is the factory specified timing chain on it?
3. What's your compression? Does it burn oil?
Does your vehicle start warm OK? Does it ping? If there is no problem with the way it runs, then don't fix it!
20 inches is high, in my opinion. Many say that's OK. I think running around 16 inches is better. Set your timing according to factory and then set your vacuum to about 16 inches. Then enrichen your idle circuit to get a nice idle. I start my idles screws at 1-1/2 turns and enrichen each equally until I get the idle I want. If the idle is too high at 1-1/2 turns then close your throttle down to get the right idle. If this happens remeasure your vacuum and report back. I'm curious to see what you end up with.
KingFisher
