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I have a recently built 383 in my 72 that has HEI ignition. I believe I have a miss and started diagnosing. When I pull #8 plug, I don't see much change in the idle, so I'm suspecting that may be it. I haven't pulled the plug or checked the wire yet, but believe I may need a valve adjustment.
I did try to diagnose with a vacuum gauge, which I haven't used before. I connected to the front of the carb, where the vacuum advance connects and only measured 7 in. I connected to the manifold and measured 14 in. I'm not sure if those two should be consistent, but I was expecting at least 18 in. My carb was rebuilt by Lars, so I don't suspect carb issues, although I do get a bog off acceleration. I have the timing set, at least I think it's correct. I'm currently at 10 degrees. I don't know how to set advance timing, so this is initial.
So my question is, where do I need to connect my vacuum gauge to get the correct reading? If I am only getting 14 in, what could be the problems?
I'll pull the plug and check the wire next, but it may be a few days before I have time to get to it.
Is that 10 degrees BTDC with the vacuum line going to the distributor disconnected?
Did you adjust the idle speed and mixture screws when you installed the carburetor?
From: At my Bar drinking and wrenching in Lafayette Colorado
Any port other than that ported vacuum port. Your 10-degree timing setting is also way out of whack for that engine - you're running grossly retarded. Get that total timing up to 36 and verify your initial (without vacuum) is falling in the mid-teen range. Then limit the vacuum advance to 12 degrees and run it off manifold vacuum as noted so your actual timing at idle is near 30.
Lars has some excellent papers on how to set timing, both initial and total.
Send him a PM and he'll email them to you.
Since you have a new engine you must get the timing correct, or it will run horrible, or you could hurt it.
It's probably the number one cause of poor running engines
You want something like this:
12-15 degrees initial (no vac)
36 degrees total, all-in at 3000rpm (no vac)
Hook vac can up to manifold (14" port) and timing at idle should jump from around 15 to around 30 degrees.
#2 is the most important to get exactly correct.
Last edited by leigh1322; Mar 11, 2021 at 06:40 PM.
I took Lars advice and got my total timing set. I also had to adjust my air mixture a bit. Now, it is night and day different. I think I still need some fine tuning, but this made a huge difference. Thank you, everyone.
So, next question, does this fuel line look to be too crimped, restricting fuel?
Terrific news!
Yes timing and idle mixture and idle speed is a critical combination to get right.
On the fuel line: It's probably OK for normal driving.
If it is restricting fuel enough, it will "nose over" on a high rpm WOT pull.
A new one would certainly look better.
Low engine vacuum help
