Technical Vacuum Nightmare

Slack tube monometer?

 

Yep.

 

OP you said you "Checked Camshaft Timing". How did you check? A cam chain that has slipped a tooth will run xactly like yours is.

 

Did you ever see the engine run in its previous version? In other words, can you confirm with certainty that you didn’t just buy someone else’s problem?
When I read your original post, the very first thing that came to mind is cam timing. If you in fact did see this engine run previously, then you can file that in the “unlikely” category.
I like pretty much all of the posted suggestions.
Now, if you did not see this engine run before buying it, then start with Hemi Joel’s suggestion of confirming TDC/timing mark.
My personal suggestion is to do a open throttle compression test to see if you are in the ballpark on what you would normally expect out of a typical engine. If you are seeing low numbers, you can stop looking at carb/ignition for problems, and dig into cam timing or simply to big of a cam for your wants/needs.

 

Violence is traditional!

 

so what should I do about my 396 that only has 6 in vacuum, but I think it's perfectly normal?

 

Sounds like you have the perfect engine to give cranking compression numbers for comparison.
We know your cam is a bit outlandish for a daily… If your numbers are comparable to his, either he has a long duration cam, or his cam timing is off.

 

My cam isn't that big, really....

 

I would guess that cam in a 396 (in neutral) at 1000 rpm would generate about 12”. Surprises me that your vacuum is only 6”.

 

Yeah pretty mild cam for a big block huh?
I know the OP said he's changed intake manifolds but I will relate this- I had an engine- a 396 by coincidence -that wouldn't idle, manifold vacuum very low. I couldn't figure it out. Fresh Freshin Up too. I had the intake off yet again looking & had it on the floor rolling it around & a piece of Iron fell outta the exhaust crossover!
Long story short that manifold musta sat thru a winter with water in it & froze & busted that chunk outta the bottom of the intake into the exhaust crossover! Instant un-seeable vacuum leak!

 

Not a good idea. If it backfires it will turn your hand into hamburger, WELL DONE!

 

Rag or an oven mitt?

 

Have done that hundreds of times and NEVER had one backfire while doing it.

 

I've done it many times to clear up a rough running carb. Sometimes if a little piece of crud will be stuck in a p***age, and of you put your hand over the carb and rev it, vacuum will **** the obstruction out. (Or for the scientists a**** us, atmospheric pressure will push it into the low pressure area in the intake manifold.)

 

Not scientist, engineer...

 

For a internal vacuum, he could just do smoke test at oil cap/ breather and with throttle plates open see if smoke comes out of carb . Or do same thing with a spray of real carb cleaner at idle. If Kahill double checks engine with correct initial timing, say 15 btdc, or a tad more,and known good vacuum gauge, and results are still same, he must be running a fairly hot cam, he needs to update with better info, before gang can offer anymore useful thoughts.

 

So is prison!

 

Never mind that he has a 6-71 blower on it, with two Carters.

Also, his LSA is 114º.

The OP's cam likely has a much tighter LSA.

 

You're talking about my 427, aren't you? I'm talking about my 396 in my old truck, which has a single plane intake and a Holley 650 on it.

 

What intake are you using? I tried a Torker on my ‘70 LS5 and never got out of its way until maybe 3500rpms. I used a 750 Holley vac sec carb.
Engine really woke up down low with the edlebrock dual plane I changed to. Maybe the 3.07’s and L60’s affected things?
Anyways, never checked with the Torker, with with the dual plane I had 16/17 inches at idle if memory serves.
I know it was over 15c because 15 was centered in the middle of the gauige

 

I don't like how the intake works, will probably go back to a two plane. I recently got a few more Qjets to play with, so I may just put the iron intake back on, and then it'll run fine.

 

Ok, good to know. So as long as you don't start talking metric system, we can still be friends.

 

People are talking about two different ways to measure pressure. One is gauge pressure (the one most of us are used to) where zero gauge is set at atmospheric pressure (14.7 psi). The other is absolute pressure where the zero is actually 30 in mercury vacuum when measured in gauge pressure. The absolute group is correct when they say there is pressure in an intake if you are using the absolute scale.

It is a bit like measuring temperature in Celsius (used to be Centigrade) or Fahrenheit. Water freezes at 32 degrees F, but at 0 degrees C. Water still freezes at the same temperature you are just looking at two different ways to measure that temperature.

 

I'm kind of sorry I ever said anything, I was just remembering a guy I used to work with and relating how he would scowl about it everytime somebody used the term "vacuum". I just thought it was a funny story, I didn't intend to create a controversy.

 

Check your timing chain if it is a tooth off you will have low vacuum been there.RLS

 

I can't keep track of your stuff.

 

That's understandable.

 

That's called "negative pressure".

 

Wait a minute, didn't I just read in your thread about your latest adventure that the Rambler vacuum wipers didn't work all that well. I'm so confused by all this that I've got a headache.