Vacuum Reading - shakey

[Purple440]

When you guys use check your vacuum, does the needle in the gauge flop back and forth like at least 4 degrees either way, rapidly?  I'm reading a big block rebuilding book and the author mentions the needle should show a steady vacuum reading.  When I'm reading the gauge it's impossible to get a good reading because the needle never stabilizes.

Thanks,

[firefighter3931]

The vacuum needle should be steady. Try advancing the idle timing and see if it calms down. How much vacuum are you reading Doug ?


Ron

[Purple440]

Hi Ron - Idle in drive is somewhere between 6 and 10 and in neutral it's somewhere between 12 and 16.  So I figured about 8 in drive and 14 in neutral.  I can't get a steady reading so that's my best guess.  I've got the idle screws adjusted for max vacuum and the timing is at about 17 initial. 

- Doug

[firefighter3931]

Sounds like it's a carb issue....the idle vacuum should be higher with the cam you're using. What is the idle speed in park/neutral and what is it in gear ? Refresh my memory on the carb again....i forgot what carb you have 



Ron

[Purple440]

Idle speed in park is about 850 and in drive it's about 600.  The carb is your favorite...BG Speed Demon 750.

[jg68]

.  The carb is your favorite...BG Speed Demon 750.

[supserdave]

Just this summer we had a 400 we rebuilt do the same thing, but worse. It would fluctuate from like 0-15". Ended up being a broken valve spring.

Doesn't sound like your issue though, but I would bet it's the carb causing problems.

[69chargeryeehaa]

good info here:
http://web.ncf.ca/ag384/vacuum.txt

[Purple440]

69chargeryeehaa  this backs up a couple of articles I read last night, pointing to the valves, valves guides,  or head gaskets.  Thanks! 

Ron, three days ago I bought a new 1'' 4 hole spacer with new gaskets so the carb is now sealed up real good.  The old spacer was warped and compressed and the gaskets were fubar.  I also fixed my plug wire problems that were identified in a previous thread.  So I'm leaning towards something else.  Unless the carb is just no good...but I also tried a different BG carb that I had laying around and had the same problem.

[firefighter3931]

Unless the carb is just no good...but I also tried a different BG carb that I had laying around and had the same problem.

Was the other BG carb also a Speed Demon ? From my experience, the Speed Demon is a poor choice with a cam that has some overlap....the fuel curve is just too conservative. Joe (JG68) just went through this trying to dial in a Speed Demon on a mild 440 with less cam than you have in yours. 

I'm sure you've seen Alan's thread on the Holley 750dp and how well it's working on his car. The fuel curve on the Holley carbs is better suited to your type of build, inmo. A BG Mighty Demon or Race Demon would probably work OK but the cost is higher than a comparable Holley. 


If you're thinking it might be something mechanical just run a compression and leakdown test to see what you have. The motor is pretty fresh so it should be fine but it wouldn't hurt to eliminate these possibilities. 



Ron