Low vacuum reading

-
cu440da

cu440da

Well-Known Member
Joined
Jun 12, 2011
Messages
278
Reaction score
13
Location
Cottage Grove, MN
What is a normal vacuum reading at idle for such a mild cam?

383/ .474 purple shaft/ Holley 600 DP/ stock distributor

Trying to get the 383 in my new Dart tuned right. Got timing set to 32* total. Hooked up vacuum gauge and was shocked to see 5" on the gauge. Adjusted mixture screws by idle speed as the vacuum did not change. Looked for vacuum leaks and found none.

Rebuilt the Holley and replaced the 6.0 power valve with a 3.5 power valve because of the low vacuum. I assume the power valve was open at idle before rebuild.

Motor has what I think is a screw in pcv on the pass side valve cover. I plugged that and the carbs pcv inlet and was able to get a 10" vacuum reading. Can a bad pcv drop vacuum readings like that?
 
Sure it could, but where is your timing set?
 
Was you hooked to full manifold vacuum? Your power valve should be half of inches of vacuum.

Hook to full manifold vacuum, set your vacuum reading as high as you can by turning the air/fuel screws back off 1". Adjust idle accordingly.

All of this should be done AFTER you have your timing set.

Timing first, carb second.

What initial? What mechanical? Set timing with vacuum advance unhooked and plugged.
 
When you blocked the PCV off, did the idle speed go up also? Blocking of that air and getting a better result looks to me like your idle mixture is lean.
 
FYI, 5" is excessively low with that cam but I'm lucky to get 10-11" at idle (440). Don't expect big numbers because of the overlap.
 
I bumped the initial up to 12*, which brought my total back to the 45* where it was before I started this whole thing... Just ssounds too high- all you hear and read about is how BBM's like 32-36 total. Seems to like it, vacuum has bumped up to 12" Still get an occasional spit out of the carb off idle, though.

I really need to limit the mechanical advance to get my total advance a bit lower so I can get my vacuum advance hooked up again. Thinking of going with one of those E Curve type distributors instead of welding up the slots in the advance mechanism.
 
My opinion.....However you decide to get there, get the total back to at least 32-36 and still allow 12-16 initial. Gives some room to play with it. Then put the 6.5 PV back in and jet the primaries up a number or two. You can always jet the secd. to get good plug readings or the highest mph at the track.
 
Fix the distributor mechanical advance so you can run sufficient initial timing.

I'd run anywhere from 16-20 if the starter will handle it when warmed up.

It will run like a different car when you get it right.
 
Listen to crackedback it will run like a hemi if you follow his instructions. He certainly helped me and thats not too much timing. im running 22 initial 32 total and it works great about 54 with advance hooked up, However Im gonna try to recurve tomorrow or wed to try to achieve 20 initial and 34-36 total
 
Timing, as they say, is everything. This 383 likes 16* initial- vacuum is at 15" now. Got mentally prepared to weld up the advance slots and start trial and error adjustment to limit the mechanical advance. Pleasantly surprised to find out the motor has a MP distributor and adjustment is as easy as loosening 2 screws and moving the limiting plate. Sweet! Got total advance dialed in at 36* and it runs sooo much better now. Thanks guys....
 
Nicely done!

Those newer MP distributors are nice for limiting mechanical advance. A neat little trick to avoid the curve kit is use drill bits with the proper size shanks. Got a drill index, you can limit mechanical...
 
70Duster440 said:
FYI, 5" is excessively low with that cam but I'm lucky to get 10-11" at idle (440). Don't expect big numbers because of the overlap.
Click to expand...

I have a 440 based 493 with a MP '509 cam. At 950 idle it shows 10 unsteady inches of vacuum.
 
You must log in or register to reply here.
-

Back
Top