Low Vacuum

[StreetWedge65]

I'm running a fairly radical bump stick in my 493. I realize that vacuum will be low, but 3.5-5hg seems too low. I've done all the usual test and things check out fine. Presently all four mixture screws are completely closed and the engine idles fine. Smells a little rich. RPM is low enough to stay on the idle circuit, changed PV's to 4.5's. It's getting fuel from some where to idle. Just can't find where it's coming from. Carb is a Holley 850 dp.

 

[StreetWedge65]

I removed this engine from my 68' Dart. Ran 6.50's - 6.80s. No more than 1500 miles total.
The only changes I've done is freshen the carb, set valves and using two valve cover breathers instead of one breather and PCV.
It always idled around 8.5 hg vacuum.

 

[Fran Blacker]

Do you have a vacuum leak. I'm no expert tuner but PV is suppose be half of what your vacuum is. Power valve leaking from lack of vacuum?

 

[StreetWedge65]

It probably is now, with only 3.5 hg. I just can't figure out why the engine has gone from 8.5 if to 3.5 hg. Will open breathers effect vacuum that much?

 

[snakeoil24]

First thingis take a propane torch, crack open the nozzle and move around the intake and see if you get a rise in rpm. If you do then vacuum leak.. Another issue is check to see if the throttle shafts and plates are ok. Meaning any signs the shaft is bent or the throttle plates are warped. Again carefully gas the baseof carb with torh and look for leaks. Are the carb metering blocks warped?? Do you have a spare carb to use to confirm this?? Put the Power vale back in that was stock. While in their look at gaskets and see if they are cut. Place metering blocks on glass and check for warp.

 

[dvw]

It was fine before in the other car. Go back to what was changed. Carb assembled correctly? Possible wrong metering block gaskets? No vacuum leaks? Is the timing the same? Valve lash to tight holding the valves off the seat? Unless the power valve is ruptured or missing it's gasket there will be no effect on idle mixture.
Doug

 

[miller]

StreetWedge65? Sooo, that's where OSDart's been hiding!

 

[StreetWedge65]

Shuuuuuuu! Be very quiet. There are hackers among us.

 

[65-440]

If your engine is still running with all mixture screws all the way in then you need to look at that carb 1st.... take it apart, clean it out good then try this again !

 

[Bill Monk]

If your engine is still running with all mixture screws all the way in then you need to look at that carb 1st.... take it apart, clean it out good then try this again !

I agree, if you have all idle screws in all the way, and you are still smelling rich I think your problem is in the carb rather than a vacuum leak. With all 4 turned in, if you had a vacuum leak it likely wouldn't run at all. You said you freshened the carb. Not sure what that means but I would start there for sure. Also, why no PCV? Unless this is a race only engine, you will always benefit from a pcv valve

 

[moper]

You touched the valves and the carb. It's one of those...lol. IMO, run the valves again and run it again. Then reset the carb.

 

[PRHeads]

I’d check the timing first.

Not enough initial timing will cause low vacuum, and you’d have to open the throttle blades farther to get the correct idle speed.

If the throttle blades are open too far the motor will be running on the transfer slots and the idle screws will be ineffective.

 

[Bill Monk]

I’d check the timing first.

Not enough initial timing will cause low vacuum, and you’d have to open the throttle blades farther to get the correct idle speed.

If the throttle blades are open too far the motor will be running on the transfer slots and the idle screws will be ineffective.

Good point.

 

[StreetWedge65]

Freshen up meaning; cleaned, checked PV's, ensured transfer slots set at .020" , checked acc pumps, etc,etc,etc. I did use Demon gaskets. That's what I had on hand. I sure thought Demon and Holley used the same bowel and metering block gaskets. Maybe not. I'll take it off tonight.

 

[Bill Monk]

Freshen up meaning; cleaned, checked PV's, ensured transfer slots set at .020" , checked acc pumps, etc,etc,etc. I did use Demon gaskets. That's what I had on hand. I sure thought Demon and Holley used the same bowel and metering block gaskets. Maybe not. I'll take it off tonight.

10.4, Might be worth the time to do a quick timing check before you go to the trouble but u didn't mention changing it so....

 

[StreetWedge65]

I've been adjusting the timing by ear, drag, and ease of crank. I don't want to advance too much and knock the nose off the starter. After getting it to start easy and run,
I checked with light and it's back real close to 18°.

 

[Bill Monk]

I've been adjusting the timing by ear, drag, and ease of crank. I don't want to advance too much and knock the nose off the starter. After getting it to start easy and run,
I checked with light and it's back real close to 18°.

Engines are different but that should be about right, definitely not causing your problem

 

[Bicylindrico]

Ruptured power valve? Where is it getting fuel?

 

[StreetWedge65]

Removed carb tonight. Didn't see anything obviously wrong. PV's ok. I thought maybe the Demon gaskets weren't compatible with a Holley carb. Everything looked good. The secondary blades were at .042" . I had set them at .020" earlier . Seems something has moved.

 

[miller]

RPM is low enough to stay on the idle circuit, changed PV's to 4.5's. It's getting fuel from some where to idle. Just can't find where it's coming from. Carb is a Holley 850 dp.

I've only been in a few Holley carbs, but know on most of them, using the correct gaskets everywhere, will make, or break 'em. Just curious how far you went, while freshing it...all gaskets changed? Keep the old gaskets, to compare? Worth getting a new gasket kit?
That's on top of everything the other guys have mentioned. Sure sounds like an internal leak.