Vacuum issues need diagnostics.

[Chevys10dr]

I have a fresh 350, heavily worked 487 heads with a 272/272 .454/454 comp cam. it idles at 900 in park and 700 in drive and running a little rich, she only makes between 0-7 inches at idle of vacuum but when it revs up to 2000-2200 she pulls about 15 inches. What could cause this?

Thanks

[swamp rat]

Check for vacuum leaks:

Do you have a power booster on the brakes? make sure that hose isn't leaking any where, i replaced the grommet in mine and it made a world of difference, You can remove that hose from the carb and plug the port to see if the vacuum go's up go's up.

Is it an automatic TH350? is the vacuum line hooked up that go's to the tranny?

[Chevys10dr]

10-4 its a TH350 the line is hooked up, I do have a brake booster, will check the grommet, I checked all vacuum lines, intake and carb but everything is sealed tight.

[swamp rat]

Ok, try unhooking the booster line at the carb and plug it. That grommet is ofter overlooked.

You didn't mention what carb but make sure all the connections are to the correct spot, ie, manafold vacuum and not ported vacuum

[Chevys10dr]

Its a rochester 4 BBL, I will definitely check the grommet to night after work.

[geezer#99]

You need to look at your timing. That cam combo needs at leasr 14 degrees initial or more.
It smells rich because you have the idle set too high and you're trying to idle on the power circuit in the carb.
Better timing will bring your vac reading up.
What's your timing set at now?

[Fitz]

Originally Posted by Chevys10dr

I have a fresh 350, heavily worked 487 heads with a 272/272 .454/454 comp cam. it idles at 900 in park and 700 in drive and running a little rich, she only makes between 0-7 inches at idle of vacuum but when it revs up to 2000-2200 she pulls about 15 inches. What could cause this?

Thanks

In two words? YOUR CAMSHAFT! That's normal vacuum for a 272 degree duration cam. My 572 is a 254/264 (10 degree overlap) .620 cam and it does about the same. I needed to install a separate vacuum pump to run the brake booster when I drive around town. I never get the revs high enough to keep the brake booster working in city traffic.

Look at it this way: It's just a consequence of "More Power" that you can live with!

[Lee H]

Originally Posted by geezer#99

You need to look at your timing. That cam combo needs at leasr 14 degrees initial or more.
It smells rich because you have the idle set too high and you're trying to idle on the power circuit in the carb.
Better timing will bring your vac reading up.
What's your timing set at now?

After you pull the booster line and plug it, this is the next step.
Posted via Mobile Device

[Coley]

Forget the vacuum for a moment.....one thing I didn't see you mention is how the engine is actually running and performing?
Any Info?
Coley

[DT1]

Originally Posted by fitz

in two words? Your camshaft! That's normal vacuum for a 272 degree duration cam. My 572 is a 254/264 (10 degree overlap) .620 cam and it does about the same. I needed to install a separate vacuum pump to run the brake booster when i drive around town. I never get the revs high enough to keep the brake booster working in city traffic.

Look at it this way: It's just a consequence of "more power" that you can live with!

x2

[msgdsrf]

Pretty much all 272 Advertised Duration cams will make way more than 7", 272 @ .050 is a different story but then you'd have much more lift than .454. And you wrote "0-7". Is it bouncing that much?

But, you asked, what could cause this? Ignition Timing, Valve Adjustment, Vacuum Leaks, Cam Timing.

Fitz, BTW cam overlap is from intake open BTDC to exhaust close ATDC, not the extended exhaust duration your split cam has. Just saying..

[swamp rat]

I understand the camshaft deal too, but it does sound like he may have a leak issue, check that first, then reset timing and carb.

[Fitz]

Originally Posted by msgdsrf

Fitz, BTW cam overlap is from intake open BTDC to exhaust close ATDC, not the extended exhaust duration your split cam has. Just saying..

You are right & I just got lazy!

[Chevys10dr]

Hey guys,

geezer#99: I am running 12 degrees initial timing, still got to do a little learning on the rest of the timing stuff.

Coley: The engine runs good no rough idle strong sounding engine, I haven't taken it out for a drive I just finished the rebuild. Noticed this issue on the break-in.

msgdsrf: no it doesn't bounce I should have said 7 in idle and 0 in drive.,

Thanks for some advice on what to look for, I would have checked it today, I had to go work on a F**d which sucked!!!!

[msgdsrf]

Add 4 degrees initial. That will help with the richness you reported and give a bit more vacuum too likely. Then curve the mechanical to not exceed 36 (you'll not be reving past 3000 for awhile so can probably drive to break in until you do). But still very low on vacuum numbers, some break in will help as well though, Setting the rings during break in.

[Chevys10dr]

Update:

I'm still stumped today I worked on finding the vacuum leak and found nothing. I did bump it to 16 degrees now i have 10" at idle and 5" in gear. What is a good vacuum to hunt for with this cam? and how much vacuum will be good when using a brake booster?

[geezer#99]

Where's your vac advance hooked to?

[Chevys10dr]

This is a 71 rochester Quadrajet carb

[geezer#99]

THat's ported!
Try the other one on the front upper left. Should be manifold. Pop the cap off while idling. It should suck! LOL!! Idle will speed up. Turn it back down. Adjust your idle mixture screws for best idle.
And now for all those guys who jump up and say that won't be manifold vac, the passage inside comes out in the base of the carb. And the choke pull off pot is attached right below it. Another manifold vac source.

And kudo's for the clean motor with a nice steel tubing line for the fuel. Well done!!

[Chevys10dr]

I read somewhere that was manifold. Well guess I was wrong, Will try the other port tomorrow. Thanks

That steel line was a pain in the A** to build.

[geezer#99]

Put a vacuum gauge on them to be sure. I've been wrong before!
Just remember if the one you're presently hooked to shows vacuum it could be because your idle is too high exposing that port to manifold vacuum in the plenum.
I bet on your fuel line you wasted a few pieces getting it right. I learned how to bend lines repairing steam tracing in a gas plant.

[Twisted78SS]

Some good basic info at this link for interpreting vacuum readings
http://www.classictruckshop.com/club...ts/vac/uum.htm
Posted via Mobile Device
Posted via Mobile Device