I installed a Comp cams XE256H ( Grind # 12-256-4 ) camshaft into a LG4 and was wandering what my vacuum should be at idle in and out of gear and at WOT, The cam specs out as follows: 256 / 268 advertised duration, 212 \ 218 duration @ 0.050, .449 / .456 I & E lift and 112° lobe separation.

The engine is not stock. Heads are World Products S/R 1.94 intake, 1.50 exhaust with 58cc combustion chambers. I also installed under cut valves into the heads along new springs.

The exhaust is composed of Hooker headers and Y-pipe attached to the stock exhaust.

I believe that I am having a problem because I have around 15” of vacuum in park but it drops to bellow 10” in gear. At WOT the vacuum will go to zero. If I let off the gas the and cost a while the Vacuum will go up to around 20”. Does this sound right?

 

I would more vacuum at idle.

 

Whoa, I caught Apeiron with a grammatical error - mwahaha.

I agree, a bit more vacuum at idle. Also when engine braking, you have mid 20's right?
Check for vacuum leaks, use an unlight propane torch around hoses and whatnot when you're idling.

0" at WOT is normal.

How much timing are you running at idle? Is this the stock CC distributor?

 

Syntax, not grammar.

 

What goes around comes around right?

 

Yes, it is the stock CC distributor. Right now my base timing is set at 10 degrees. I have checked for vacuum leaks and found nothing yet.

Also, is this setup going to require any computer mods?

 

It won't require computer mods. I ran a lot more cam than that, at high altitude no less, without any problems. Everything else basically the same as you're running (heads were ported).

Check for vacuum leaks. There are many things run off of vacuum, and any one of them could cause you problems without being obviously busted.

 

Sounds rather like what happens when you set the ignition timing to the factory spec, but without unplugging the plug you're supposed to unplug.

 

The computer wasn't even in the car when I set the base timing.

 

uh huh... So then what's controlling your cc-distributor? Or your cc carburetor?

 

I was testing a theroy that possibly the computer had something to do with the problem. I was swapping out the computer when I decided to check the base timing. It was easier to do it without the computer plugged in since it was out anyway. The computer is now back in the car and it wasn't the problem.

 

So what is the timing set to with the plug disconnected?

What does it go to when you plug the plug back in?

 

Right now the timing is set at 8 degrees advanced with the plug disconnected. With it connected it advances off the marks, if I had to guess somewhere between 12 and 16 degrees. The timing will also advance further when the engine is reved. I do still have the stock catalitic converter and thinkig that this could be plugged. Is there a way to tell with it still attached to the car?

I also have another question. Could a bad VSS sensor cause the speedometer to quit functioning?

 

Found out that the cable was broken on the tranny side.

When I started it to put it on ramps the vacuum wouldn't go above 10 inches. I must have a leak that is gettin worse.

 

I found part of the vacuum leak and the vacuum was back up to 15". Somethig is still wrong because when I put the car in gear the vacuum dropped almost to nothing. I am running out of ideas as to were to look. Checked to make sure the carb was bolted down tight. Removed all vacuum lines to the cruise control that wasn't even in the car. Checked and repaired or replaced all the vacuum lines. Any othe ideas would be greately appreciated.

 

What's your idle speed in gear?
In park, if you hit the brakes, does the vacuum drop?

In gear, if you use the ebrake, and no brakes, is the vacuum still near zero?

Is it stumbling when it has near zero vacuum, like it wants to stall?

Did you try the propane bottle trick for vacuum leaks? You don't hear any hissing when listening around?

 

Idle speed in gear is bounceing around and doesn't stay stable. It is like the computer is trying to compensate for something. Idle speed in park is just under 1000 rpm.

I haven't noticed the vacuum drop when the brakes are pushed. At least not while it is in park. I would like to try the e-brake thing but they do not work. Something about a defect in the rear calipers.

When the vacuum is near zero it doesn't stumble or stall but the idle is jumping around, can be seen on the tach. May be a stumble.

I don't have a propane bottle so I used starter fluid instead. There was no signs of a leak. The idle didn't change.

I also destroyed my timing light today but had the timing set at 8 degrees with the computer unplugged from the distributor.

 

You could disconnect the rubber hose and remove the check valve at the exh. manifold A.I.R. pipe (drivers side) and connect a fuel pump pressure gauge. Run the RPM's up to 2500 and you shouldn't have more then 2.75 psi. If you do there is a restriction in the exh. system some where.

Auggie

 

I have headers on the car, no manifolds. Also the passenger side header isn't completly connected an is leaking exhaust. Found the exhaust leak earlier. A bolt had come loose between the header and the y-pipe.

 

Yah, I had that same problem when I checked my exh. back pressure, so I drilled and taped a hole just in front of the cat.

Auggie

 

Was the exhaust the problem? I have holes just in front of the muffler and plan on getting a new cat back system soon but right now I don't have the money.

 

No, the exhaust isn't the problem, this is;

Either there's a vacuum leak, or you've got an intermittent miss or something.

Is this how it's run ever since you put in the cam? I'd pull the distributor and rotate the engine over to #1 compression, drop in the dist and put the plug wires back on. That's the equivalent of "take two advil and call me in the morning" or "reboot it".

 

I have a similar setup...1987 Monte Carlo SS HO 305: Herbert Cams 217-223 @ 0.050", World Heads 1.95 int ported, bowl blended, port matched. Headers, Stock Aluminum Duel Plane and new CCC-q-jet. I got 15" of vac @ idle in park 800rpm and 12-13" in gear. I built a DIY voltage regulater to boost up the vac volts; since I'm still runnung the computer, the voltage was a little low causing timing to be pulled out when in the throttle..also the TCC lock-up wasn't right...this little device fixed both! I think the timing is set to 16deg base..but not positive. Nice strong choppy idle though!!

 

Not a bad idea Rods. The Differential Pressure (VAC) Sensor does affect engine timing. This sensor measures the pressure differance between the atmosphere and the manifold. High vacuum = high voltage and more spark advance and low vac = less voltage and reduces ign. advance. Do you have any spec's on your voltage regulator?? You know, I don't even know what the stock engine vac. should be on a tight 305?

Auggie

 

http://http://www.national.com/mpf/LM/LM317.html
http://http://www.maplin.co.uk/modul...=24m7#overview
I was running a HO305-190hp, Cam spec was 180*-210*@0.050" with 115* spread. Vac at idle was 20" @ 4.3volts and would drop down to 17" in gear.

Thank for the response! I'll post more on this later!
http://www.national.com/ds/LM/LM117.pdf

 

I found some free vacuum by accident when I switched to some gapless rings from Sealed Power. My set is on the second compression ring and really makes a difference if you got a big cam. They even warn you that if you run a stock cam you may need to switch your pcv valve to one with a stiffer spring to compensate.

 

You know a good reason for loss of vac. after a cam change could very well be misadjusted valves. He said that the vac. gauge is jumping around, right. Readjust the valves.

Auggie