fluctuating vacuum?

[Bronco Paul]

Ok...so I've got a problem with the vacuum on my 302. I'm getting a solid 15 at idle but the needle fluctuates quickly. It fluctuates throughout the power range. I've got an Edelbrock performer cam 2122, Edelbrock intake 2121, Edelbrock 5896 valve springs, Edelbrock 600 cfm carb, and either an Edelbrock 7811 or 7814 timing chain...I can't remember which. I've looked for vacuum leaks but haven't found any...the only thing I can think of that would cause the fluctuation is the timing being too far retarded. I've tried setting the distributor a tooth off but it gets worse so I'm pretty sure that it's in correctly. Could I have the wrong timing chain? I'm not sure which chain I have...it does have the three key notches and it is set on the "O" one...which is for the straight up position. According to Summit Racing the 7811 has:

Maximum Advance/Retard at Camshaft None 2 degree
Maximum Advance/Retard at Crankshaft None 4 degree

where the 7814 has no advance. Is there way to visually tell them apart? Or does anyone have any other idea as to why my vacuum is fluctuating?

 

[rguest3]

Is the vacuum fluctuating a lot at idle? Vacuum will change dramatically as you rev engine or increase RPMs. That's normal.

 

[Bronco Paul]

it's a quick fluctuation at idle...not much though...like a little over a half inch each way. So it reads fluctuating from 14.5 to 15.5 at idle...then continues all the way through the rpm band. I figure its got to be either a timing chain or vacuum leak issue.

 

[rustbucket]

Could be a stuck valve. Is it a quick down and up?

 

[73azbronco]

http://www.international-auto.com/f...uges/tips-on-reading-gauges-vacuum-gauges.cfm


http://www.gregsengine.com/using-a-vacuum-gauge.html


My take is weak springs or sticking valves.

Point to ponder, I just had my machinist hone my brand new edelbrock head valve guides because they were sticking out of the box.

 

[Bronco Paul]

I thought the same thing so I put new springs in.

 

[rustbucket]

Can you post video of how the vacuum gauge is behaving?

 

[broncnaz]

Most likely sticky or to tight of valves. could be igintion miss but you should notice the miss at idle. worst case you have a burnt valve or two.

 

[Explorer]

Not familiar with your cam, but could be normal due to valve overlap. (intake and exhaust valves open momentarily at same time due to how it's ground) This could also account for vacuum being a little low.

 

[cldonley]

Is the engine broken in, or is it a fresh build? Do you have any other symptoms? Noises? Hard to start? Hesitations? Vacuum tells you a lot, but don't "chase the needle". And an aftermarket cam with changes in lift, duration and overlap will cause it to read much differently than stock.