Shdggsdv

Newly completed top end build. I've plugged all the vacuum hoses. I've also sprayed carb cleaner along the intake and the carb base.

I have a comp XE274 with a CR of 10:1. I should be getting 11" at 800rpm, but I'm getting a rapidly fluctuating 5-6"

What else should I be checking?

Edit:

I check vacuum using a full manifold vacuum port.

I checked and photographed when I did the chain. The cam gear and crank sprocket line up perfectly.

The timing I have messed with, but not a ton. The best I've got so far is about 11" at 1k RPM at about 14 degrees initial (Fluctuates quickly between 10" and 11"). Have not messed with timing past initial, however.

My mechanic is the one that did the valve lash. Supposedly to the specs on my cam card, but I guess shops can make mistakes, so I'll check that out next.

The carb is not tuned properly, because I can't figure this out. I can't get more than an inch of extra vacuum through carb adjustments

HeadsU.P.

Where are you taking your measurement? Ported vacuum?

JoeMinnesota

Is the carb all tuned up? The mixture will affect vacuum. Not sure by how much.

TimAT

Where is your timing? More aggressive cams like more advance.

terry82

https://itstillruns.com/erratic-fluc...-12227649.html

claysmoker

Here's a good tutorial on vacuum gauge reading:

http://www.secondchancegarage.com/public/186.cfm

zwede

You may have tightened the rockers too much holding a couple valves open.

stumpshot

Check with another gauge if you can. I was always given to understand that a jumping gauge indicated valve problems. Just went through this with a mite-vac. Hooked up a regular vac gauge and vac was steady.

7T1vette

Timing and valves over tightened are good candidates to eliminate. You could also have a leak in the intake gaskets/seals that allows more air to leak in. The 'pulsating' indication is of concern.

Crimson Thunder

I have the same cam and I'm getting a steady 15" at idle. I agree its probably a valve adjustment and timing issue.

CanadaGrant

You said you did a top end build. If you changed the cam and timing chain is it possible you are out one tooth on the cam gear? 5 inches of fluctuating vacuum is nowhere near right for any cam let alone that one. Running and tuned properly it should be up around 15 as posted by Crimson Thunder.

Shdggsdv

I check vacuum using a full manifold vacuum port.

I checked and photographed when I did the chain. The cam gear and crank sprocket line up perfectly.

The timing I have messed with, but not a ton. The best I've got so far is about 11" at 1k RPM at about 14 degrees initial (Fluctuates quickly between 10" and 11"). Have not messed with timing past initial, however.

My mechanic is the one that did the valve lash. Supposedly to the specs on my cam card, but I guess shops can make mistakes, so I'll check that out next.

The carb is not tuned properly, because I can't figure this out. I can't get more than an inch of extra vacuum through carb adjustments

Tonio

Please post a photo of your crank and cam sprockets to show how they were aligned.

I'm in a similar situation, chasing a low vacuum condition on a '72 LT-1.

Subscribed.

Shdggsdv

JayRay

11" seems a little low to me. Tune the Carb, fuel mixture screws. Tuning the carb brought my vacuum from a 16 to 22 after a carb rebuild. I use the vacuum port that is connected to the choke pull off to measure.

Do one side at a time, get it to the highest vacuum reading then adjust the other side.

Tonio

My low vacuum issue turned out to be valves that were WAY too tightly adjusted. I set them up with the 0.024/0.030" gaps and now am getting a full 20-inches of steady vac at the manifold. Check your valve adjustments again.