Help! Pulsating Engine Vacuum?

[vette70cdg]

1970 454 running good, but rough idle. I started hearing a pulsating noise somewhere on top of the engine. I put my hand on the distributor vacuum advance and can feel it very rapidly pulsating. It stops few seconds and starts again. I disconnect the hose and connect it to a tee on a vacuum gauge and see it pulsating between 14-15 very rapidly. I disconnct the hose, plug it and the vacuum advance stops pulsating. Whats going here? :confusedHow do I fix it? I am not sure when it started? ;help

 

[Hib Halverson]

I'm thinking engine mechanical issues.

Run a compression test.
If you find any cylinders which are low, check those cylinders for worn camshaft lobes or burned exhaust valves.

If you find one or more bad cam lobes the engine is "toast" and will need to be overhauled. If you just have some burned valves, you need to pull the heads and do a valve job. With a 70, when you have the engine apart, carefully inspect all the valve seats for valve seat recession.

 

[duke94]

This link might help you.

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

 

[vette70cdg]

Thank you for your reply! I forgot to mention that this is a rebuilt engine with only 8,000 miles on it since rebuild, so I am hoping this might not be the issue.

 

[vette70cdg]

This web site about how to read Vacuum gages looks great! I will look into this!
Thank You

 

[Hib Halverson]

If it's a fresh motor and there was any deviation for proper procedure for camshaft installation and break-in or the cam was broken in with an ILSAC GF-4 oil, camshaft failure is possible.

If your tests with the vacuum gauge indicate that big fluctuation is valve-realated. I'd check valve lift first by visual inspection. If you have a valve or valves that look like the rocker is not going down as far as it should, then I'd look up the cam lift and measure it at the pushrod end of the rocker arm using a dial indicator.

 

[vette70cdg]

If it's a fresh motor and there was any deviation for proper procedure for camshaft installation and break-in or the cam was broken in with an ILSAC GF-4 oil, camshaft failure is possible.

If your tests with the vacuum gauge indicate that big fluctuation is valve-realated. I'd check valve lift first by visual inspection. If you have a valve or valves that look like the rocker is not going down as far as it should, then I'd look up the cam lift and measure it at the pushrod end of the rocker arm using a dial indicator.

If it's a valve related problem, than why is the engine vacuum only pulsating after a 15 minute warm-up at resting idle? The entire time that it's fast idling during warm-up, it's smooth as silk! I flick the throttle to release fast idle and then I can then hear the vacuum advance flickering servel times a second! I just replaced all the spark plugs because there were 2 bad ones, firing inside the case and not at the electrode tip. I checked cap and rotor. I am using Pertronix breakerless ignition. I have not done a compression check.

 

[GerryLP]

70,

Sounds like you have a non-stock cam in your vette's engine.

However, does the car seems to drive fine other than that? If so, then if you're using a non-dampened gage, then you'll see the gage needle flux rapidly.

Does the fluxing affect the rpm?

GerryLP:cool