Vacuum bouncing at idle - steady at higher RPM

[70Bronco]

Idling at 700 RPMs, my vacuum is bouncing rapidly 2-3". It smooths out with higher RPMs and gets rock steady at 1,500 RPMs. What could be wrong and how do I check it? It's a '66 Belvedere 361 .

I checked for an intake leak and everything seems OK there.

 

[70Bronco]

OK. After wrestling with this half the day, I threw some Sea Foam in and drove it a little bit. I checked the vacuum again and everything is normal now: 18.25" steady. That was weird. No clue.

 

[oldbee]

Sounds like you had some carbon buildup on valve seats IMHO. Good now & dont worry(for awhile,lol).

 

[oldbee]

70bronc just got done running a cup full of water thru mine to get rid of an abnormal "pocking" noise in exhaust. Even though I try to "blow the carbon out", sometimes you have to try something else.

 

[khryslerkid]

70bronc just got done running a cup full of water thru mine to get rid of an abnormal "pocking" noise in exhaust. Even though I try to "blow the carbon out", sometimes you have to try something else.

Boy, you are old!

We'll keep that little trick between you and I

 

[oldbee]

Boy, you are old!

We'll keep that little trick between you and I

And. did you look at new posts lately?lol

 

[oldbee]

KK and others, always remember that some of these 'tricks' need an explanation for the younger members. Hate to break a piston/head by doing it wrong!

 

[Yatzee]

Idling at 700 RPMs, my vacuum is bouncing rapidly 2-3". It smooths out with higher RPMs and gets rock steady at 1,500 RPMs. What could be wrong and how do I check it? It's a '66 Belvedere 361 .

I checked for an intake leak and everything seems OK there.

What you are reading is pulses from a /some pistons on the intake stroke usually present in stroked engines but in your case it could be a piston ring or two that's not expanding properly. If blow-by gases aren't an issue then it would be valve related. If the engine runs rich, carbon build up and play havoc on both the rings and the valve seats. Pull and check the plugs appearance to get a better idea of what's going on in the cumbustion chamber.