vacuum leak comes and goes with heat

[thistle3585]

I have tried everything to try and track down a vacuum leak. I have sprayed various fluids around every crevice with no result. I replaced all the gaskets on carburetor. What I have noticed is that the vacuum pressure is better after the engine is hot. My guess is something is expanding with the heat? Any suggestions on what would cause that?

 

[Posi]

What is your vacuum readings?
How many vacuum lines are you running, and where do they go?
It's harder to detect a vac leaks on V8 engines using fluids.
I would recommend using propane with a hose attachment, mush easier, at least on both china walls.

 

[thistle3585]

Its been running 15-17 then runs 19-20 when hot. I haven't tried a propane tank but I can try it.

 

[Lumpy Grits]

When was the last time you ck'd how tight the intake & exhaust manifold bolts are?
Do you have powder brakes?

LG

 

[Hedgehog]

Choke open or closed?

 

[thistle3585]

choke opened. Haven't tried exhaust manifold bolts but will do that. No power brakes.

 

[Lumpy Grits]

Re-torque the intake manifold bolts.
Disco the vac line to the dizzy and plug it-any change in vac readings??
LG

 

[thistle3585]

Here is a six second video of my vacuum gauge. Does this help? https://www.youtube.com/watch?v=PhlZohZ6rS0

 

[Lumpy Grits]

Sez video is private, and I have to sign in-
LG

 

[Posi]

Could be sticking valves, hydraulic lifters pumping up because the oil is too thick and cold oil pressure too high would be my first thought.

 

[thistle3585]

I just made the video public. https://www.youtube.com/watch?v=PhlZohZ6rS0

 

[Lumpy Grits]

Try a different vac. gauge.
That vac. drop can also be a cyl leak. Have you done a comp test or leak down test?
LG

 

[Hedgehog]

Now I'm confused. Do you have less vacuum at a constant engine speed when your engine is warm? Or are you worried about the drop in vacuum pressure when accelerating the engine? Loosing vacuum is absolutely normal when accelerating, it is supposed to happen. Once you are at cruising speed the vacuum should return to normal pressure.

 

[Lumpy Grits]

Now I'm confused. Do you have less vacuum at a constant engine speed when your engine is warm? Or are you worried about the drop in vacuum pressure when accelerating the engine? Loosing vacuum is absolutely normal when accelerating, it is supposed to happen. Once you are at cruising speed the vacuum should return to normal pressure.

Good point-AFTER I turn'd the sound on-I could here the engine speed up.
OP-Did you speed the engine up??
This could also be a carb issue if you did not-Example, is unbalanced idle mix from the screws not turned out the same amount-
LG

 

[thistle3585]

That is a brand new vacuum gauge and its idling at about 750 RPM. I didn't touch a thing to make it jump around. The vacuum is a little higher when the engine is hot but still fluctuates as it is in that video. I'll have to go buy the setup to test the compression. I'm thinking I might be as well off to just take it to a mechanic.

 

[Lumpy Grits]

If your not touch'n the throttle and it 'dips' like that. It very well could be a timing chain-cam issue or sticky lifter.
Did you ever re-torque the intake manifold bolts, like I suggested??
LG

 

[thistle3585]

No, not yet. Been under the weather and haven't had a chance to get in the shop. I hope to do it tomorrow.

 

[Lumpy Grits]

Get well and don't push yourself.
LG

 

[thistle3585]

Update: I torqued all the bolts and saw no change. I decided to mess with a couple things and ran the idle mixture screws on the carb out a couple turns. From 1.5 turns out to 3 turns out. The vacuum went up and held steady at 22. When I triggered the gas the vacuum dropped to zero then up to 26 then back down to 14 where the engine stumbled until slowly working its way back up to 22 where it held firm while idling. I drove it and it stumbled a bit and seemed to have a heavier exhaust smell.

One thing I noticed... I had the negative wire running to one of the intake bolts, so I moved it to a bolt that was holding the alternator to the block. I noticed that the long bolt that runs through the alternator, a spacer and a bracket and into the block next to the exhaust manifold was stripped out and just turning in the hole. I wasn't sure if that may have an impact on any of this. I have used propane and starter fluid to find leaks but can't find anything.

 

[Lumpy Grits]

Reset you mix screws to 1 3/4 turns out and see what happens. Your way to rich in mix now at 3 turns.
Your gonna need to do a compression test and a leak-down test.
How many miles on the engine?
LG