Still chasing low manifold vacuum--best way to seal intake manifold?

[Goldhawg]

Well, going to take the recommendation to pull the intake off today and reseal the manifold. I ordered a set of Mr. Gasket ultraseal gaskets which are thicker. ONe question, when I sealed it the first time I did not use the front or rear gasket that came with it, but went w/the recco to just use a bead of RTV. Any ideas whether I should use both, i.e., a small amount of RTV on each side of the gasket, or just use the gasket solo, or continue with a single bead of RTV? I previously used Permatex's ultra black.

Thx for your thoughts.

 

[Justafordguy]

I have always used RTV only on the front and back. I have installed tons of manifolds on many different V8s and V6s this way and have never had any leaks.

 

[broncnaz]

X2 RTV only on the valleys.

 

[Pedestrian]

X3 just a nice bead of Grey RTV

 

[Broncobowsher]

RTV in the valley x4.

Manifold vacuum is related to the intake runner ports. Silicone on the ends of the valley do not affect this. You could seal the intake runners and put nothing in the valley ends. It will drool oil like mad but still seal the runners.

 

[Goldhawg]

Looks like the consensus is to just do it the same way again...

thx

 

[DonsBolt]

RTV only in the vally X 5 or whatever number I am

Something elso to concider if your chasing a vacuum leak, maybe one, or both of the intake manifold gaskets have shifted a little when to placed the intake back down.

So take your time,
After you really clean both the heads, and the intake

Place the gaskets down on the heads, line them up real carefully, then take a marker, and mark where they line up on each head. You may need to use some masking tape between some of the ports to keep it from moving on you in the next step.

Then sit the intake down, making sure the gasket stays where you marked it, and take the marker and make a line at the top edge of the intake on each the gaskets.

Then remove the intake, remove the gaskets, the line you just drew on the gaskets will be a good guide when you do your final instal.

If the Gaskets slips even a 16th of a inch could cause a vacuum leak

Another thing you could do is get a set of intake manifold studs, one good thing about our 302's or 351's is the manifold bolts screww straight in, not at a angle like most other motors, so we are about the only guys that can use studs on out intakes. With studs you can place the intake down a lot straighter, though the one downside to studs is you have to remove the distributor anytime you remove the intake.

 

[Goldhawg]

Well, don't know if these pics suggest anything, but the gaskets staying on the intake when I removed it. Seems like they were ok, but I'm open to thoughts.

In any case, I'll be putting on the ultra seal gaskets and cross my fingers.

 

[1970mule]

well ill be odd one out, i had problems when i just used rtv, then i used both sealed like a champ. but i have to say the key is to wait at least 24 hours before you start it, 36 is best i waited 12 and every time i blew the rtv out the manifold.

 

[Bronco73]

Try to pin point the leak. Why do you think it's the intake gasket? It could be several things, carb, a vac line, carb base gasket and so on. Check everythig for flatness while it's apart.

 

[Goldhawg]

Try to pin point the leak. Why do you think it's the intake gasket? It could be several things, carb, a vac line, carb base gasket and so on. Check everythig for flatness while it's apart.

Well, I have been tracking this on another link here
http://classicbroncos.com/forums/showthread.php?t=164037&highlight=goldhawg

Checked w/propane all possible above manifold sources of a leak, and adjusted the valves again just to be sure they were not too tight, and the cam is a mild RV cam (.472 lift). So if this isn't it, I'm really stumped.

 

[Quick & Dirty]

I use High Tack to keep gaskets from moving around during installation. On intakes, I use it on both sides to provide a bit of extra sealing ability.
It brushes on, sort of like rubber cement, but is impervious to gas and oil.

 

[broncnaz]

Hard to tell but I'd almost say that 5,6,7 were leaking and maybe 1,4. Also could be leaking from the other side of the gaskets as well.
Cam lift has nothing to do with vacuum duration is what affects vacuum. But we've already determined that you should have more vacuum than you do.
You cant always tell if a gasket was leaking but if there is oil/carbon/varnish staining in the intake ports I would say thoses ports may have been leaking.

 

[Bronco73]

Have the heads or intake been milled at all?