Intake manifold leakage is the number one cause of maladies ranging from hard starting to holes in pistons. Shadetree tests to find leaks, such as spraying combustibles around the nuts, are often inconclusive, as well as sometimes risky. Ether should never be used!
The following is a method that shows exactly where leaks exist, as well as an indication of their size.
With two Chiefs in for service, I set about testing the manifolds before disassembly. The apparatus I use is a plate cut to fit the manifold as the carburetor does, although a simple cork or stopper with a tube through it would do:
You will note that I use a low pressure regulator and gauge, as no more than 15 psi is normally equivalent to complete vacuum. Usually 10 psi will demonstrate any troublesome leaks.
A simple squirt bottle filled with diluted dish detergent is all that is needed to douse the manifold nuts and quickly show foam where any leaks might be.
With the intake valves shut and pressure introduced, it soon shows where the leaks are on this '51 Chief: The volume of leak was so great that it blew away the water faster than I could photograph it.
The same procedure on a '46 Chief showed a more typical result from a minor leak: Here you can see how the leak foams continuously on the left. Note also a another small leak on the right. Even this degree of leakage can cause tuning problems.
Another source of leakage that I have found occasionally is a casting flaw below the top bolthole on aluminum manifolds. Here it is found on a manifold that had also been previously fitted with brass sleeves to repair ferrule wear:
This becomes apparent when grinding away the usual distortion from bolt torque, as can be seen as the shadow between the bottom holes. This particular manifold was easily ground to clean up these imperfections, but the others had such a deep flaw at the top hole leading into the bore that I feared the carb lip would bottom out if I removed too much stock. I filled the void with a urethane compound, and hope that a modern thick gasket will do the rest. Ultimately weld repair would be in order. Or a new manifold.
Grinding can easily be accomplished on a common stationary disc grinder, the kind that uses adhesive paper abrasive discs.
The most feared source of leakage is when it occurs around the nipple in the cylinder itself, which is unfortunately common with Sport Scouts. Leakage can even occur around the rivet, although I have found this rare with Indians.
These circumstances ultimately require tedious removal of the nipple. It is best to cut and crush the nipple to remove it, as backing it out can destroy the threads in the cylinder. The rivet often has distorted the nipple's threads, and causes this damage. Re-installing a new nipple and rivet is equally demanding, and a high temperature sealant such as "Seal-Lock" is recommended. A cam-ground anvil is used for peening the rivet:
With the rivet inserted from the inside, the anvil is then placed so that rotating it with the wrench will jam the rivet tightly against the nipple, and allow it to be peened without forcing it back into the bore.
The face of intake nipples must be smooth. Any flaws at all will prevent the ferrules from sealing. Often they can be dressed with a flat stone.
And of course, the manifold and ferrules themselves must be in serviceable condition. Brass ferrules work-harden, and should be annealed if re-used. Gland nuts that have been over-torqued until they are oval, or distorted from chisels and punches often prevent an adequate seal.
Use no sealers when reassembling the manifold, only grease or anti-seize compound upon the ferrules and threads. Align it with the carburetor on it's support, and center it between cylinders before tightening.
Following these simple steps for an airtight
intake assembly will make starting and tuning much easier.
Inexpensive testplate, available for various manifold configurations
|Comments from the VI Mailing
From: Alan N. Campbell
Now I'm curious. I want to know more, like how to avoid getting a leaky intake manifold to begin with. What causes it, do you think? And what choices do we have for repairs? How bad does it have to be before something terrible happens? And do some of the leaks just go away after the engine heats up to operating temperature? Perhaps some leaks get much worse when the engine heats. Are there some sounds, smells or vibrations that might hint at what is going on?
The causes of leaks are inescapeable with our motor design.
In service, the manifold is subjected to cyclic stresses from the thermal expansion and contraction of the cylinders. As the cylinders get longer when hot, the V-configuration means that they spread farther apart. When cooled, they crush back, and this tends to force the ferrules to erode into the manifold spigots.
With many cycles of of heating and cooling, and the cinching up of the nuts to keep them tight, this can cause deep annular grooves. Add the effect of vibration from the motor itself, and the eccentric mass of the carb hanging off to one side ... and something's gotta move.
Remember that the ferrules seal at two interfaces: where they butt up against the nipple in the cylinder, and where the gland nut crushes it down upon the manifold spigot. If either of these mating surfaces lose their conformity, you lose the seal. Eventually the vibrations work-harden the brass to where it will no longer conform, and since it is crushed into the spigot, the entire assembly must often be replaced.
Minor leaks are compensated for with carburetor adjustments, but this makes for an uneven burn between cylinders, hard starting, etc.
And, of course, there are extraneous causes of leaks not ordinarily encountered with a well-maintained machine. These include Sport Scout nipples pushed out by using too long of a headbolt, over-torqued carb screws that deform the flanges, falling down on the left side and knocking the carb loose, etc.
I agree. I always try to use new seals whenever I have to pull the manifold off. When I have to put the manifold back on I screw each nut a couple turns each making sure everything is lining up correctly. Then begin screwing the nuts by hand I try to turn each nut the same amount. That is I don't put one nut up all the way. I tighten them up the same amount. I start the engine and let it warm up and check with a spray bottle also. If I notice a leak I tighten up whichever side is leaking.
If I have to put the cylinders on I always put the manifold on loosely while I'm drawing the cylinders down I keep working on the manifold nuts at the same time and that way I'm pretty sure all is lined up correctly. I found out a long time ago if you put the cylinders on without the manifold and nuts you will most likely have to loosen the cylinders to get the manifold to line up. I hope this info helps someone out.
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