Exhaust leak sound

[Bob from Northport]

Just started a new rebuilt engine on a 30 Sport Coupe. The engine runs very well, but there is a sound like an exhaust leak in the center area of the manifolds. Anyone have any tricks or methods to locate the leak area???
It just sounds loud compared to other engine builds we have done

 

[Synchro909]

Shield the carburettor intake against overspray and squirt some starter fluid around the area. When you hit the location of the leak (assuming there is one), you will hear the engine smooth out and sound like it should. Don't forget to spray from underneath too.
Now that yo have confirmed the problem is a vacuum leak and where it is, the rest is easy.

 

[nkaminar]

To find an exhaust leak, use a short piece of hose, such as a garden hose. Hold one end up to your ear and move the other one around the area where you suspect a leak. You will hear the leak when you get to it.

 

[Omar Morales]

To find an exhaust leak, use a short piece of hose, such as a garden hose. Hold one end up to your ear and move the other one around the area where you suspect a leak. You will hear the leak when you get to it.

I will have to try this! I believe I also have a leak, but I am unsure how to listen for it. I did buy replacement gaskets from Snyders but have not gotten around to changing them out.

 

[Tom F OHIO]

Bob, my brother-in-law had a sound like that also and we squirted some soapy water
to the manifold and you should have seen it bubbling. Definitely found our leak.

 

[Marshall V. Daut]

The only manifold gaskets I have had success at sealing leaks are the copper ones. The green cardboard gaskets blow out in short order and the one-piece aluminum-clad gasket is worthless. The copper gaskets are of course the most expensive, but they are worth it.
Unless your intake manifold is also leaking, generally exhaust leaks do not affect how the engine idles or runs. Such leaks are more irritating than anything else. The main problem with pesky exhaust leaks is that the rearmost portion of the exhaust manifold droops over time from repeated heatings and coolings. Unless gland rings are used, this will happen eventually to even new exhaust manifolds.
To avoid disappointment with your new manifold gaskets, remove the manifolds and lay a METAL straight edge across the tops of the ports. The straight edge should touch the tops of the ports equally. Chances are, though, the rearmost port will be lower than the other three exhaust ports = drooping manifold. No gaskets ever made for the Model A will seal a leak there - for very long. The solution is a new exhaust manifold in tandem with gland rings to keep the manifold from drooping in the rear.
Marshall

 

[old ugly]

i don't know if i would squirt starter fluid on a hot exhaust manifold. probably not.

soap and water may work but the the bubbles maybe misleading as that hot manifold will boil the water. i would try that tho.

the hose and listening through it sound good. i would try that.

the manifold is not hard to remove and check. use a good gasket like said above. use the gland rings if there is a recess for them. put a straight edge on the manifold or set it on something perfectly flat to check for being warped. check for cracks.

the connection at the exhaust pipe is a common spot for a leak. a manifold warp or droop is common. gasket issues are common.
i say if it sounds like a leak pull the manifold your going to have to do that anyway.

 

[JayJay]

Coupla comments:

1. I've had two cars now that I had to replace the exhaust manifold with new because the droop was excessive.

2. Like Marshall, I've not had success with the green paper manifold gaskets. I use the two-piece late 31 copper style and they work fine. I've had trouble with the one-piece copper buckling because the stud holes on the manifold do not line up with the studs.

3. I also use glands, which are pretty straightforward with a new manifold. (Interestingly, the new Burtz block does not have the relief for glands cut into the block. Not sure why.)

4. Be sure to check if your intake and exhaust manifolds are dead flat to each other at the block sealing surface. Any decent machine shop can deck that assembly. (See this from Snyders for a discussion on this point.)

 

[nkaminar]

It is also important to properly torque the nuts and use the proper washers. The proper washers are slightly conical. The concave side goes towards the manifolds. It acts as a bridge to apply proper pressure to both the intake and exhaust manifolds.

If the exhaust manifold and intake manifold are flat and co planer then you can apply 55 foot-pounds of torque. If they are not flat and co planer you should make them so. A mill is the proper tool but they can be sanded flat and co planer with gobs of elbow grease. Emery paper glued to a flat granite block works best. After machining check to see that the ears on both the intake and exhaust manifolds are the same thickness.

The manifold nuns should be re torqued after the first heat and thereafter when the head bolt nuts are re torqued.

 

[Marshall V. Daut]

Here's how I check the tops of the manifolds for drooping in the rear as well as whether the two manifolds are even across their faces. Oftentimes when manifolds are mated from other manifolds, the intake flanges are too short, creating a gap that gaskets cannot close = vacuum leak and poor idling.
Marshall

 

[JayJay]

Since we're diving into "lessons learned" with manifolds, a couple of other hard-earned words of wisdom* to pass on:

1. When checking the manifolds for coplanar across the mating surfaces as Marshall shows in his first picture, I believe the general consensus is that up to 0.002" is acceptable gap.

2. After an intake/exhaust manifold assembly has been decked, either by using a mill or by sanding as NK suggests, it's important to check whether there is enough recess left in the manifold to still accommodate the glands (if you are going to use them). A simple dial micrometer can measure the depth. Also measure the depth of the recesses in your block, and add the two together. That should be equal to or only slightly less than (say 0.010") the width of the glands. If not, you can grind or sand down the glands.

3. How much "droop" is acceptable for the exhaust manifold? If you want to use the glands, virtually none. If you can forego the glands, then I've been successful getting a manifold with a bit less than 1/8" droop to seal properly. Any more than that...

*Wisdom is gained by experience. Experience is gained by making mistakes.

 

[Synchro909]

Common opinion is that the droop is caused by the weight of the exhaust hanging on a manifold that gets VERY hot but I believe there is another cause. If the exhaust clamp at the back of the exhaust is too tight and the pipe cannot slide through it as it expands, it will push the back of the manifold forward. That will cause it to droop.

 

[nkaminar]

Very hot cast iron is somewhat malleable. Weight and pressure over time can cause it to bend while hot. It also expands over heat cycles. Old, well-used, cast iron cook stoves are warped, especially in the fire box. The Model A exhaust manifold is not symmetric at the back and this can cause it to warp. The design on the very last ones was changed to reduce the warping.

 

[Fullraceflathead]

I also believe 1 of the reasons the manifolds tend to droop is if you forget to advance the ignition timing you generate much much more heat in the manifold in fact it will glow red hot!!!