TheSamba.com :: View topic - Engine noise self healing magic, need explanation

Sun Sep 21, 2025 4:26 pm

I'll explain as much as I can.

I have a pretty decent 1972 VW Bus with a type 4 engine. The bus was in OK shape when I bought it, and I’ve since made it even better. I replaced the wires, reorganized the engine compartment, swapped the single carb for duals, installed a smart distributor, plus lots of miscellaneous small improvements.

One of the forum members (thanks, aeromech) helped me with a lot of this work.

The bus was running great. By “great” I mean there were no issues other than oil drips I still haven’t been able to fix. I’ve made multiple attempts, but it honestly doesn’t bother me too much.

Things changed after a light two-day rain. I left the bus outside the garage, and after those two days I tried to start it. Right away I noticed the sound was different.

I was surprised—how could the sound change so much just from sitting in the rain? It sounded like there was a hole in the muffler. I knew it couldn’t actually be the muffler, since those don’t just fail from two days of rain. You’d need a real hole or crack.

Also, the RPM was low when it first started. I had to adjust the idle quite a bit to hold it at around 1000.

The sound was terrible when I started driving. I thought, “Dang, the muffler must be broken and needs replacement.” My bus sounded like an old Harley with an untuned engine.

After warming it up a bit, I drove about a mile across the city. I didn’t notice a big loss of power, but the sound stayed bad—still like an old Harley. Then, about 1.5 miles in (on the way back home), I gave it some acceleration and… the engine fixed itself.

The sound went back to normal, which confirmed it wasn’t a muffler issue. It immediately became quieter and normal again. The bus runs great now, starts fine, and I keep it in the garage (we moved recently, and that’s my only option). Also, the idle RPM jumped to 2000, since I had adjusted it upward right before this short trip.

So I’m curious: why did this happen? How did it magically self-heal? Distributor you say? But the engine compartment was dry. The bus was not running, the rain was light.

Also, if anyone can recommend someone around the San Francisco Bay Area who can take a serious look at my oil leak, I’d really appreciate it—you’d be the third person to try.

Sun Sep 21, 2025 4:49 pm

Check the exhaust for missing or broken studs.

Sun Sep 21, 2025 4:52 pm

Fouled spark plug came back to life after it warmed up

Sun Sep 21, 2025 6:49 pm

Do you have hydraulic lifters?

Sun Sep 21, 2025 8:17 pm

If you are running ethanol laced fuel the stuff goes rancid if left sitting in the tank for more than a few weeks and the gum that causes can accumulate on the valve stem causing a valve to stick in its guide leading to problems in how the engine runs. If the problem eventually corrected itself the engine might instantly run normally again.

For now add a cup (8oz) of synthetic oil to your gas, and if you can't find a source of ethanol free fuel, add a half cup (4oz) of synthetic oil with each further fill up.

Sun Sep 21, 2025 10:41 pm

Type IV engines are subject to pitch change due to atmospheric conditions fluctuating. Sometimes they just sound 'stronger'.

BUT also the '72 cylinder heads feature three air injection ports fed through the cooling shrouds, three stubs into the heads. There are spigots that feed air in via an old smog pump for the 72-74 dual carb 1700-1800s.
They get crimped off but CAN unthread at give you explosive exhaust gas sound in the engine compartment which then acts as an echo chamber.
Yes they can slowly work loose. yes some people thread valve adjusters in those ports because that's the thread pitch in the heads.
Check that. One head has one port, the other has two. If you have the crimped spigots, the factory air feed tubes, you just tighten them with a Crescent wrench. They feature a bulge at the end that snugs against the cylinder cooling tin. And right into the head. Near the outside studs for the intake manifolds.

Sun Sep 21, 2025 11:19 pm

"Things changed after a light two-day rain. I left the bus outside the garage, and after those two days I tried to start it. Right away I noticed the sound was different.

I was surprised—how could the sound change so much just from sitting in the rain? It sounded like there was a hole in the muffler. I knew it couldn’t actually be the muffler, since those don’t just fail from two days of rain. You’d need a real hole or crack.

Also, the RPM was low when it first started. I had to adjust the idle quite a bit to hold it at around 1000.

The sound was terrible when I started driving. I thought, “Dang, the muffler must be broken and needs replacement.” My bus sounded like an old Harley with an untuned engine. "

If the rain has changed things, I assume, water went inside of cylinders, through airfilters, carbs.
Otherwise I´m also thinking about hydraulic lifters, but they are not original for 72. What engine, number do you have?
You have enough oil in your engine (and not too much, maybe also water?)

Mon Sep 22, 2025 6:50 am

Loose spark plug lead ? flashover through damp and cracked insulation?
Condensation and a layer of dust inside the distributor cap ?

Once it warmed up and dried out it fixed itself.

Mon Sep 22, 2025 7:59 am

smart distributor and many improvements ,could be the source of the problem.