This is a small Fiat 500 (classic), two cylinders, air cooled engine I have partially rebuilt. The person that previously touched the engine said changed the distribution sprockets and chain, so I didn't inspected that area. I checked the valves for leak, no leaks. Then calibrated the valves, set the ignition, etc. so the engine starts and adjusted the carburetor to get a steady and even idle. Mounted the engine in the car and did a few trips, no problems whatsoever; the engine starts well, turns off ok, power is felt as normal i.e., normal compression, nice idle, so far so good. BUT: when I crank it and right before it starts working I can hear a sucking noise I compare with the one that air compressors do when you manually turn them. Something like "foop" "foop" "foop" in low tone. Then it immediately starts and the noise either disappear or gets mixed with the engine's regular noise. As something I'm considering a side effect: while driving, when releasing the accelerator I get backfire noise "pop" "pop" "pop".

Would it be the chain off by 1 or 2 tooth? Jus asking before unmounting the engine again...Since all the rest seems to be ok: valves calibration, ignition timing, carb adjustment, engine starts right away, has power, etc...

  • Welcome back :o) Dec 16, 2021 at 1:06
  • My first thought is valve timing, but really it'd be hard to tell. Dec 16, 2021 at 1:08
  • You mean distribution sprocket positions? Dec 16, 2021 at 1:09
  • How did you verify valve timing (calibration)? You state it's good, but how do you know it's good? Dec 16, 2021 at 1:16
  • In this engine, calibration is intake 0,20mm, exhaust 0.25mm which I checked at least three times before and after heating the engine. Dec 16, 2021 at 1:28

2 Answers 2


Deceleration backfiring is often from too rich a fuel mixture and unburned fuel igniting in the exhaust side. So before before taking the engine back out, I'd look at the carb and timing.


So after a couple of days pulling my hair I found the reasons to both issues:

a) Sucking noise was made by a metal tab in the air filter box's intake. I realized that by removing the breathing hose from the valves cover to the filter box the noise disappeared. So I removed the kind of deflecting plate right where the hose gets connected and noise is gone now.

b) backfire: just installed a new ignition coil and problem solved. I did it accidentally when I found that the engine suddenly stalled without a previous notice, a clear sign of bad coil.

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