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The bike is a Hero Honda Splendor Plus. When riding the bike I noticed a "Tshch" sound which I estimate happens immediately after the compression stroke of the engine. I took the bike to a local mechanic who removed a couple of tubes near the engine while having the engine running, but he couldn't figure out what the problem was. He did say that it seemed like there was an air leak somewhere and that it would go away after I rode the bike for a while. After riding the bike for a while it did seem like the sound stopped, so I assume it was because of any of the metal pipe's shrinkage during winter which gave some gap for air to leak.

Today morning I noticed that when I raised the kickstart lever to start the bike, if I just pushed the lever down by 2 centimeter with my foot, I could hear the "Tshch" sound from somewhere near the engine. Brought the lever back up and moved it 2cm down and the sound could be heard again.

Service centers tend to inflate bills and give bogus reasons, so I wanted to know what the problem might be before taking it to an authorized service center.
Also because the local mechanic didn't have a clue as to what the problem was.

What in your opinion might be the possible causes of such a problem? Images of the engine area are here: Troubleshooting fluid leakage onto the engine area?

EDIT:
Removed the spark plug and fitted it back as per the user manual. The sound persists. This time I moved the kickstart pedal with my hand and positioned my ear near the engine. The sound persists, and it's not from near the spark plug area.

enter image description here

I can see why the mechanic couldn't figure it out. It's hard to locate the source. At first the sound seemed to come from point1 in the picture. Then it seemed to be from point2. Then it seemed to come from the general area between 1, 2, 3 and 4. I need to debug this like how I debug software. Is it safe to yank out any of these tubes, plug them with my finger and push the kickstart pedal to check if the sound still exists?

  • Have you checked to see if the spark plug is fully seated and torqued to spec? – Pᴀᴜʟsᴛᴇʀ2 Jan 7 '17 at 11:17
  • Checked. That's not the problem. Edited my question with more details. – Nav Jan 8 '17 at 8:12
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If it really is only audible when the engine is at its compression stroke or the subsequent power stroke, in theory these are the only possible options:

-worn piston rings
-leaking head gasket
-leaking valves
-loose spark plug(s)

If you turn over the engine slowly, like when pushing down the kickstarter slowly, it's not uncommon to hear air leaking past the valves or the piston rings. That doesn't have a significant effect when the engine operates at normal speeds. I'd check if the compression is up to spec, it gives a little more insight in the situation.

The spark plugs are seated correctly you say. An exhaust gas analyzer at the local garage will tell you if the valves are leaking(high HC reading I think), when that is not the case, the piston rings and the head gasket are left. If it's the head gasket I suppose sooth marks should be visible somewhere around the gasket, and your coolant would get contaminated with oil an such. Then only the piston rings are left. If the compression looks OK, don't worry. If the compression reads too low, it's time to replace your piston rings.

OP Editing the answer to add to what was found to be the problem:

enter image description here

The red arrow points to where the sound originated from.

You estimated the problem and the cause correctly. The head gasket was damaged, and it was damaged exactly because they didn't replace the gasket with a new one after doing the engine head cleaning. The service person at the authorized service center today listened to the sound of air leaking and immediately concluded that the gasket was the problem. He admitted that it is the first time he's ever heard such a sound though, and proceeded to move the kickstart levers of other bikes just to check for such noises. The dismantling was done in front of me, and this is what the damaged gasket looked like:

enter image description here

  • Perhaps this could be it. The person at the authorized service center had recommended an engine head cleaning when I gave the bike for a carb tuning. Looks like the result of a poorly done job. I'll be escalating the matter and getting it confirmed at the service center. Thank you very much for helping. – Nav Jan 10 '17 at 0:49
  • Engine head cleaning as in: remove head, flatten head and block surfaces, fit new gasket? Then i hope they indeed also fitted a new gasket, otherwise that could be your problem. It could be a poor job or cheap service that resuses the old gasket. But i wouldn't suspect that immediately, first check the compression. If that's up to spec, i wouldn't worry too much. – Bart Jan 10 '17 at 7:52
  • Not fitting a new Aluminium gasket was exactly the problem. I've edited your answer with more details from my side. I'm amazed at how you figured out the problem seated miles away! Kudos and thank you very much for taking the time to help! – Nav Jan 10 '17 at 14:30
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    @Nav You're welcome! It's just logical thinking. If there's a sound during the compression or powerstroke, the options are limited. The air is inside the cylinder, and can escape only via the spark plug, gasket, piston rings or valves. Now you can easily rule out the spark plug and gaskets by spraying some foam on it and see if it moves. That's also how you can detect air leaks. A gas analyzer will tell you if there's leakage along the piston rings and (exhaust)valves. A compression test will tell if it's serious. And that gasket's showing some serious damage, i wouldn't go back to that shop. – Bart Jan 10 '17 at 21:23
  • Don't forget to clean the head and the block to prevent the new gasket from leaking. It may be necessary to (re)flatten the head and/or the block, with emphasis on 'may'. – Bart Jan 10 '17 at 21:26

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