Author Topic: M21  (Read 183 times)

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M21
« on: 21 October, 2020, 12:41:30 »
on my M21, which is running nicely, i recently found that after revving up as the revs go down there is like a couple of bangs in the silencer. This is a new symptom and it's getting worse. Sounds like the baffles have started to disintegrate and are no longer silencing effectively. Maybe making exhaust gases not operating under the correct pressure. Would this sound about right? It does seem a bit far fetched that baffles would just break but everything was running so well, lovely tick over, mixture set correctly and no other mis-firing. I recently cleaned the carb and jets plus contact breaker and spark plug. Any suggestions would be welcome please.
john

BEESY

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Re: M21
« Reply #1 on: 21 October, 2020, 13:55:44 »
My understanding is that its unburnt fuel burning in the exhaust and its can be caused by a few things. Running a bit lean, perhaps timing slightly off or air an air leak somewhere. E.g where exhaust joins the head or perhaps one of the valves.

I would check exhaust joint 1st.

craigc11g

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Re: M21
« Reply #2 on: 21 October, 2020, 15:29:32 »
sounds like a air leak

Re: M21
« Reply #3 on: 22 October, 2020, 07:07:19 »
thanks. I had a closer look and discovered it is leaking between the head and exhaust pipe. Did read that these are push fit and can be tapped in. Maybe the end could benefit from opening slightly which i think i read in an old magazine somewhere, can be done using a suitably sized "drift" to slightly open the bit that pushes into the head. I shall deal with that first and hope this fixes the problem. Thanks for the help.
john

craigc11g

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Re: M21
« Reply #4 on: 22 October, 2020, 07:31:30 »
gun gum will sort this

BEESY

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Re: M21
« Reply #5 on: 22 October, 2020, 15:52:35 »
I've not done it but old school method would be to tap a cone shape piece of wood to expand the hole. I used silicone  to seal mine.

Greenfield

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Re: M21
« Reply #6 on: 22 October, 2020, 16:27:19 »
Looking at the problem from another angle, you could try retarding the ignition by a very small amount, just a couple of degrees and see if the popping stops. If there's no difference you can simply return it to where it was and concentrate on the air/carburation. If the baffles are a bit iffy, this will change the exhaust backpressure which can also play a part. I think the solution will be a bit of trial and error.

BEESY

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Re: M21
« Reply #7 on: 22 October, 2020, 18:21:06 »
I don't think dodgy baffles would cause popping in the exhaust. It would just sound loud.

It could be the timing a bit late. If the exhaust valve was to close before all the fuel had burnt then it would burn in the exhaust.

JulianS

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Re: M21
« Reply #8 on: 22 October, 2020, 18:34:59 »
PTFE tape is useful for sealing leaky exhaust port to exhaust pipe joints.