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LM1 clone Backfire/Misfire on overrun?

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Jonathan View Drop Down
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    Posted: 10 Mar 2020 at 14:03
Hi
Just joined, see intros section, and I have one query for the moment.
On overrun with throttle closed I have a lot of back firing, poppling noises(perhaps more so once hot). The bike is an 850t converted to LM1 with head and flywheel mods apparently done. Is this normal? If not where should I start? Do I remember something about unburnt fuel from past reading?
Many thanks for thoughts
cheers Jonathan
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Brian UK View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post Posted: 10 Mar 2020 at 15:59
A leak in the exhaust system can cause this, check that first.
Brian.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken-Guzzibear Quote  Post ReplyReply Direct Link To This Post Posted: 10 Mar 2020 at 19:49
Any air leak a the exhaust or inlet will cause this you often find old exhaust gaskets in the zorst port .... any tiny air leak will cause some loud and spectacular backfires 3ft long flames even lol 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sardineone Quote  Post ReplyReply Direct Link To This Post Posted: 10 Mar 2020 at 19:57
Not normal if properly tuned for sure.  Are your carburetors in sync?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote -HJB- Quote  Post ReplyReply Direct Link To This Post Posted: 10 Mar 2020 at 20:24
I had a small hole beneath where the crossover pipe is welded to the headers at one side. Made pedestrians dive for cover assuming a drive-by until I fixed it! Makes you realise how much fuel is still coming through those big Del Ortos off throttle!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Guzzis3 Quote  Post ReplyReply Direct Link To This Post Posted: 11 Mar 2020 at 00:56
Neutral cure silicon. I know I bang on about it but it really is the cure all for exhausts. Take it all apart, wipe silicon (NOT acid cure, it will be written on the tube) either side of the exhaust gasket, wipe it into the female side of every joint. It will seal but also prevent it all rusting together  when you next want to disassemble.

On threaded collars wipe generously into the thread. Stops them coming loose and acts as a lubricant next time you unscrew.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jonathan Quote  Post ReplyReply Direct Link To This Post Posted: 11 Mar 2020 at 07:17
Many thanks everyone.
My parcel arrives from Gutsibits tomorrow so I will do a full exhaust disassembly and refit and post the results. I’ll see if I can post some piccies too.
Cheers Jonathan 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TheWrongTrousers Quote  Post ReplyReply Direct Link To This Post Posted: 11 Mar 2020 at 08:08
Originally posted by Guzzis3 Guzzis3 wrote:

Neutral cure silicon. I know I bang on about it but it really is the cure all for exhausts. Take it all apart, wipe silicon (NOT acid cure, it will be written on the tube) either side of the exhaust gasket, wipe it into the female side of every joint. It will seal but also prevent it all rusting together  when you next want to disassemble.

On threaded collars wipe generously into the thread. Stops them coming loose and acts as a lubricant next time you unscrew.

That sounds like a good tip, i have not heard of that before. Does anyone else do this ?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Throwley Quote  Post ReplyReply Direct Link To This Post Posted: 11 Mar 2020 at 08:53
Has the bike always done this, or is it something new?

Could be that idle jetting needs looking at, lean jetting will cause exactly these symptoms.  If there's no apparent exhaust leak, try going about five points up on the jet.  It's likely you'll be able to tune the rest out on the screws.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brian UK Quote  Post ReplyReply Direct Link To This Post Posted: 11 Mar 2020 at 09:13
In the intro he said he had just bought it, so who knows what it was like before.

Brian.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Throwley Quote  Post ReplyReply Direct Link To This Post Posted: 11 Mar 2020 at 09:30
I'll confess to being just too lazy to check that, Brian.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Guzzis3 Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 2020 at 00:20
Originally posted by TheWrongTrousers TheWrongTrousers wrote:

Originally posted by Guzzis3 Guzzis3 wrote:

Neutral cure silicon. I know I bang on about it but it really is the cure all for exhausts. Take it all apart, wipe silicon (NOT acid cure, it will be written on the tube) either side of the exhaust gasket, wipe it into the female side of every joint. It will seal but also prevent it all rusting together  when you next want to disassemble.

On threaded collars wipe generously into the thread. Stops them coming loose and acts as a lubricant next time you unscrew.

That sounds like a good tip, i have not heard of that before. Does anyone else do this ?

I served my trade as a motorcycle mechanic in the 80's. I've been doing it on all my bikes since and shared the tip about the place. I have had feedback from people who have taken it up and were happy with the results.

One of my guzzis has the threaded exhaust retainer and silastic cures all the problems with that. There is a tremendous sense of relief knowing next time you take your exhaust apart it will be easy, not the nightmare it can become.

And there is something you can squirt on the silastic to make it swell and come off easily. I can't remember, alcohol, petrol, acetone. Something like that. So it's easily reversible.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ian T Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 2020 at 07:07
Such a great tip and soo cheap to buy. 

Wonderful and thanks

Ian
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TheWrongTrousers Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 2020 at 08:06
I would have thought that the heat would melt it/burn it up. it's very counter intuitive, but I am prepared to take your word for it. It so happens that my exhaust system is currently off the bike and will be put back on in the coming days, so I am off to buy some. Your tip is very timely. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jonathan Quote  Post ReplyReply Direct Link To This Post Posted: 12 Mar 2020 at 09:22
Yes, thanks for the tip Guzzis3. 
I looked up neutral cure silicon on eBay and it appears to be simply a tube of soft plastic silicon sealant at a cost of about £2 a tube and advertised as a double glazing sealant. Is this definitely the stuff? It seems very counter-intuitive and you would think it would either melt or burn(as TWT says). I also found Dow Corning red Silastic at £37 a tube and wonder if this is what you mean? Appreciate clarification as I intend to follow your advice
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