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Post Info TOPIC: Back firing and popping from exhaust on deceleration/over run


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Back firing and popping from exhaust on deceleration/over run
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 Hi all,

During deceleration/over run I get a popping/backfiring.  The engine also hunts when I'm at low load/low revs. It also jerks when I'm just rolling the throttle - such as rolling up to a T junction and gently pulling away without stopping. Everything is fine when I'm loading up the engine with some revs. I never had this issue before with the bike.

So far I've changed all the jets, diaphragm and seal and o-rings - everything on the bottom of the carb(I got the Carb Kit from the TTR shop. I've also changed the inlet rubber from carb to cylinder head(Old one appeared fine- cheap Chinese replacement went on ok). I also tried new fuel. I still have the problem. I seem to remember reading the choke can leak air in if seals are damaged - but I can't find anything on this when I look on this forum again. I had suspected the inlet/exhaust valve/seat had burned out/leaked- but I get loads of power. I've done a compression check on a suitably warm engine and it came back at only 70 psi - worrying- but as I say I'm getting loads of power above 1200rpm- I suspect I'm not getting the o-ring on the end of the compression tester to seat and seal properly. I'm going to clean this area and see if I'm going to get a better seal and then have confidence in any compression reading I get. But I’ll be amazed if there is any major issue with the engine(knackered valves, bores, head gasket) because my bike pulls like a train above 1200rpm.

Unless someone else has verified such major issues on strip down when their bike has still had loads of power above idle? If the compression reading still comes back low after I’m certain I’m getting a good o-ring seal I’ll strip down the engine during winter but in the meantime if anyone has any ideas or guidance I’d appreciate it.

Also I’ve seen the cheap Chinese 4 valve set for £40 and complete gasket set for £14 on eBay – has anyone any experience of them? They are £160 and £100 respectively elsewhere

Thanks

Tony

 

 

 

 

 

 

 

 



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I recently had similar symptoms with a couple key differences:

   1. Compression was in spec (I expect your compression is too given the power you have)

   2. The problem occurred instantly, i.e., it was running well then had symptoms on the next startup

Ultimately, my issue was caused by a blockage in the supply circuit in the float bowl - see photo (courtesy of DC and scam):

 

float bowl passage mystery.jpg

 

Compressed air cleared the blockage and the symptoms disappeared. I have added this step to the carburetor service routine from now on.



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Thanks Fat Leopard I'll take a look there.



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every 4th tank of gas or so I put carb/ injector cleaner in the fuel.....

my carb always looks like new inside..


..

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Bikes... 06 TTR250 / 09 XT660R / 1977 Montesa Cota 348 MRR / 1979 Montesa H6 125 Enduro...

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petenz wrote:

every 4th tank of gas or so I put carb/ injector cleaner in the fuel.....

my carb always looks like new inside..


..


 I do the same  the carb is always spotless 



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I'd put money on it being the carb stub from engine to carby, letting air in.

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Changed that Leigh after I done the diaphragm and lower carb stuff.

Just zapped the circuit Fat Leopard suggested but that was already clear with a six inch jet of carb cleaner firing out of it.   



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Do you remember which jets you used as replacements? A 52.5 pilot seems to help your condition. You may also need to fine tune the needle. By symptom I would guess you are lean near idle but you may want to move your needle up one and down one clip land to test. Keep us posted.



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ttboof wrote:
petenz wrote:

every 4th tank of gas or so I put carb/ injector cleaner in the fuel.....

my carb always looks like new inside..


..


 I do the same  the carb is always spotless 


 How much cleaner do you guys add? A good idea to do it so i think i shall.



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1996 TTR 250 OE, 1996 YAMAHA WR 200, 1990 YAMAHA FZR 1000,2001 HONDA DEAUVILLE 650, 2005 TTR 250, 1998 Honda CR125R( in pieces) 1994 Kasawaski KX 250.

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